REVIEW

Emerging infectious diseases of crop plants in developingcountries: impact on agriculture and socio-economicconsequences

Maurizio Vurro & Barbara Bonciani & Giovanni Vannacci

Received: 10 February 2010 /Accepted: 29 March 2010 /Published online: 16 April 2010# Springer Science+Business Media B.V. & International Society for Plant Pathology 2010

Abstract Emerging infectious diseases (EIDs) caused byplant pathogens can develop into unexpected and very seriousepidemics, owing to the influence of various characteristics ofthe pathogen, host and environment. Devastating epidemics,having social implications by increasing the rate of urbani-zation, occurred in the past in Europe, and many other EIDsstill occur with high frequency in developing countries.Although the ability to diagnose diseases and the technolo-gies available for their control are far greater than in the past,EIDs are still able to cause tremendous crop losses, theeconomic and social impact of which, in developingcountries, is often underestimated. In the present article, fourof the most important EIDs in developing countries areconsidered from the standpoint of their origin, characteristics,symptoms, mode of spread, possible control strategies,economic impact and the socio-economic consequences oftheir dissemination. They are Cassava Mosaic Virus Disease,

capable of reducing yields by 80–90% and causing thesuspension of cassava cultivation in many areas of EastAfrica; Striga hermonthica, a parasitic weed affectingcereals in an area of at least 5 million hectares in Sub-Saharan Africa; Xanthomonas Wilt of Banana, a bacterialdisease that caused around 50% yield losses at the beginningof 21st century in Uganda and is threatening the foodsecurity of about 70 million people owing to its impact onan important staple crop; and race Ug99 of the rust fungusPuccinia graminis f. sp. tritici, which is having a tremen-dous impact on wheat in Uganda, and is also threateningmost of the wheat-growing countries of the world.

Keywords Emerging infectious diseases . Cassava MosaicVirus Disease . Striga hermonthica . Banana XanthomonasWilt .Wheat Rust Ug99

Introduction

“Emerging infectious diseases” (EIDs) are those caused bypathogens which, for a number of different reasons,develop into epidemics that may be both unexpected anddevastating. Some of the best known epidemics appearedduring the 19th century and coincided with the advent ofmore intensive agriculture and reduction in the duration ofsea voyages. The latter allowed increased internationaltrading and, as a corollary, the introduction into new areasof foreign species of plants and parasites, resulting in morefrequent upsetting of agro-environmental balances.

Sometimes EIDs developed into pandemics over wholenations and even continents, causing famine and favouringhuman diseases, socio-economic disasters and technicalcrises for the management of whole agricultural communi-ties. For example, Phytophthora infestans, the Oomycete

Authors have contributed equally to the preparation of the manuscript.Dr. Bonciani prepared the paragraphs relating to the economic andsocial impact of the case-studies considered.

M. Vurro (*)Istituto di Scienze delle Produzioni Alimentari,Consiglio Nazionale delle Ricerche,Via G. Amendola 122/O,70126 Bari, Italye-mail: maurizio.vurro@ispa.cnr.it

B. BoncianiDipartimento di Scienze Politiche e Sociali, Università di Pisa,Via Santa Maria 46,56100 Pisa, Italy

G. VannacciDipartimento di Coltivazione e Difesa delle Specie Legnose“G. Scaramuzzi”, Università di Pisa,Via del Borghetto 80,56100 Pisa, Italy

Food Sec. (2010) 2:113–132DOI 10.1007/s12571-010-0062-7

agent of potato late blight, was the primary cause of thegreat Irish famine of the nineteenth century. The pathogenoriginated in the Andes and was observed in North Americain 1843 (Gomez-Alpizar et al. 2007). Due to intense trade itreached Europe two years later (Fry et al. 1992). Thedisease caused significant yield losses, which were partic-ularly catastrophic in Ireland owing to the wetness of theclimate and the almost total dependence of a largeproportion of the population on potato (Large 1940). Outof a population of 8 million approximately one million diedof starvation and 1.5 million emigrated, of which about aquarter died in transit (Klinkovski 1970). Thus, the firstmassive migration of modern history was caused by a plantdisease.

The most recent of the great famines occurred in EastBengal in 1943, where the failure of the rice crop causedthe starvation of an estimated 2–3 million people. Theaetiology of the disease is disputed but many attribute it tothe fungal pathogen, Cochliobolus miyabeanus, the dis-semination of which was favoured by the environmentalconditions pertaining at the time (Padmanaban 1973).Others suggest that high levels of iron or aluminium or anoutbreak of the brown planthopper were the cause of theproblem (Strange 2003).

The great Southern Corn Leaf Blight epidemic wascaused by a variant strain of the fungus, Cochliobolusheterostrophus, named race T, which was specificallyvirulent for maize containing a cytoplasmically inheritedgene for male sterility (Tcms). Because of the advantageconferred by the gene in breeding this self-fertile crop, ithad been incorporated into about 85% of the American cropby 1970. As a result and aided by favourable climaticconditions a pandemic developed with the epicenter in the“corn belt” causing enormous damage in 1970–71. Thepandemic was halted by the withdrawal of susceptiblevarieties and the establishment of new hybrids. SouthernCorn Leaf Blight is infamous for having shocked the worldfeed market and for having set a record in terms ofeconomic losses produced on a single agricultural crop ina single season (Scheffer 1997). Subsequent to the disaster,the reason for the specificity and high virulence of C.heterostrophus race T for Tcms maize was determined to bethe production of a so-called host-selective toxin by thefungus.

Land use by English settlers and population growth hadled, in Ceylon (now Sri Lanka), to an enormous expansionof coffee cultivation during the first 50 years of thenineteenth century. In 1868, prosperity from the crop hadreached a maximum but then Hemileia vastatrix, a rustfungus, was found which was likely to have spread fromEthiopia, the centre of origin of both the plant and its rust.Initially damage was thought to be light but may have beenunderestimated by the British planters who could compen-

sate for lower production with increased prices. But thedisease spread to all the plantations and production lossesquickly became economically unsustainable. By 1905 thearea planted to coffee in Ceylon had shrunk from 275,000acres in 1878 to around 3,500 in 1905 (Mills 1964).Because of the pandemic, coffee had to be replaced, luckilywith success, by tea.

Thanks to technological advances in, for example,diagnostics, agronomic practices and the use of specificdisease management strategies, the risk of epidemicsoccurring with catastrophic consequences has been sharplyreduced in developed countries compared to developingcountries (Waage et al. 2009). Unluckily, the stability of theagricultural systems reached with great difficulty is veryoften upset by the sudden appearance of novel parasites andpathogens, as well as by environmental and technologicalalterations in management practices. If we consider themany factors relating to the pathogen, the host or theenvironment that can affect a disease, it is easy tounderstand that the “emergence” of a disease is thecoincidence of a number of unfortunate events. Further, ifwe also include consideration of socio-economic conse-quences in the evaluation of the seriousness of a plantdisease, we will find that some devastating epidemics,reported only in history books for Europe, are stilloccurring very frequently in many developing countries.

As with pathogens of humans and domestic or wildanimals, the emergence or re-emergence of phytopathogen-ic agents is very often due to man’s activities, such as theirintroduction into novel areas as a consequence of masstourism, global trade, farming changes and environmentalchanges. Although only a fraction of a pathogen commu-nity is introduced together with a newly-introduced plantspecies, this seems to be the most important cause ofexpansion of an emerging disease to a new area. Moreover,if the pathogen responsible has a wider host range than theplant species introduced it may infect indigenous plantspecies, which may be particularly vulnerable as they willnot have co-evolved with the pathogen.

Although introductions of alien pathogens may occurowing to the trading of vegetables, germplasm, grafts orwhole living plants, introductions via international seedtrading is a particularly important vehicle for pathogenintroduction and dissemination. For example, it has beenestimated that at least 2,400 different plant pathogens werecontained in the seeds of 380 plant genera (McGee 1997),and that up to one third of the plant pathogenic viruses aretransmissible through seeds to at least one of their hosts(Stace-Smith and Hamilton 1988).

The simple introduction of a pathogen into a new area isa necessary but not sufficient requirement to produce anepidemic. Many factors influence the success or failure ofan introduced pathogen to cause an epidemic, such as the

114 M. Vurro et al.

environmental conditions or the genotypes of the potentialhost plants. In particular, in the case of pathogens which aretransmitted by vectors, it is the introduction of the vectorinto a new area that may be the “real” cause of theoccurrence.

