UV Radiation Effects on Pathogens and Insect Pests of Greenhouse-Grown Crops

Photochemistry and Photobiology, 2004, 79(3): 21 9-226

Invited Review

UV Radiation Effects on Pathogens and Insect Pests of Greenhouse-Grown Crops

Michael Raviv*’ and Yehezkel Antignus2 ’Department of Environmental Horticulture, Newe Ya’ar Research Center, Agricultural Research Organization, Ramat Yishay, Israel 2Department of Virology, Volcani Center, Agricultural Research Organization, Bet Dagan, Israel

Received 6 June 2003; accepted 2 January 2004

ABSTRACT

Production of high-value crops is often performed under protected cultivation. In recent years various spectral mod- ifications have been made in greenhouse covers. Two of the main reasons to modify the spectral characteristics of greenhouse covers have been to suppress the proliferation of several foliar diseases and to protect crops from insects and insect-borne virus diseases of greenhouse-grown crops. These goals were achieved by complete or partial absorption of solar UV radiation, which interrupts the life cycle of several fungal pathogens and alters the visual behavior of many insects. Examples of these management strategies are described in this article.

INTRODUCTION Pathogens and pests play a major role in determining plant performance in both agricultural and natural settings. The involvement of UV radiation (28WOO nm) in the interaction between plants and their pests is of major importance and was the subject of numerous papers. Plant production under various types of covers is unique because spectral characteristics of greenhouse covers can be manipulated so that plants are no longer exposed to unscreened solar radiation. It is possible, for example, to reduce the total intensity of UV radiation reaching the plants, to screen UV radiation with various cutoffs and to change the ratio between UV radiation and that of other parts of the spectrum (Fig. 1). This involvement in the nature of light reaching the crop may shift the balance between hosts and pests and is the subject of the current review.

*To whom correspondence should be addressed at: Department of Environmental Horticulture, Newe Ya’ar Research Center, Agricultural Research Organization, P.O. Box 1021, Ramat Yishay 30095, Israel. Fax: 972-4-983-6936; e-mail: [email protected]

Abbreviations; CYSDV, cucurbit yellowing stunt disorder virus; IF’M, integrated pest management; PAR, photosynthetically active radiation (400-700nm); PC, polycarbonate; PE, polyethylene; PVC, polyvinyl chloride; TYLCV, tomato yellow leaf curl virus; ZYMV, zucchini yellow mosaic virus.

0 2004 American Society for Photobiology 003 1-8655/04 $5.00+0.00

PATHOGEN-RELATED UV EFFECTS ON PLANTS The depletion of the stratospheric ozone layer and the resulting increase in UV-B radiation (280-320 nm) reaching the earth is of worldwide concern. The increase in UV-B radiation prompted numerous researchers to study the effects of UV and especially those of enhanced UV-B radiation on plants (1-7). Contrary to earlier predictions, Allen et al. (8) argued that induced UV-B inhibition of photosynthetic capacity can be expected only at levels of radiation that are much higher than those predicted to occur. On the same lines, Caldwell et al. (9) concluded that most direct impacts of UV-B on agricultural crops are not of importance under predicted UV-B levels. On the other hand, they suggested that indirect effects on secondary metabolism, insect pests and fungal pathogens, and interactions with pollutants may be of greater significance. Lavola et al. (10) suggested that biochemical changes induced by enhanced UV-B may affect the consumption of plant tissues by insects. Caldwell et al. (1 1) corroborated this conclusion.

Enhanced UV-B radiation effects on plants that may be related to their susceptibility to pests include DNA damage (12); modification in gene expression (13,14); changes in secondary metabolism (especially the production of UV-absorbing molecules (15-21) and changes in leaf anatomy, leaf thickness and cuticle thickness (22,23). The production of UV-absorbing compounds such as flavonoids and other phenolics is the plant’s most important mechanism of UV adaptation (24-26). However, exceptions do occur: in some cases long-term adaptation to enhanced UV-B resulted in phenolic levels similar to those of plants grown under ambient conditions (27). The responses were found to be clone specific (28). These compounds are localized mainly in the epidermis and mesophyll and often function also as phytoalexins or antifungal compounds (29-3 1).

Some of these UV-B-mediated responses are postulated to increase plant resistance to fungal diseases. UV-B radiation induced an accumulation of pathogenesis-related proteins in leaves of several species (32). Flavonoids are frequently associated with cultivar-specific resistance to fungal diseases, either as a constitu- tive molecule or as one induced by fungal challenge (15-18). Some anthocyanins and carotenoids exhibit similar responses (19-21). The thickness of the cuticle and the composition of epicuticular wax may affect plant susceptibility to fungal attack (22). It can

21 9

220 Michael Raviv and Yehezkel Antignus

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Figure 1. Example for spectral characteristics in the UV waveband of a set of PE covers, provided for a study of the effect of Botrytis cinerea on sweet basil (I. Shamay, N. Dudai and M. Raviv, in preparation).

therefore be predicted that increased UV-B radiation should confer greater disease resistance to plants, and in some cases this seems to be a valid assumption. However, in other cases the situation is reversed, suggesting elevated host susceptibility to pathogens induced by UV-B radiation (33,34). It is possible, for example, that different phenylpropanoid metabolic pathways are induced by UV- B than those that are elicited by fungal attack. If this is the case, the UV-B-mediated pathway may compete for substrates with the fungal-mediated one, thus leaving the plant more vulnerable to infection. Naito et al. (35) demonstrated the concept of UV-B- induced plant vulnerability to pathogen attack in an elegant manner: they showed that spinach (Spinaciu oleruceu L.) was more susceptible to the soil-borne fungus Fusarium oxysporum Schlecht.: Fr. when exposed to enhanced UV-B. Because the fungus is naturally protected from UV-B radiation by the soil cover, they suggest that the defense response of the aboveground parts of the plant may be negatively affected by UV-B exposure.

UV RADIATION EFFECTS ON PLANT PATHOGENS The effect of UV radiation on plant pathogens can take two forms: either through direct effects on various stages of pathogen development, such as spore germination and germ tube extension (3&38), hyphal development (39) and sporulation (40), or indirectly through its above-described effects on the host (1 1,14). In spite of the tendency to associate these UV-related changes with a putative decrease in disease severity (41), no such consistent trend is evident, and in many cases UV induces or at least enhances plant pathogenicity (33,34,42,43). Any prediction is therefore of a limited value because of the inherent difficulty in separating the effects of UV radiation on the host from those on the pathogen. The common method of distinguishing between UV effects on the host and those on the pathogen involves the use of pre- and postinoculation radiation, a method with intrinsic faults. The effect of preinoculation treatment may not be apparent until the full effect of the pathogen is expressed, whereas postinoculation treatment affects both the host and the pathogen.

In the following section several relevant pathogen responses to UV will be reviewed. The direct effects of UV on the causal organisms of plant pathogenicity and the indirect effects on host- pathogen interactions on the host will be discussed. Additional conclusions will be drawn from studies conducted under protected cultivation, where steps were taken to modify the solar spectrum reaching the plant by altering the spectral characteristics of the cover.

Since the early sixties the effects of monochromatic light on many fungi have been thoroughly investigated both in vitro and in vivo. Several comprehensive reviews have documented these responses (44-46). Many fungi are most susceptible to UV-B damage during the spore germination stage of their life cycle. Inhibited germination of conidia and high spore mortality in response to UV-B radiation have been found in fungal species such as Septoriu tritici Rob. (36), Sclerotinia sclerotiorum (Lib.) de Bary (37) and Venturia inaequalis (Cooke) Wint. (38). Few studies have been conducted on the effect of UV-B on hyphal growth. Fourtouni et al. (39) found that mycelial radial growth and dry mass production of Alternaria solani (Ell. and Mart.) Jones and Grout were reduced when exposed to moderate doses of UV-B.

Most studies on the effect of UV on fungi have focused on reproductive development. In many of these studies fungal propagules showed sensitivity (47), but some exhibited enhanced sporulation in response to elevated UV-B radiation (48,49). Most studies of action spectra for UV-B-enhanced sporulation have used a monochromatic UV source; in various fungi these generally reveal maxima at 280-290 nm. It should be noted that other important effects of light on sporulation exist. Blue, red and far-red wavelengths all exert well-documented effects. A discussion of these latter effects, however, is beyond the scope of this review.