Lacking the elements favouring their further dissemina-tion, some pathogens may remain restricted to their area ofintroduction, making very limited impact. For example,Citrus Tristeza Virus (CTV) was probably introduced intoSouth America in the 1920s but only became economicallydevastating in the 1950s owing to the introduction fromAsia of a very efficient vector, the aphid Toxopteracitricidus. Since then, in Brazil, more than 6 million citrustrees have been destroyed (Bar-Joseph et al. 1979). Pierce’sDisease (PD) of grapevine, caused by the bacterium Xylellafastidiosa, was reported in California as being not seriousfor more than a century but in 1997 a new vector,Graphocephala atropunctata, was introduced into Califor-nia. This allowed the rapid development of the disease inthe vineyards, with damage estimated in 1999 at 6 milliondollars (Anderson et al. 2004). In some cases the reciprocalsituation can occur, i.e. plants species introduced in novelareas are affected by endemic pathogens. For instance,cassava mosaic viruses are not known in South America,the centre of origin of cassava, but as will be describedbelow, they have caused havoc in the crop in Africa towhich the plant was exported in the 16th century (Strange2003).

Climatic changes have very often been connected withthe appearance of epidemics in humans and animals, butvery little is known about their effects on plant EIDs(Garrett et al. 2006). Changes in the incidence and severityof plant diseases are likely to occur and these will varyaccording to the particular pathosystem. In addition,incidence and severity will also be influenced by otherfactors, such as the use of transgenic plants, the availabilityof new chemicals and changes in land management.

Changes in farming systems have determined theappearance of a number of EIDs both of crop and wildspecies. In some developing countries, the lowering of thevalue of traditional crops and the higher demand for non-traditional crops has caused an increased cultivation of thelatter. The introduction of highly productive selectedmonocultures has reduced genetic variability, increasingthe risk of exposure to pathogens. For example, in 1970,outbreaks of Southern corn leaf blight and yellow corn leafblight destroyed 17% of all US maize crops, 85% of whichwere of the same variety, susceptible to these diseases(Pring and Lonsdale 1989).

In the next sections, four EIDs of great importance todeveloping countries, in particular to Sub-Saharan Africa(SSA) will be considered as models, chosen for theirdifferent origins, agents and means of dissemination.

Species of the genus Striga, although often referred to asparasitic weeds, are actually pathogens and are thereforeconsidered. Although there is an extensive bibliographyavailable regarding the biology, symptoms, distribution andcrop losses of some pathogens, data on their economic andsocial impact are scarce. As a result, several of theestimates of impact on crop production losses due todisease cited in this paper are at best based on assumptionsrather than certifiable data. This is much more evident forpathogens affecting crops which are neither widely grownnor exported and are therefore of interest only to localpopulations. Here the lack of a network of technicalassistance for monitoring, surveying and controlling diseasemeans that such information is far from complete and thistherefore represents a considerable handicap for cropprotection. These problems are particularly severe indeveloping countries where when a disease is reported as“new”, it is often already widely distributed in theenvironment without any control, increasing the risk ofpandemics.

Cassava Mosaic Virus Disease

The plant host

Cassava (Manihot esculenta Crantz) (Fig. 1) is a shrubbyperennial plant belonging to the Euphorbiaceae family. Ithas been cultivated in South America and particularly in theAmazon basin for millennia for its starchy roots, but it wasonly introduced into Africa by the Portuguese in thesixteenth century. Thereafter, its drought tolerance andability to produce yields on even marginal lands wasappreciated. As a result, its cultivation spread slowly acrossAfrica, largely by way of the river trade in Central andWestern Africa (Legg and Thresh 2000). There was a

Fig. 1 Manihot esculenta (cassava) - Unprocessed roots - Wikipedia

Agricultural and socio-economic impact of infectious crop diseases 115

considerable expansion of the area devoted to the cropduring the colonial period, reaching its present distributionduring 1920–1930 as a consequence of the authoritiesencouraging its cultivation as a food reserve for periods offamine and drought. Moreover, cassava (as well as banana,considered in another section of the article) is ideally suitedto SSA, because it requires minimal field management.Cassava now constitutes a major source of food in SSA, anda significant source of revenue from the sale of the fresh orprocessed crop. The status of cassava cultivation today ischanging from subsistence farming to an industrializedsystem designed to process cassava into a diverse spectrumof products, including starch, sago grains, flour, chips,animal feed and, potentially, biofuel, all derived from a cropthat has the ability to grow in poor soils (Thresh 2006).

Origin, distribution and impact

Cassava Mosaic Virus causes the most important disease ofcassava in Africa. Symptoms consist of mosaic yellow oryellow-green chlorosis, leaf deformation and stunting(Fig. 2). The disease was first reported in Tanzania(Warburg 1894) and it was assumed to be caused by avirus, as the pathogen could be transmitted mechanicallyand was not visible. Only in more recent times has the exactaetiology been determined, with the agent of the diseasebeing identified as a geminivirus (Bock and Woods 1983).There are some reports of its spread in the early decades ofthe last century, but none that were “alarming” as regardseither disease severity or rate of spread. However, therewere some sporadic outbreaks during the period 1920–1940that led to the initiation of some crop protection pro-grammes, especially the introduction of resistant varieties.

Nevertheless, until the mid-1980s, Cassava MosaicDisease (CMD) was considered to be just one of severaldiseases affecting cassava (Otim-Nape 1987). The situationchanged suddenly in 1988 when a serious outbreak wasreported in northern Uganda. Owing to social and politicalinsecurity and instability at that time, following the flight ofthe dictator, Idi Amin, 10 years previously, it was notpossible to obtain accurate analysis of the situation. Basedon a series of observations, it was hypothesized that highertemperatures and lower humidity in that area of the countryhad favoured the spread of Bemisia tabaci, a polyphagouswhite fly insect and vector of the virus, thus indirectlyfavouring the spread of the virus itself (Otim-Nape 1993).However, this hypothesis soon became untenable because,in subsequent years, the disease spread southward to areasthat were more humid and colder at the rate of 20–30 kmper year (Otim-Nape et al. 1997; Legg and Ogwal 1998).Also, symptoms were more severe at the disease front,whereas vector populations were more numerous where thedisease was already present.

The effects of virus disease on the farming communitiesin Uganda became evident in the early 1990s. The initialimpact was greatest in the north-eastern areas of thecountry, particularly because of the cultivar grown, Ebwa-nateraka, which later proved to be the most susceptible tothe virus. Here, cassava production between 1990 and 1993was reduced by 80 to 90% and many farmers suspended itscultivation (Thresh and Otim-Nape 1994). In 1993, thefailure of the crops of maize, beans and other food crops,owing to drought, compounded the lack of cassava as afood reserve, leading to widespread food shortages andfamine-related deaths (Thresh and Otim-Nape 1994). Acommon reaction to this situation was the cultivation ofother crops, mainly sweet potatoes. The impact of theepidemic in central and western regions of Uganda was lessacute, mainly due to the use of a greater range of varieties,and therefore to the presence of certain varieties moretolerant to the disease. But the effects were still extremelyserious. Several attempts have been made to quantify thelosses due to the virus, the most reliable estimate beingaround 600 thousand tonnes per year valued at 60 milliondollars (Otim-Nape et al. 1997, 1998).

When the impact of the epidemic became very clear, thereal causes that had led to the outbreak of a disease that hadhitherto been relatively innocuous were investigated.Serological and molecular techniques demonstrated theexistence of virus variants which differed in virulence.Two are prevalent in Africa, African cassava mosaic virus(ACMV), and East African cassava mosaic virus (EACMV)(Swanson and Harrison 1994). However, in Uganda, avariant, which appeared to be a recombinant hybrid ofEACMV and ACMV has been detected and is referred toeither as the Uganda variant (UgV) (Zhou et al. 1997) or as

Fig. 2 Cassava plant with the typical symptoms of cassava mosaicvirus on the right and a healthy plant on the left. Courtesy of Dr.Danny Coyne, IITA, Kampala, Uganda

116 M. Vurro et al.

a distinctive strain of EACMV (EACMV-Ug) (Deng et al.1997). The enhanced severity of the disease was relatedboth to higher virus titres in cassava plants infected withUgV and the widespread occurrence of plants with mixedACMV/UgV infections in which symptoms were mostsevere (Harrison et al. 1997). The whitefly vector, althoughpolyphagous, was much more prolific on infected plants.Also, in seeking less crowded areas, whiteflies enhancedthe rate of dissemination of the virus. Moreover, the declinein cassava cultivation increased the capability of whitefliesto spread the disease to non-infected areas (Legg andThresh 2000).

The severe CMD epidemic subsequently expandedrapidly to Kenya, affecting in a few years (from 1995 to1998) virtually all the areas of cassava cultivation (Legg etal. 1999). Field observations estimated yield reductions ofapproximately 140 thousand tons per annum in those areas.The disease subsequently spread to the Sudan and Congobut, probably because of political instability in theseregions, an accurate assessment of the magnitude of thepandemic was not possible, although it seems very seriousin these areas (Legg 1999).

Social and economic aspects

Cassava is a main staple food in tropical areas and itsproduction is extremely important in the poor central,eastern and southern African region for the role that it playswith other food crops in the local diet. Its adaptability todifferent environments as well as its tolerance to longperiods of drought make it one of the most important staplefoods in many parts of the world where soil stresses andhuman conflicts constrain production.

CMD is one of the most serious and widespread diseasesthroughout cassava growing areas in those African regions.It has threatened food security of millions of people throughits impact on cassava production. According to FAO, inAfrica cassava is the primary source of food for anestimated 70 million people, contributing over 500 kcalper day per person (FAO 2009a). Cassava has been definedas ‘the crop of the poor’. Its contribution to ruraldevelopment and poverty alleviation of marginal popula-tions is an important reality (Howeler et al. 2001).

In many Asian and African countries, cassava is the realcatalyst for development of rural areas, its productionrepresenting the main source of income for the poorest ruralhouseholds. In those areas in which food security con-ditions remain alarming, further development of the cassavaproduction sector and of disease control and preventioncould be very important contributions to the achievement ofpoverty reduction (FAO 2009a).

CMD is the most important disease of cassava in Africa,Sri Lanka and southern India (Otim-Nape & Thresh 2006).

Currently the presence of the disease has been registered inmany countries of SSA, such as Angola, Burundi, CentralAfrican Republic, Democratic Republic of the Congo,Gabon, Kenya, Malawi, Mozambique, Rwanda, SouthernSudan, Tanzania, Uganda, Zambia and Zimbabwe. Accord-ing to FAO (2009a), CDM threatens the whole cassavaproduction system in the Great Lakes regions. In thoseareas the disease has reduced cassava yields of affectedfarms by up to 80 percent. The highest levels of the diseasehave been reported in Burundi, Malawi, North and CentralUganda, North Zambia, Tanzania and the central areas ofKenya. In the Democratic Republic of the Congo, it isestimated that the disease can cause losses as high as 90%(FAO 2009a).

There are several reasons for cassava’s importance forfood security in African countries. In order to understandthe socio-economic impact of cassava mosaic disease onthe continent we have to consider that 50 percent of thecurrent world production of cassava takes place there.Cassava is extremely important for the poorest smallholderfarmers cropping on marginal and sub-marginal lands. Firstof all, it provides them with their main source of income.Secondly, it contributes to their living standard as cassava isthe source of simple food products which are cheaper andmore accessible than those of rice, wheat and maize(Nweke and Ezumah 1988). Cassava plays a major role inthe alleviation of famine because of its efficient productionof food energy and year-round availability. The crop istolerant to drought and does not require the use of fertilizeror purchased seeds that are very expensive and notaccessible to poor farmers (Nweke 1995). Another reasonfor cassava’s importance is that it can remain in the soiluntended for up to 2 years without losing its nutritionalproperties, an important property in the event of temporarycivil strife (Nweke et al. 2002).

The current widespread existence of CMD on theAfrican continent is alarming the whole internationalcommunity (developed countries, research and humanitar-ian organizations) because of the precariousness of live-lihoods there owing to the high incidence of civil unrest andthe reduced capability to face natural crises. These haveresulted in millions of refugees crossing borders, putting astrain on the recipients’ food supply and also contributingto the spread of the disease by the transport of infectedvegetative material.

Today, food security and livelihoods of millions ofpeople are under severe threat because of CMD. For a longtime, the cultivation of cassava as an economic activity inthose regions was neglected with little research beingcarried out to improve the crop and control disease. Today,greater attention from the international community isneeded in order to increase cassava production and controldisease. Investing in cassava production would make a

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considerable contribution to the achievement of sustainablefood security and poverty alleviation.

There have been interesting political interventions tobuffer the impact of the disease indirectly. In Nigeria greatstrides have been made, as a result of a Governmentinitiative, through collaboration with the InternationalInstitute of Tropical Agriculture (IITA), to penalize impor-tations of wheat flour and promote the of use of cassavaflour. All products using wheat flour now contain a definedpercentage of cassava flour, hence providing a market forlocal producers (personal communication). This is impor-tant considering that the population of Nigeria amounts to130 million (20% of all Africans). This has bufferedNigeria’s food security from fluctuations in global wheatprices. Further, huge strides have been taken to developprocessing technologies to turn cassava into chips forhuman and animal feed and, perhaps more novel, intoindustrial starch for the textile and food industries (it haseven been used in oil drilling activities). This exampleprovides a positive indication of what is possible if thepolitical will is present to create policies and an associatedenabling environment for the exploitation of cassavavarieties selected for market orientations accompanied byappropriate processing technologies. This approach alsoreduces the current constraint of massive post harvest lossesdue to the sale of perishable goods and transport onappalling roads by poorly maintained vehicles.

Striga hermonthica

Origin, distribution and impact

Striga hermonthica (Family Scrophulariaceae) is a hemi-parasitic weed, currently the main biotic “problem” forcereal crops in the SSA region (Fig. 3). It is verywidespread, infesting land in western, central and easternSSA, from Gambia in the west, to Ethiopia, Kenya andTanzania in the east (Parker 2009), and Sahel in WestAfrica, including northern areas of Cameroon; besides S.hermonthica, economically the most important, there arethree other main species of Striga in SSA which areagriculturally important: S. asiatica is economically signif-icant in eastern and southern regions; S. forbesii is limitedto certain areas of Zimbabwe; S. gesnerioides, is found inareas of Nigeria and Tanzania. Sorghum, millet and maizeare particularly susceptible to S. hermonthica, whereas allgrasses are attacked by S. asiatica and S. forbesii. Legumesare attacked by S. gesnerioides, with cowpea (Viciaunguiculata) being very susceptible.

S. hermonthica is native of the tropical grasslands of the“old world” and reached its highest biodiversity in regionswhere it co-evolved with cereals, especially sorghum, millet

and rice. It then spread widely and became a scourge for theproduction of cereals (including maize) in areas wherefertility is low and water availability is limited or erratic.Maize was introduced into Africa many years ago, replac-ing sorghum and millet species more tolerant and betteradapted to the scarce water resources available. The reasonsfor this introduction are many, such as increased produc-tivity compared to sorghum, at least in favourable years,consumer preference due to greater palatability, and acovered panicle reducing the risk of predation by theweaver bird, whose flocks of millions can destroy entireplantations of sorghum (Doggett 1988).

S. hermonthica seeds germinate only in the presence ofthe host plant, owing to the release of germinationstimulants by the roots of hosts. The germ tube growstowards the root, attaches to it by a haustorium, and beginsto steal nutrients and water from the host. The small seedssurvive for many years in the soil, and therefore croprotations have little effect on control once a certainthreshold of the seed bank has been reached. Wheninfestation is heavy, the plant seems also to be “poisoned”during the underground phase, making the injury moreserious than the “mere” withdrawal of nutrients. It is notclear whether the compounds responsible are directlyproduced by S. hermonthica, or are the result of metabolismby the crop. Plants that initially appear healthy suddenlyturn yellow and wither, as if they had been bewitched—giving the parasite its common name “witchweed”. After aphase of underground growth, when the parasite accumu-lates nutrients, it emerges from the soil, where its ownphotosynthetic activity allows it to complete its life cycle.

Fig. 3 Maize field destroyed by infestation of Striga hermonthica inBenin. Courtesy of Dr. Fen Beed, IITA, Kampala, Uganda

118 M. Vurro et al.

An estimate in 1991 showed that there were at least 5million hectares infested in six countries of Central-WestAfrica with an average loss in production of 12%(Sauerborn 1991). In northern Ghana, the estimated lossesof the sorghum and millet crops reached an average of 20%.Overall losses in economic terms were estimated at morethan $300 million although, considering the incompletenessof information, this may be an underestimate by a factor ashigh as ten. Recent estimates report a rapidly deterioratingsituation with an increase in the total area infested to almost50 million hectares. Nigeria is the country worst affected,with over 8 million hectares infested. According to theseestimates, the infested area in Ghana would be between 12and 27%, while in Central and Eastern Africa more than 6million hectares of maize are infested by Striga. A report onthe distribution of S. hermonthica in 25 African countriesestimated that infestation of maize fields varied from 20 to30% of the total in Togo, Mali and Nigeria and up to 65%in Benin (De Groote et al. 2008). In the province ofNyanza, in Kenya, clean fields planted with maize yieldedabout 1.5 t/ha, whereas the yield was about 750 kg/ha inmoderately infested fields and only about 300 kg/ha inseverely infested fields (Manyong et al. 2007).