To determine photomorphogenetic responses of fungi, studies must be conducted under realistic, polychromatic conditions, in which varying levels of both supplemental UV radiation and normal solar radiation are applied, as reviewed by Paul (42,43). The sensitivity of many fungi to UV-B is not always reflected as reduced infection in plants. In fact, the spread of several prominent disease-causing agents such as A. soluni Sorauer (Ell. and Mart.) (A. tomato), A. cichorii Nattrass, A. soluni (Ell. and Mart.) Jones and Grout, Botrytis cinerea (Pers.: Fr.), Cercospora heticola Saccardo, Helminthosporiurn oryzae B. de Hahn, S. sclerotiorum and Stemphylium hotryosum Wallr. can be reduced by filtering out UV-B radiation (37,39,50-52).

On the other hand, several phytopathogenic fungi such as S. tritici, the causal agent of wheat leaf blotch, and Exobasidium vexans Massee, the causal agent of blister blight in tea, exhibit reduced infection under elevated UV-B (36). In reality, however, even these sensitive pathogens usually escape the effect of UV-B. For example, the negative effect of UV-B on wheat infection by S. tritici can be seen in spring but not in summer, probably because of the ability of the fungus to complete the infection during the night in spring but not in summer (42). The effect of UV-B attenuation by the canopy and the clouds also facilitates the spread of these diseases under elevated UV-B conditions (53). The situation is further complicated by genetic variability expressed by fungal strains exposed to identical UV radiation conditions (54) as well as genetic variability of the hosts (33,34).

THE EFFECT OF SPECTRAL MODIFICATION

RELATIONS OF SUNLIGHT ON HOST-PATHOGEN

Western society desires a year-round supply of fresh produce, which can be achieved only through protected cultivation. Greenhouse-grown plants are prone to several fungal diseases because warm, humid greenhouse environments favor fungal development. Disrupting the life cycle of the pathogen is a key requirement for any effective integrated pest management (IPM) strategy. This can be achieved by direct action against the causative

Photochemistry and Photobiology, 2004, 79(3) 221

Table 1. The effects of UV absorption by greenhouse covers in ameliorating disease development in several greenhouse-grown plants ~~ ~ ~~

Pathogen Host Effect Reference

Alternuriu porri (Ellis) Cif, root Welsh onion Reduced sporulation, slower disease 38

A. soluni Sorauer. early blight of Sweet pepper, red pepper Reduced sporulation, slower disease 38

A. soluni (Ell. and Mart.) Jones Tomato Reduced sporulation, slower disease 38

B . cinereu Pers.: Fr., gray mold

B . squuniosu Walker, onion leaf blight Chinese chive Reduced sporulation. slower disease 38

rot pythium development

potato development

and Grout, early blight of tomato development

incidence

development

increased plant resistance

effect on sporangial production

Cucumber, tomato, sweet basil, rose Reduced sporulation, reduced disease 33,36,37,39.40,64

Fusuriurn oi~ysporum Schlecht.: Spinach Lower disease incidence caused by 21 Fr., fusarium wilt

Curtis) Rostovtsev, downy mildew

white mold disease incidence

Pseudoperonosporu cubensis (Berk. & Cucumber Enhanced colonization, no consistent 41

Sclerotiniu sclevoturiurn (Lib.) deBary, Eggplant, cucumber Reduced apothecia formation, reduced 31

organism. e.g. the use of prophylactic fungicides, or by manipulation of the ambient conditions. For psychrophilic fungi this can be achieved by reducing the air humidity and by preventing the formation of a water film on the canopy by using internal air circulation, heating or ventilation (or all). This approach requires both accurate humidity and wetness-sensing devices and investment in suitable systems to prevent condensation of water on leaf surfaces, which may require a substantial amount of energy.

An additional method of changing ambient conditions is to alter the spectral properties of the greenhouse cover on the basis of an understanding of the role of the light spectrum in plant pathogenicity (55,56). There are a growing number of successful examples for this strategy (Table 1). This approach to IPM is passive but economically attractive and is based on modifying the solar spectrum reaching the plant to exclude the transmission of certain wavelength bands in order to affect the life cycle of the pathogen. Specifically, it is possible to exclude part or all of the UV radiation and thus to reduce sporulation and thereby minimize the spread of epidemics, with no or minimal negative effect on the crop. This approach is especially attractive to low-technology greenhouses, particularly those in subtropical climates, where frequently no ventilating or heating systems are installed.