Management

Generally speaking, one of the most widely used conven-tional solutions for the management of weeds is the use ofherbicides. There are systemic herbicides that, whensprayed on leaves, are absorbed and move through thevascular system until they reach the roots where they maycome into contact with parasitic plants, controlling them.Initially herbicides were applied at low doses, so as not toaffect the crops, but without positive results. Then the useof hybrids of crops resistant to herbicides was attempted(Joel et al. 1995). This strategy, although extremelyvaluable, would still require supply and adoption ofcommercial seeds and machines for treatments, whichwould be too expensive for most African farmers. Recently,the use of seeds pre-treated with herbicides has beenproposed. The herbicide spreads systemically in the plantafter germination, protecting it from attack by Striga. Theadvantages of this approach are that it does not requiremachinery or technical knowledge and the consumption ofherbicide is much lower, and therefore more environmen-tally compatible (Gressel 2008). Herbicide seed coatingcould also be combined with biocontrol agent coating, toimprove the efficacy and reduce the risks of resistanceoccurring due to selection pressure. A study on theacceptability of this technology in which treated seed wasdistributed has recently been done with the help ofnongovernmental organizations (De Groote et al. 2008).The success was resounding, with the result that companies

providing the treated seed were not able to meet thedemand in the following year. Even greater benefits werereaped by supplying farmers with bags containing fertilizeralong with the treated seed. Another advantage of thistechnology, in addition to its affordability, is that thecultivation of legume intercropping is not precluded, aswould be the case with traditional herbicide treatments(Kanampiu et al. 2003).

Researchers have tried for decades to find resistance genesin maize effective against S. hermonthica but as the centresof origins of the two species are on different continents, theAmericas and Africa, respectively, they have not co-evolvedand it is therefore probable that maize has no intrinsicresistance (Gressel and Valverde 2009). Only recently, anindigenous species Zea diploperennis, similar to maize, hasbeen found to have a modest level of resistance (Amusan etal. 2008). In the case of sorghum the situation is different asits centre of origin and diversity, like that of Striga, is inAfrica (Ethiopia) and consequently there are likely to beresistance genes in wild populations. Recently, significantprogress was made as plants were found and characterizedthat produce small quantities of stimulants that hamper thedevelopment of the haustorium or block the penetration ofthe germ tube. The combination of a few of these factorsallowed sorghum to have normal yields, even if some Strigaplants grew and set seed. Breeding these traits into localcultivars has been facilitated by the use of genetic markers,resulting in sorghum lines with high resistance (Ejeta et al.2007; Gressel and Valverde 2009).

Often African farmers have intercropped legumes withmaize, both as a nitrogen source for the crop and toensure a food supply in the event of total loss of themaize crop due to drought or Striga. Legumes aregenerally not antagonistic to S. hermonthica but recentwork has shown that the legume shrub, Desmodiumuncinatum, originally investigated as a catch crop for themaize borer, Sesamia cretica, had an excellent effect onthe control of Striga. Unfortunately, owing to its lack ofadaptability it has the disadvantage of only being able togrow in restricted areas and can only be used, if freshlyharvested, as animal feed (Khan et al. 2007). Studies areunderway to identify the factors responsible for control-ling Striga in order to identify other crops with similarproperties which are better suited to the different culturalneeds and environmental characteristics of African regions(Hooper et al. 2009).

Interesting results were achieved with the use of isolatesof Fusarium as mycoherbicides specific to Striga, particu-larly strains of F. oxysporum (Ciotola et al. 1995; Elzeinand Kroschel 2004). The conidia of Fusarium can beapplied in the form of a powder to the soil, or directlymixed with the seeds of the crop. Initially it was thoughtthat the material could be manufactured in small local

Agricultural and socio-economic impact of infectious crop diseases 119

industries and programmes were initiated in this direction.However, as the culture of the microorganism is not easilyaccomplished, recent trends favour the organization of acentralized unit for the production and distribution of thematerial, which may provide higher quality and reliabilityof the microbial product (Venne et al. 2009).

Social and economic aspects

In the colonial period, the spread of Striga (and of S.hermonthica in particular) had been limited for severalreasons: in the fertile soil symptoms of the host were notserious, local labour was used to remove Striga plants andprevent the production of seeds, and crop rotations helpedto reduce its impact and spread. After the colonial period,the situation progressively worsened. National governmentswanted cheap grain for the inhabitants of cities, andtherefore prices were kept low by decree or by importinggrain donated or discarded by Western countries. Fertilizershave never been used traditionally, and Striga plantscompete better in low-fertility soil (Ransom et al. 2007).Once the seed bank of the parasite reaches a critical levelthe situation becomes hopeless: restoring soil fertility isineffective because both the parasite and the crop takeadvantage; weeding by hand-pulling becomes impracticable(Ransom et al. 2007).

In the developing countries in general, and especiallythose in Africa, control of weeds is delegated to women,who are subjugated to a life in the fields, spending asmuch as 80% of their available time there performingthis manual practice (Akobundu 1991). This is whywomen often prefer to accept other jobs, where these areavailable, even if they are poorly paid, rather than workingin the fields. Weeds are a major reason why land becomesunproductive and when this occurs, men leave manage-ment of the land in the hands of women, children or theelderly. The abandonment of land and the proliferation ofweeds lead to a further worsening of the situation. Menmove to cities in search of work, with the consequence ofthe spread of sexually transmitted diseases. Men andwomen living with HIV are debilitated, and thus have evenless ability to manage the lands, and the situation becomesworse still when their children are orphaned. There is thusa vicious circle: no fertilizer to limit the initial spread ofStriga, fewer manual workers to remove the parasiticplants because the men leave the farms for the cities,spreading diseases such as HIV-AIDS and malaria, lessfarm work owing to disease and consequently increasinglylarger areas becoming more severely infested with Striga(Ejeta 2007; Parker 2009). This situation is furthercomplicated by the fact that the level of damage isunpredictable, some years being worse than others. Thus,farmers cannot produce more than 80% of the minimum

caloric needs of families, well below the level of survival(Gressel and Valverde 2009).

Banana Xanthomonas Wilt (BXW)

The plant host

Bananas and plantains (Musa spp.) are the fourth mostimportant staple food in the world, after rice, wheat andmaize. The annual world production is estimated at 100million tonnes, of which only 10% enter the commercialcircuit, demonstrating how this culture is more importantlocally than for export (FAOSTAT 2006). Approximatelyone third of world production is concentrated in the SSAregions, where it provides about 25% of the food to over 70million people. Banana is a common feature of theagricultural and cultural landscape in Africa, despite beingintroduced from Asia only several hundred years ago.Moreover banana is a perennial, providing continuousground cover and preventing soil erosion that wouldotherwise occur if annuals were cultivated, especially thosethat are uprooted for their roots and tubers.

Fruits are used at different stages of ripening and fordifferent purposes. For this reason they are named: dessert,plantain, cooking and juicing bananas. Dessert is a snackand in some countries exported (Ghana, Ivory Coast,Cameroon, Kenya, Malawi, Zambia and under irrigationin Somalia). Plantain (West and central Africa) is a stapleand commands a higher premium as it takes longer toproduce than cooking bananas. Cooking tends to be grownin the Great Lakes region and is a key staple. Juicing is alsogrown in the Great Lakes region and is processed intoalcohol and used for income generation.

The eastern regions (Burundi, Kenya, Rwanda, Tanzaniaand Uganda) are the major producers and consumers ofbananas in Africa. Uganda is the second largest producer inthe world after India (FAOSTAT 2004). Bananas have hugeeconomic and social importance in the Great Lakes area asthey represent both a source of food security and of profit(Edmeades et al. 2007). In countries such as Uganda andBurundi, they provide more than 30% of daily caloricneeds, reaching even 60% in some areas. Also, for someagricultural areas they are the main export crop andtherefore constitute a very important source of income(Abele et al. 2007; Okech et al. 2004). Annual bananaconsumption is about 190 Kg per person in Uganda,140 Kg in Rwanda, 90 Kg in Kenya and 20 Kg in Tanzania(FAOSTAT 2007). Bananas constitute an important sourceof income for approximately 30% of the farmers whogenerally sell from 25% to 50% of their yield, especially inthe west of Uganda (Okech et al. 2004). More than sevenmillion Ugandans depend on bananas for survival, so it is

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not surprising that they use the term “matooke” to indicateboth “food” in general as well as “boiling banana” inparticular.

Origin, distribution and impact

Among the many threats to banana plantations, such asreduced soil fertility, insects or phytopathogenic agents, thedisease caused by the bacterium Xanthomonas campestrispv. musacearum, known as Banana Xanthomonas Wilt(BXW) is one of the most important emerging risks. Thisdisease was initially reported in Ethiopia about 40 years agoon Ensete sp. (Yirgou and Bradbury 1968), a genus closelyrelated to Musa. It was reported in Uganda in 2001 onbanana and from there it has spread rapidly to all regions ofAfrica where the crop is grown. No varieties have completegenetic resistance but they differ in degree of susceptibility(Tripathi et al. 2008). The cultivar Pisang Awak, originatingin Malaysia, is the most susceptible (Tushemereirwe et al.2006). Symptoms consist of a progressive yellowing andwithering of leaves, and rapid and premature ripening offruits (Fig. 4), which suffer internal brown staining (Fig. 5),leaf necrosis (Fig. 6), and rotting of male flowers, rachisand the bunch. Finally, the plants wither and rot. Symptomappearance is rapid, becoming evident as early as 3 or4 weeks after infection. This, however, depends on the typeof infection, the environmental conditions, the state of theplant and the cultivar. Infection can occur: in inflorescences,when the bacterium is carried by insect vectors (stingless

bees, fruit flies) (Tinzaara et al. 2006); by mechanicaltransmission due to the use of infected tools; in the rootswhen the soil is contaminated by infected plant debris(Mwangi and Bandyopadhyay 2006; Tripathi et al. 2008);and by raindrops containing the bacterium. It can also bedisseminated by planting infected propagating material.

The disease has a devastating impact because it developsvery quickly, giving rise to severe symptoms, leading to thedeath of entire plants, including those used for propagation(Tripathi et al. 2007). Moreover, infested fields cannot bereplanted with banana for at least for 6 months, owing tothe persistence of the pathogen in the soil. Once thepathogen has initiated infection, damage limitation isextremely difficult and the disease is impossible to cure(Eden-Green 2004). Since 2001 the disease has spread insome areas in an impressive manner, causing yield losses of

Fig. 4 Early ripening and rotting bunch caused by Banana Xantho-monas Wilt. Courtesy of Dr. Fen Beed, IITA, Kampala, Uganda

Fig. 5 Typical symptoms on fruit transects showing brown staining,caused by Banana Xanthomonas Wilt. Courtesy of Dr. Fen Beed,IITA, Kampala, Uganda

Fig. 6 Foliar symptoms caused by Bacterial Xanthomonas Wilt onbanana plants. Courtesy of Dr. Fen Beed, IITA, Kampala, Uganda

Agricultural and socio-economic impact of infectious crop diseases 121

up to 60%. This has led the government of Uganda, forexample, to set up a task force for the eradication of thedisease by cutting down and destroying by fire diseasedplantations, removal of male buds to prevent infestationfrom insects and stopping traders from coming to farms toharvest bunches unless tools are sterilized (Tushemereirweet al. 2006). Although these interventions have led to areduction in the incidence of the disease, there has beenlittle support for them because of the high costs and thedifficulty in convincing farmers of their necessity.

It has been estimated that, if not controlled, the pathogencan increase the area infected at a rate of 8% per year(Kayobyo et al. 2005). The damage caused by the diseaseeach year is estimated at 2 billion dollars, and at least8 billions if projected over a period of 10 years. A recentstudy estimated 53% yield losses in banana production inUganda in 10 years. Production losses caused by thedisease threaten the food security of about 100 millionpeople and the income of millions of farmers in the GreatLakes region of Central and Eastern Africa (Tripathi et al.2009).

Management

The scenario described in the previous paragraphs has someimportant consequences for disease management. Usually,disease control measures are based on an economicthreshold and are put into practice when the lossesoutweigh the costs of disease management (Peterson andHunt 2003). In this regard, management of bacterialdiseases presents several problems, such as mild symptomsin the early stages of an epidemic, and thus reducing thepropensity of farmers to take action as the losses resultingfrom the destruction of plantations would, in the short term,outweigh the benefits. This, allied with the rapid onset ofsevere symptoms and spread of the pathogen mean thatfarmers only begin to take action when it is already too late(Biruma et al. 2007).

The management of tropical diseases in perennial cropssuch as bananas and plantains is a continual challenge.Management measures include a combination of: preventa-tive interventions to reduce disease establishment; curativecontrol through destruction of infected plants where thedisease is already present; and, rehabilitation of areas thatwere previously infected. Information campaigns, technicalassistance and financial input from national governmentssupported by international and transnational organizationsare crucial in these circumstances. From this point of view,the results in different countries have been different. Incountries such as Uganda and Tanzania, with activepolitical leadership, disease reduction of more than 90%was achieved. In other countries with different social andpolitical situations such as the Democratic Republic of the

Congo, the spread of the disease has, on the contrary,almost quadrupled (Mwangi et al. 2008).

Thanks to a suite of international donors, a Task Forcewas set up over the period from 2005 to 2008, which hashelped the poorest countries to mitigate the effects of thedisease in terms of both social security and food security,so that the reduction of banana production in countriessuch as Burundi, Kenya and Tanzania has had a lessdevastating social impact. This Task Force implementeddiffering interventions according to severity of the diseasein order to: reduce the spread in areas where it was notyet widespread; provide opportunities for cultivatingalternative crops in areas where it had had devastatingeffects; and, prepare for a gradual replacement of bananaswith other crops, where the situation was graduallyworsening. Properly informed and trained, the majorityof small farmers have shown willingness to replacebanana cultivation with annual crops resistant to Xantho-monas, such as beans, cassava, maize and potato, whichare viable alternatives for cultivation and consumption(Tushemereirwe 2001).

The use of certain farming practices and cultivationoperations can reduce the spread of the disease. Forexample, timely removal of male buds prevents dissemina-tion of the disease by insect vectors, which in some areas isthe most important means of spread. However, this practicehas found little application in some areas as farmers, owingto ignorance, refuse to adopt it, considering it to bedetrimental to the quality of the crop (Kagezi et al. 2006).Once the disease is established, there is no remedy otherthan to remove and destroy all plants and debris, in whichthe pathogen can survive for a considerable time. If this isdone carefully, the crop can be reintroduced after a fallowperiod of several months (Turyagyenda et al. 2007). Theplants are propagated by detaching the many suckers thatare formed, and replanting them. Selection of plantingmaterial from fields known to be free of BXW can do muchto prevent the re-establishment of the disease. Diagnosticmethods are also valuable such as the use of semi-selectivemedia or PCR-based diagnostics, preferably controlledthrough a robust regulatory system (Mwangi et al. 2007;Tripathi et al. 2007). A current constraint to the re-establishment of banana is the lack of systems to supplymicro-propagated (tissue culture) or macro-propagated(suckers) planting material, or certification systems toensure material is disease free.

There are no cultivars with complete resistance to thebacterium, but there are some which escape infectionbecause they have flowers with persistent inflorescencebracts, making them difficult for the insect vectors topenetrate, or flowers that do not produce exudates and aretherefore less attractive to insects. It is not therefore cellularresistance, but simply anatomical or physiological charac-

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teristics that make some varieties less exposed to the risk ofinfection (Mwangi et al. 2006).

The use of resistant varieties would be extremelydesirable and economically viable. Germplasm of somelocal varieties has resistance characteristics (Tripathi et al.2008) but these have no value as food so inclusion inbreeding programs may not yield commercially viablegermplasm. Also the genetic diversity of banana in Africais very narrow. It would be necessary to include germplasmfrom elsewhere in the world for an effective breedingprogram.

Other possibilities under study are based on geneticengineering and the transfer of resistance genes that inducethe hypersensitive response through the use of embryogeniccell suspensions and the development of meristematic tissueculture (Ganapathi et al. 2001; Hernandez et al. 1999; Weiand Beer 1996). Of particular interest are the ferredoxinlikeamphipathic protein (pflp) and hypersensitive responseassisting protein (hrap), isolated from sweet pepper(Capsicum annuum), which can intensify the hypersensitiveresponse. The International Institute of Tropical Agriculture(IITA) in collaboration with the National AgriculturalResearch Organization (NARO) in Uganda has generatedseveral transformed lines of banana cultivars and these arecurrently being evaluated for disease resistance underlaboratory conditions (Tripathi et al. 2009).

Social and economic aspects

In countries where the disease is present, BXW causes adrastic reduction in banana production, with consequenteconomic and social problems, which differ from country tocountry. Today, cultivation of Cavendish bananas isparticularly risky because of the simultaneous presence ofBXW and Banana Bunchy Top Virus (BBTV). Thesepathogens can seriously compromise the economies ofsome countries (FAO 2009b). The most alarming economicand social situations have arisen from the spread of BXW inthe countries of central and west Africa, seriously affectingthe livelihoods and food security of the people there.