The first researchers to demonstrate the feasibility of this approach were Honda et (I/. (50) and Honda and Yunoki (51). They compared two common agricultural vinyl films, one with a cutoff of transmission at 300 nm and the other a UV-absorbing film with a cutoff of transmission at 390 nm. Gray mold of cucumber and tomato, caused by B. cinerea, and sclerotinia disease in eggplant and cucumber, caused by S. sclerotiorum, were greatly reduced by the use of UV-absorbing film. In laboratory tests the UV cutoff wavelength for preventing sporulation in several isolates of B. cinerea was determined to be 345 nm; there was no sporulation at wavelengths above this value. Sasaki ef al. (52) extended the range of fungi that can be partially controlled using UV-absorbing covers to A. hrussicue (Berk.) Sacc., A. dauci (Kuhn) Groves and Skolko, A . porri (Ell.) Cif, A . soluni (Ell. and Mart.) Jones and Grout, Botrytis syuumosa Walker and S. botryosum Wallr.

Many countries no longer use vinyl films agriculturally because of the phytoxicity of their plasticizers and problems with recycling the films. UV-stabilized polyethylene (PE) films, which have replaced them, are recyclable. They are inherently transparent to UV and have a transmissivity spectrum similar to that of Film 1 (Fig. 1 )

The use of various UV absorbers in PE films as a means to control gray mold of cucumber and tomato was shown to be effective by Reuveni et al. (57). Sporulation of B. cinerea is apparently further inhibited by absorption of part of the yellow waveband (580 nm) (58). It was shown that the efficiency of the filters was proportional to their blue-UV-B transmittance ratio and that complete removal of UV is not required to control gray mold effectively. In contrast to these results, Reuveni and Raviv (59) showed that filtering out UV- B enhanced colonization of Pseudoperonosporu cubensis (Berk. & Curtis), the causal agent of downy mildew in cucumbers. Adding a blue pigment to the film significantly reduced colonization of P. cubensis (59). The response of sporangial production to UV-B radiation was not consistent; partial UV-B absorption promoted greater sporangial production than did either complete absorption or no absorption. The addition of blue pigment greatly inhibited sporangial production. As a result of the above findings, PE producers are now frequently introducing UV-absorbing compounds into films produced for greenhouse covers.

INSECT-PLANT COMMUNICATION-VISUAL CUES Insects perceive light signals through their compound eyes. These organs of vision are connected to the visual centers of the brain, where light signals are processed and translated to behavioral reactions. The anatomy and physiology of the compound eye is adapted to sense UV wavelengths alone or a mixture of UV and visible radiation (60). The UV part of the solar spectrum plays an important role in the ecological behavior of insects, including orientation, navigation, feeding and interaction between the sexes (61.62). The manipulation of the UV vision of insects by using UV-blocking greenhouse cladding materials has been shown to be effective in preventing the immigration of a wide range of insect pests (whiteflies, aphids, thrips and leafminers) from the external environment into the protected crop (63,64). The use of UV- absorbing plastics as greenhouse covers also reduces the spread of insect-borne virus diseases (64).

Evolutionary contacts have created mechanisms that enable insects to detect and select their favorite plant hosts for feeding and oviposition. Vision (colors, shape, size) and olfaction (host odor) are the primary cues used by insects to orient to their plant hosts; sometimes the two cues work in concert (65,66). Insects’ visual


behavior is linked to a chain of events, which begins with their orientation to the plant from a distance and ends with their establishment on plants for feeding and oviposition (62). By interfering with different links along this pathway, we may prevent a contact between the vector and the plant, thereby preventing plant infestation and viral infection.

The UV component of the solar spectrum (280-400 nm) plays an important role in the ecological behavior of insects (62). Understanding how these behavioral parameters are affected by the solar spectrum in economically important insects is a prerequisite for the design of innovative control means.

Phototactic action spectrum for whiteflies

Mound (67) correlated the reaction to UV of Bemisia tubaci Gennadius to the induction of migratory behavior and showed that yellow wavelengths induced vegetative behavior. which may be part of a natural host selection mechanism. Coombe (62) found that the greenhouse whitefly took off more readily and walked faster when exposed to 400 nm of radiation than under 500 nm. He confirmed Mound’s suggestion that the two types of radiation are complementary, with an apparent balance between migratory behavior induced by UV wavelengths and the landing reaction controlled by yellow wavelengths. These results were later supported by the finding of Antignus et al. (68), who showed that filtration of UV light in the range of 280-380 hindered the ability of whiteflies to disperse inside tunnels covered with UV- absorbing films.