In Ethiopia and in the countries of the Great Lakesregion (Kenya, Rwanda, Tanzania and Uganda) losses ofproduction are thought to be serious but are difficult toquantify (FAO 2009b). In Uganda, the disease wasregistered in a total of 39 districts in 2008 and has spreadfrom the centre of the country, where the subsistenceeconomies are largely dependant on the production ofbananas, to the west, where banana cultivation is intensiveand largely dedicated to local markets. The greatest numberof registrations was from the central areas of the country,where exotic, susceptible varieties such as Pisang Awak aremainly cultivated. In other areas of intensive production,such as the southwest, the disease has had less impact as, in

these areas, “cooking bananas”, which are less susceptibleare the principal cultivars grown (Tushemereirwe andOpolot 2005). In the period between 2001 and 2004, croplosses were estimated to be between 30 and 50% (Karamuraet al. 2006). These losses have dramatically affectedfarmers’ families as 60% of their income is derived fromthe banana crop. As a result, many families have abandonedthe cultivation of bananas. It is estimated that over a periodof 10–15 years the cumulative loss in yield may exceed 5billion dollars, with annual losses of food and income foreach farmer of over $200, enormous sums for sucheconomies (Kalyebara et al. 2006).

Banana production has almost halved over the last10 years because of lack of nutrients, water and diseasemanagement, but the population has nearly doubled. Thishas led to a substantial increase in demand over productionwith consequent price rises, prices sometimes quadruplingin a few years (FAOSTAT 2007). The farmers initially triedto compensate for crop losses by raising prices on localmarkets (FOODNET 2006). However, the losses became sohigh that this approach became unsustainable. As a resultlocal consumers with the low incomes that are prevalent inweak economies like those of most central Africancountries, could no longer purchase their main source ofnutrition, giving rise to enormous problems in providingfood, social tensions and political instability (Abele andPillay 2007; Kayobyo et al. 2005). In the DemocraticRepublic of the Congo, banana is one of the staple foods andis responsible for 90% of farmers’ incomes. Here BXW ismost devastating in the provinces of North Kivu, near theborder with Rwanda. The economic activity in theseprovinces is now in crisis, exacerbated by conflicts andviolence. Banana plantations occupy 23% of arable land inRwanda with an annual yield of around 2.4 million tonnes.Here it is also the staple food and accounts for 60–80% ofincome (Okecie et al. 2004). The most dramatic consequen-ces of BXWof banana will probably occur in countries wherefood security depends on consumption of the crop and wherethere are already high or medium levels of food insecurity.

Wheat rust - Ug99

Origin, distribution and impact

The microorganism responsible for stem rust of wheat is amicroscopic fungus, Puccinia graminis f. sp. tritici. Thepathogen is also known as black rust owing to abundantproduction of glossy black teliospores (Fig. 7) formed at theend of the summer. It is considered to be a very seriousdisease in many wheat-growing areas around the world andhas been known since Roman times (Savastano 1890). Thefear of this disease is due to poor knowledge of its

Agricultural and socio-economic impact of infectious crop diseases 123

biological cycle and unpredictability of its occurrence andimpact.

Indeed, a field apparently healthy during kernel enlarge-ment and maturation may be quickly reduced to a pile ofdark stalks and broken shrivelled kernels a few weeksbefore harvest (Figs. 8 and 9). Only in the last century havestudies allowed a better understanding of the pathogen and

the recognition of the existence of different races of thepathogen differing in ability to cause disease on differentgenotypes of the host. Successful breeding programmes forresistance have created resistant cultivars and reduced theharmfulness of the disease but, over time, the pathogenoften overcomes the resistance, necessitating further breed-ing. Some major epidemics occurred in the 1940s and1950s in Australia and the United States (Saari and Prescott1985), but were then managed. In other areas appearance ofthe disease from time to time is a very serious matter.

This rust is particularly important in the final growthstages of the wheat plant, on late-sown or late-maturingcrops, and at low altitudes. In warm and humid areas suchas parts of Africa, the disease can survive from year to yearon infected crops and wild grasses. As for all the rusts,spores of P. graminis are dispersed mainly by air. Most ofthe spores move only short distances, contributing to thelocal development of epidemics. However, low numbers ofspores can be transported over long distances and causenew infections. For example, Watson and de Sousa (1983)reported transport of spores from South Africa to Australia.

The rust has a complex life cycle (Fig. 10), that requiresbarberry (Berberis vulgaris) as well as a cereal species. Inthe spring and summer, stem rust infections on cereal plantsproduce urediniospores, which are spread by the wind tonearby cereal plants, where they germinate and infectcereals by penetration through the stomata, the infectionsproducing a new crop of urediniospores in as short a timeas a week. As a result, this phase can rapidly increase theseverity of attack and spread the infection over a wide area.Towards the end of the growing season, the rust converts toproducing teliospores, which remain dormant until the nextspring when they produce basidiospores, that cannot infectcereal plants, but are instead carried in the wind, and infectyoung leaves of common barberry. The basidiosporepenetrates the leaf epidermis directly, and the resultinginfections produce specialized structures called pycnia,

Fig. 9 Symptoms caused by race Ug99 of stem rust on wheat stems.Courtesy of Prof. David L. Hansen, University of Minnesota

Fig. 8 Wheat plants field severely affected by race Ug99 of stem rustin Kenya. In the background healthy resistant plants. Courtesy of Prof.David L. Hansen, University of Minnesota

Fig. 7 Typical “black” symptoms caused by race Ug99 of stem ruston wheat stems, showing the dark teliospore stage. Courtesy of Prof.Zakkie Pretorius, University of the Free State, South Africa

124 M. Vurro et al.

which are the sexual stage of the fungal life cycle. Theresulting fertilized hypha forms an aecium which producesaeciospores that are carried by the wind and infect cerealsby penetration through stomata. Development of theseinfections gives rise to urediniospores, completing thelifecycle.

In the case of wheat in tropical areas, the alternatebarberry host is no longer considered important from theepidemiological point of view as inoculum survives on“volunteer” wheat plants, i.e. self-sown plants growing outof season. The fungus remains in the repeating uredinio-spore stage on these (Fig. 10). Given a high density ofsusceptible hosts over a large area, serious epidemics arelikely to ensue.

In recent years, a race named Ug99 after its discovery inUganda in 1999 has caused serious epidemics in somecountries of East Africa and around the Horn of Africa(Ethiopia, Kenya, Sudan, Uganda). In 2001 the epidemicreached Kenya and arrived in Ethiopia only two years later.Today Ug99 has reached countries such as Yemen and Iranand is endangering the whole of Central Asia and theCaucasus (Mackenzie 2007). These areas of the worldtogether account for 37% of the world’s wheat production(FAO 2008a). The ability of Ug99 to overcome theresistance conferred by many genes effective against otherraces of the fungus makes Ug99 one of the most dangerousof emerging plant diseases. The International Centre for theImprovement of Wheat and Maize (CIMMYT 2005) hasestimated that at least two thirds of the wheat grown inIndia and Pakistan, which amounts to about 20% of worldproduction, are very susceptible to Ug99 and that at least80% of wheat varieties that are grown in Asia and Africaare potentially exposed to the fungus as its spores can becarried by wind over long distances and across continents.

FAO (2008b) identified countries at immediate risk ofinfection as Afghanistan, Eritrea, Iran, Oman and Pakistan.These are followed by the Caucasian and Central Asiancountries (Armenia, Azerbaijan, Georgia, Kazakhstan,Kyrgystan, Turkmenistan and Uzbekistan) which arethought to be at high risk of epidemics. Other countries atrisk are Egypt, Iraq, Jordan, Syria and Turkey. It is fearedthat Ug99 may reach the Indo-Gangetic plains of India, andthen threaten one of the most crowded areas in the world(Fig. 11).

Management

Currently about 50 genes have been identified that conferresistance to different races of stem rust (Sr genes), oftenobtained from species related to wheat. However, isolatesof the pathogen that can overcome the resistance conferredby them are already widespread, making them no longeruseful in breeding for resistance. Luckily there are somegenes for which there are no reports of races of thepathogen able to overcome them yet. The tremendousconsequences of the spread of Ug99 from Uganda(Pretorius et al. 2000) are mainly due to the fact that thisrace is capable of overcoming resistance conferred by thegene Sr31, which is widely used in almost all varieties. It isalso capable of overcoming resistance conferred by almostall the resistance genes originating from wheat, and also theSr38 gene, introduced from Triticum ventricosum intomany wheat varieties grown in Europe and Australia.

A very important aspect of the problem is to understandat the global level the potential for further spread of thedisease, and to determine which of the world’s germplasmis at risk of infection by Ug99. In regions of East Africa,favourable weather conditions occur and host plants arepresent throughout the year, which certainly favour thepathogen. However, there is much evidence that it may gowell beyond the borders of African and Arab countries andcould easily reach South Asia and even East Asia and theUnited States.

In order to provide continuously up to date informationon the current status and potential spread of wheat stem rustrace Ug99, the so-called “RustMapper” has been developedby CYMMYT (http://www.cimmyt.org/gis/rustmapper/)showing: the current known sites and status of Ug99 orderivatives; the country summary information on wheatproduction and susceptibility to Ug99; near-real-time windtrajectories from known sites; and, major wheat productionareas in Africa and Asia. Integration of this informationpresents a near-real-time summary relating to stem rust raceUg99 (or derivatives) using Google Earth for visualization.

Some resistance genes effective against Ug99 have beenidentified which may be introduced into cultivated varietiesand which would help to reduce the impact of the disease.

Fig. 10 Life cycle of Puccinia graminis. Wikimedia commons

Agricultural and socio-economic impact of infectious crop diseases 125

Some of these genes could be transferred quite quicklywhile others will require much longer breeding pro-grammes. However, there is a race against time to producecultivars that are resistant to Ug99 in order to prevent theoccurrence of widespread epidemics. The identification andintroduction of resistant genotypes that are adapted to theenvironment prevailing in the countries concerned, fol-lowed by a rapid and extensive production and distributionof seeds in these countries, remains the best possiblecontrol strategy. For poor farmers of Kenya and Ethiopia,for example, this is the only really affordable managementapproach. These measures would also reduce the ability ofthe disease to spread to neighbouring areas and, whencombined with the simultaneous introduction of resistantvarieties in areas where the disease has not yet arrived,would reduce the losses should the disease reach thoseareas.

Social and economic aspects

In the event of a pandemic, a large number of families ofwheat farmers would be seriously threatened, especiallythose who have few alternative crops that can be cultivated.In these circumstances, landless workers dependent onagricultural work would also be severely affected and therewould be an increase in the abandonment of small farmsand increased migration to cities. As already indicated forother diseases, this would have enormous social and

economic consequences at the level of individual nations,and would also be reflected globally.

The Ug99 epidemic has occurred at a historical periodwhen the world’s wheat reserves had reached their lowestlevel for four decades and when production of bio-fuels areoccupying large tracts of land which could be used for foodproduction. In recent years, the European Union and theUnited States have adopted policies aimed at drasticallycutting traditional grain reserves. Forecasts of world wheatproduction provided recently by the FAO are alarming,particularly in relation to countries outside the OECD area,where there is significant growth in consumption of cerealproducts. An added concern is that droughts causedreduction in cereal production worldwide by 3.6% in2005 and 6.9% in 2006 (FAO 2008a).

The reduction in cereal world stocks, and wheat inparticular, was primarily a consequence of high levels ofvolatility in the prices of these goods in local andinternational markets. This was due to different andconcurrent causes, e.g. the difficulties of stock titlesand securities that turned the commodities (raw materialand agricultural products) into safe assets; the acquisition ofthe titles of commodities by big speculative investors; theincreased incidence of bio-fuel crops which reduce landavailable for food production; the strong rise in the price ofenergy; the weakness of the US dollar as the SSA countrieshave invested their reserves in dollars. All these contributedto the phenomenon of agro-inflation (increases of food

Fig. 11 Spread of race Ug99 ofstem rust over time since its firstappearance in Uganda. Courtesyof Dr. David Hodson, WheatRust Disease Global Program,FAO

126 M. Vurro et al.

prices) which was reflected in the significant rise in cerealprices. For example, in the period between 2002 and 2007the price of rice increased by 70%, that of soybean by 90%and that of wheat by 130%, and increased further in the lasttwo years. There were dramatic consequences in countriesthat are net importers of cereals. Around the world theinflation phenomenon hits the poorest social classes worstas their income is spent mostly on food. The most seriousconsequences are for the 77 poorest countries which are netimporters of food products (Low Income Food DeficitCountries) (FAO 2010). Many of these are already affectedby Ug99 or are at high risk of its introduction.

In many countries already affected by the epidemic and inmost of those at immediate or high risk of infection, wheat is astaple food and provides about 40% of daily calorie intakerequired by individuals. In some countries where the diseaseis already present there is a food crisis due in large part to therise in prices of wheat and other cereals. FAO has recentlyranked countries which are in a situation of food crisis andthose at higher risk: the first group includes countries such asEthiopia, Eritrea, Kenya and Tajikistan. Among the countriesat higher risk is Yemen. These are countries where Ug99 ispresent or is likely to enter.

The most dramatic economic and social effects areoccurring in the Horn of Africa, where the economy largelydepends on agriculture. There, about 70% of the populationlives in rural areas and owes its survival to the productionand consumption of cereals such as maize, sorghum andwheat and also cassava. In Ethiopia and Kenya, countrieswhere the disease is already present, wheat contributessignificantly to food security, and the annual consumptionper capita over the last decade has progressively increased,reaching values of 30 and 27 kg, respectively. In thesecountries, however, domestic wheat production is unable tomeet the internal demand, and hence they have to importabout 16 and 22% of their cereals, respectively.

The small amount of data in the literature concerning thesocio-economic effects of the spread of Ug99 highlights theloss of significant production of wheat, leading to newlevels of social vulnerability (Fekadu and Gelmesa 2006).In Kenya it is estimated that wheat losses due to Ug99 insome areas are over 70% of the total production. This hasnecessitated increased imports of wheat from abroad andthus increased dependence on external supplies. Productionlosses have also led to higher wheat prices on local marketswhich affected the urban and rural population with lowerincomes. In turn, this has caused an increase in the numberof people suffering from hunger, an overall increase in thelevel of food insecurity and a general loss of social status offarmers who have had to abandon their crops.

To understand the social effects caused by Ug99 in theseareas of the world, one must take into account that the lossof production, due to the fungus, amplifies already existing

food shortages, resulting in particular from drought. Thesetwo factors are responsible for generating one of the mostalarming food crises in the world.

For example, in Kenya the particularly serious shortageof rainfall during the months of March and April 2009 inthe South East and along the coast, coupled with disease,resulted in the loss of most crops. As a consequence therewere increased imports of wheat and maize from abroad —during the period November 2008–June 2009 1.1 milliontonnes of wheat and maize were imported to meet domesticdemand. The low water availability in pasture areas of thecoastal area and south-eastern areas has dramaticallyworsened the living conditions of the population, increasinglevels of mortality due to hunger, which affected mostly thepoorest people. The situation is exacerbated by theconcomitant rise in grain prices and other food commod-ities in local markets. In Ethiopia, an estimated 4.9 millionpeople require emergency food aid. Here, the effects causedby the drought were felt particularly in the Oromyia andAmhara regions (FAO 2009a).

Norman Borlaug, whose efforts in breeding highyielding and rust resistant wheat won him the Nobel PeacePrize in 1970 denounced the delay of the internationalcommunity in taking Ug99 seriously (Mackenzie 2007).The achievements of the green revolution had encouragedan attitude of complacency and neglect of agriculture (Ejeta2009). This had translated into the dismantling of trainingcourses for local people and programmes of research onresistance of wheat to rust. The lack of seriousness withwhich this emerging plant disease was faced has threatenedthe survival of entire communities in the Horn of Africaand delayed scientific efforts aimed at seeking solutionscapable of halting the progress of the disease.

The risk of epidemics is also linked to the limitedcapacity of farmers in these poor areas of the world tomanage disease. In Africa, the increased dependence ofmany countries on emergency food aid has gone hand inhand with a substantial reduction of national governmentinvestment in agriculture, particularly in regard to pro-grammes for education of farmers, research and ruraldevelopment. This general phenomenon has affected allthe countries that have been invaded today by Ug99 or areat high risk of invasion (FAO 2008b).

Discussion

It is well known that the risks of introduction / invasion ofplant pathogens, whether into monocultures, horticulturalcrops or natural communities, are increasing because ofglobalization, increasing human mobility, climate change,and evolution and adaptation of pathogens or their vectors(Anderson et al. 2004). Despite the social, economic or

Agricultural and socio-economic impact of infectious crop diseases 127

environmental consequences caused by EIDs of plants, theyreceive less attention than those affecting animals andhumans.

The introduction of new pests and, with it, the risk ofnew outbreaks, continue to occur. The danger may bereduced in agricultural districts capable of promptlyorganizing defensive measures or adopting laborious andcomplex quarantine measures, but the risk of more seriousconsequences remains high in less technologically-advanced countries.