Phototactic action spectrum for aphids

Ln flying aphids it has been suggested that the primary function of color vision lies in distinguishing plants from the sky (69). Eastop (70) suggested that the aphid’s sensitivity to color may be related to its host range for any given species. Our recent studies (71) have shown for the first time that the flight activity of alate aphids ( M y u s persicue Sulzer) in a UV-deficient environment was dramatically reduced. Moreover the propagation rate of aphids grown under filtered light lacking the range of wavelengths between 280 and 380 nm was 1.5-2 times lower than that under normal solar irradiation. These findings may reflect changes in the insect physiology that lead to reduced motor activity (71).

Phototactic action spectrum for thrips

All anthophilous thrips are attracted to colors that match flowers, i.e. UV, white, blue and yellow, but few are attracted to green, red and black. Reflectance of UV-A radiation wavelengths (320-400 nm) is an important determinant of whether thrips land on a host. If U V reflectance is very high, anthophilous thrips in contrast to grass-feeding tllrips are repelled from the surface of attractive colors (72). However, on the basis of electroretinograms of Frankliniella occidentulis Pergande exposed to flashes of light ranging from 365 to 620 nm, it was found that there were two peaks of efficiency, one at 365 nm and the other in the green- yellow region around 540 nm. On the basis of these findings, it was suggested that flower thrips have two types of photoreceptors: one sensitive to UV wavelengths and the other sensitive to green- yellow wavelengths. There is no physiological evidence for a third photopigment sensitive to blue wavelengths (73).

MANAGEMENT STRATEGIES ASSOCIATED WITH THE MANIPULATION OF UV VISION Altering the visual behavior of insects has been used successfully as a tool in IPM programs directed to protect crops from insects and insect-borne viral diseases. These control measures include using colored and reflective plastic mulches and spraying with whitewashes (74,75). The possibility of manipulating the UV vision of insects has been discovered only recently independently by Nakagaki et ul. (63) and Antignus et al. (64). The im- plementation of this approach and the suggested mechanisms involved are described below.

UV-ABSORBING FILMS FOR PROTECTION OF GREENHOUSE CROPS FROM INSECT PESTS AND VIRAL DISEASES Spectrally modified PE is now produced commercially by the introduction of a UV-absorbing additive into the raw material. Polyvinyl chloride (PVC) and polycarbonate are also known for their UV-absorbing properties based on their chemical structure. All the above materials block the transmission of most wavelengths in the UV range below 370-380 nm without interfering with the transmission of photosynthetically active radiation (400-700 nm). The extent of the protection is dependent on the plastic’s UV- absorbing capacity; thus, PVC sheets were found to be superior to PE films in this respect (64,76).

The first evidence that UV-absorbing films may reduce insect invasion of greenhouses came from Japan. Nakagaki ef ul. (63) reported that populations of A . gossypii, Triuleurodes and Vapor- ariorum Wesnvood were lower on tomatoes grown in a plastic house made of polyethylene treated to exclude UV wavelengths than on crops grown in an ordinary plastic house. Thnps (F. occidentalis and Scirtothrips dorsalis Hood) were also found at lower densities under the UV-absorbing polyethylene, and a similar trend was found for the leafminer Liriomyza hryoniue Kaltenbach.

In later studies it was shown that photoselective greenhouse cladding materials can serve as a tool to reduce insect invasions and the spread of viral epidemics (64). The number of whiteflies (B. tubaci) trapped on sticky yellow plates under a UV-absorbing film was 4-10 times lower than the number trapped under regular PE covers. The number of aphids (Aphis gossypi Glover) recorded under UV-absorbing sheets was ca 100 times lower than the number recorded under regular films, and the invasion of thrips (F. occidenfulis) was 10 times lower under this type of film (64). These effects were further confirmed in commercial tunnels in which herbs were grown. UV-absorbing films (fR veradim, Ginegar Plastic Products, Ginegar, Israel) were found to be effective in preventing the infestation of mint with nocturnal moths (Spodop- tera liturulis, Luphygma sp.) (77). The reduction in pest populations in herbs grown in “walk-in” tunnels reduced the necessity for chemical pesticides. As a consequence, the number of insecticide applications under the UV-absorbing sheets was lowered by 50-80% (77).