Food security is threatened in countries with limitedresources when pandemics occur on important food crops.Low crop productivity contributes directly to malnutrition,and indirectly to the spread of human diseases, and even tothe collapse of the environment, since poor rural areas arebeing abandoned with the concomitant phenomenon ofurban overcrowding. A further deterioration of the situationoccurs when the EIDs are caused by organisms thatproduce mycotoxins, which can contaminate food or animalfeed, leading to serious poisoning (Leslie et al. 2008).

Newly introduced pests or pathogens are often exposedto little or no competition from natural biocontrol agents(predators, parasitoids and pathogens) to which they wereexposed in their site of origin. This may be perpetuated byenvironmentally unfriendly management strategies, whichcan be one of the main triggers causing pandemics of exoticpests and pathogens. Restoring the biological balancethrough natural pressure exerted by the indigenous “con-troller agents” is challenging, and requires the involvementof researchers specialized in this field and appropriateeconomic investments. A constant risk to crops around theworld is the emergence and migration of new strains ofpathogens capable of overcoming resistance. Geneticuniformity remains a risk element of primary importance.For example, great epidemics of the past in the USA, due tospecies of Helminthosporium parasitizing oats and maize,and concerns about the potential spread of Ug99 to theIndo-Gangetic plains should be warnings of the dangers ofreliance on single genes for resistance.

The economies of most developing countries arecharacterised by limited diversity of agricultural products.Over dependence on a single food source, such as thepotato in Ireland in the 19th Century and banana in Ugandain the 21st Century, is an important cause of foodinsecurity, as a single disease such as potato blight orBXW may devastate the crop. Moreover, dependence on asingle or limited number of sources of food is an importantcause of undernourishment.

Hunger in the world today, although related to insuffi-cient agricultural production, is caused not so much by theoutbreak of unusual or unforeseeable bio-constraints, butrather by the lack of public interventions, institutionalfragility, limited public investments in rural areas, political

and administrative chaos, war and local guerrilla action,and climate change. In this context, the effectiveness ofhumanitarian aid, in the absence of appropriate conditionsto start productive activities, is largely frustrated.

In developing countries there are clear links betweenfood insecurity and institutional fragility. The recent foodcrisis highlighted the acute vulnerability of net foodimporting developing countries, especially in SSA. In thepast two decades, those countries have reduced investmentin rural areas, exacerbating migration to cities and increas-ing the demand for food imports. This vicious circle (highmigration to cities coupled with decreased investment inrural areas) further undermines the capacity of agricultureto produce the required food and increases dependence onfood imports. Today, most of the SSA countries are netfood importers of major staple commodities, such as wheatand rice making them vulnerable to price hikes andconsequent food crises.

The recent global financial and economic crisis hasincreased food insecurity in developing countries. In theagricultural sector, the price of seeds and fertilizers hasmore than doubled since 2006. The net imports ofdeveloping countries are among the most vulnerable tofood price shocks which have pushed a large number ofpeople into hunger in 2009.

The most frequent primary cause of devastating epi-demics is the introduction of new pests into new environ-ments, or the emergence of new variants. The reasons fortheir success, besides favourable environmental conditions,include the absence of defensive capabilities of the hostsand the lack of natural biocontrol agents which are presentin the area of origin.

Early disease diagnosis and pathogen identification areessential elements in the protection of crops and naturalplant systems, and are crucial to taking appropriate action.Lack of rapid detection of pests and pathogens has aprofound negative impact on the management and preven-tion of diseases leading to deterioration in quality andquantity of agricultural products. National governments areresponsible for developing control systems, primarily forpreventing or controlling the introduction of pathogens intotheir countries, but also to avoid the spread of their endemicpathogens to other countries. Diagnostic systems must alsobe combined with effective monitoring and alarm systems,to give a timely indication of the priorities to be tested, andto identify the emerging risks and their origin. Other keyelements are: the management of these systems withininternational networks, in order to know in real-time theglobal situation; and the implementation of joint andcoordinated strategies, to prevent or better manage possiblepandemics (Miller et al. 2009).

The accurate monitoring of global plant health concernsnecessitates the involvement of all agricultural sectors:

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governmental agencies, universities and the agriculturalindustry. Combining the actvities and data of privateindustry and public national and international organizationsinto central databases can provide useful distributioninformation for specialists; improved monitoring for EIDs;and improved distribution records for state regulators toenhance their decision making (Magarey et al. 2009).

One of the main problems of surveillance of EIDs is thatthese technologies and systems are often expensive andrequire considerable technical and scientific preparation,and considerable economic investment and personnel.Furthermore, a large organization on a territorial basis,with clear roles and accountabilities, is of utmost impor-tance. In the case of western countries the development ofsurveillance systems is easier to achieve because there arecommunity networks already organized, more economicpossibilities and greater availability and affordability of thetechnologies needed. Given the cost, many developingcountries have limited or no control systems, nor can theyacquire and update lists of emerging pathogens within theirborders. This is particularly true in the case of Africa wherethe past century has seen a decrease in the number ofreports of new diseases, compared to Europe, where therewas, as expected, actually a dramatic increase (Waage et al.2006). The consequence is that many diseases in SSAsimply spread without being recognized and monitored. Inthis context, the role of international organizations such asFAO and IITA, is crucial. The lack of monitoring orquarantine in developing countries indirectly causes seriousproblems even in developed countries as it increases thechances of introduction of emerging or unknown pathogensdue to global trading.

The availability of adequate monitoring systems allowsnot only the initiation of appropriate security measures toprevent spread of diseases, but also timely intervention toreduce the impact of a pandemic should a disease spread.There is, for example, the possibility of determiningintervention strategies promptly, such as the use of agro-chemicals and the identification of resistant varieties.

In the so-called developed countries, agriculture is notwithout risks of pandemics, and there are many recent casesof severe damage caused by EIDs. For example, in theyears between 1991 and 1996 there was a serious epidemicof Fusarium head blight caused by several species ofFusarium (especially F. graminearum), which affected inparticular wheat and barley (McMullen et al. 1997). It isestimated that an area of over 4 million hectares wasaffected by the disease with an incidence of between 10 and80%. Quantitative and qualitative damage amounted tohundreds of millions of dollars. However, in developedcountries management systems are put in place that mitigatethe economic and social effects of such extremely harmfuldiseases. There are social safety nets to support those most

affected; food reserves that limit the risk of famine; researchsystems and technical support services that enable manage-ment of those diseases or diversification to alternativecrops; monitoring and warning systems that allow theprompt application of control measures. Similar systemmust be established urgently in developing countries toavert socio-economic disaster due to plant disease.

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Maurizio Vurro is a seniorresearcher at the Institute ofSciences of Food Production, inBari, Italy. His main areas ofexpertise are: the use of plantpathogens and of natural bioac-tive metabolites in the biologicalcontrol of weeds; the biologicalcharacterization of fungal toxinsand their role in the host-pathogen interactions; parasiticweed management. He has re-cently led the European project“Enhancement and exploitationof soil-biocontrol agents for bio-

constraint management in crops” and is the coordinator of theWorking Group “Parasitic Weeds” within the European WeedResearch Society. He is author of more than 70 articles in ScientificJournals and book chapters.

Barbara Bonciani is currentlyContracted Professor at the Uni-versity of Pisa of the followingsubjects: Sociology of Develop-ment and Underdevelopment andSociology of Ethics in Interna-tional Financial Market Relation-ships. She has a PhD in Sociologyachieved at the University ofPisa. In 2003, she was awardedthe Robert Schuman Scholarshipof the European Parliament whichshe spent at the DG Research ofthe Parliament in Luxembourg. In2004, she held a Marie-Curie

Research Studentship at the Centre for Urban and Regional DevelopmentStudies (CURDS) at the University of Newcastle Upon Tyne. Herresearch and teaching interests focus on sociological approaches todevelopment problems, with particular attention to food security, equalityand non-discrimination as well as the impact of international aid onpoverty and human well-being in developing countries.

Giovanni Vannacci is full pro-fessor of Plant Pathology in theFaculty of Agriculture at theUniversity of Pisa and teachesMycology and Biopesticides inthe Agroindustrial Biotechnolo-gy course in the same Faculty.He has been Director of theDepartment of Tree science,Entomology and Plant Patholo-gy “G. Scaramuzzi” of the sameUniversity, Managing Editor ofthe Journal of Plant Pathology,President of the organizing com-mittee of the 6th European Con-

ference on Fungal Genetics and member of the “Accademia deiGeorgofili”. His main expertise is biological control of plantpathogens by antagonist fungi, but in more than 35 years of activityhe has been involved in research on biology and molecular biology,genetic variability, systematics of fungi as plant pathogens, biocontrolagents and bioremediation tools. He has been leader of NationalCompetitive Projects (PRIN) and participated in international projects.He is author of more than 150 papers in scientific journals.

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