The protection effect of UV-absorbing films is not limited to blocking insect pests; it also significantly reduces the spread of insect-borne viruses. An extraordinary reduction in the spread of whitefly-borne viruses was recorded in tomatoes, cucumbers and melons grown in walk-in tunnels and greenhouses covered with UV-absorbing films (64,78,79). Tomatoes grown in walk-in tunnels covered with a UV-absorbing film without any insecticide


application exhibited only a 1 % infection rate of tomato yellow leaf curl virus (TYLCV), in contrast to an 80% infection rate in tunnels covered with non-UV-absorbing films (64).

Cucurbit yellowing stunt disorder virus (CYSDV) disease incidence in cucumbers was seven-fold lower under a UV- absorbing film as compared with regular films (79). Some observations (Y. Antignus, unpublished) also indicate that UV- absorbing films were effective against the nonpersistent aphid- borne zucchini yellow mosaic virus (ZYMV).

EFFECTS ON CROP YIELDS In Japan, no significant differences were found in growth, yield, maturation time and fresh and dry weights among tomatoes, pepper and cucumbers grown in greenhouses under conventional or UV- absorbing plastics (80,81). Similar results were obtained when the studies were conducted in Israel (82,83). However, UV exclusion had an adverse effect on certain cultivars of Lisianthus, Eustoma grandijlorurn (Raf.) Shim, by preventing the development of the normal violet pigmentation of the flower (84).

EFFECT ON BENEFICIAL INSECTS Pollinators

Bumblebees are an important part of greenhouse production and like other insects may be influenced by the effects of UV exclusion. However, no significant differences in the activity of bumblebees (start-up and foraging) were noticed in commercial greenhouses provided that the hives were located in the more illuminated parts of the greenhouse (Southern walls) (83).

Parasitoids

Kajita (85) found that parasitism of whiteflies by E. formosa was the same under both standard and UV-blocking films. In choice experiments, significantly more (two-three times) specimens of E. forrnosa were trapped under a standard film than under UV- blocking films. The parasitization level of Diglyphus isae, the parasite of leafminers, was 10% less under UV-blocking films than under standard films (86). Aphidius colemani Viereck and A . Matricariae Haliday behave normally in a UV-deficient environment (71,86). It seems that the parasitoids, like their hosts, preferred an environment with high UV radiation. However, when they have no choice, they can perform well in a UV-deficient environment. Thus, it can be concluded that UV-blocking films work in favor of pest management by discouraging pest colonization while sustaining the activities of some important natural enemies used in the biocontrol of insect pests (87).

PUTATIVE MECHANISMS FOR THE PROTECTIVE EFFECTS OF UV FILTRATION BY GREENHOUSE CLADDING MATERIALS UV-absorbing cladding materials prevent the reflection of UV from greenhouse covers. It is believed that under these circumstances the “UV compass” of the immigrating insects directs their flight to UV-reflecting zones in the area-away from the UV-absorbing plastic houses. This hypothetical model of UV protection is supported by the significantly lower numbers of whiteflies that are trapped over greenhouse walls covered with UV-absorbing films or UV-absorbing nets (68). The reduction in whitefly population density under the UV-absorbing plastics cannot per se explain the dramatic inhibition of TYLCV spread. In a UV-deficient

environment created under UV-absorbing films, the activity of insects may be reduced, hence diminishing the ability of these insects to transmit viral diseases (64,68).

Thus, there appear to be two mechanisms for the anti-insect and antiviral activity of UV-absorbing films.

First, UV absorbance by greenhouse cladding materials interferes with the flight orientation of insect pests toward UV wavelengths emitted by the sky or reflected from the greenhouse covers. As a consequence, fewer insects invade these structures.

Second, elimination of UV wavelengths in sunlight transmitted through UV-absorbing films alters the normal behavior of invading insects, resulting in reduced flight activity or feeding behavior (or both). As a result, the opportunity for virus transmission is reduced.

CONCLUSIONS AND FUTURE RESEARCH UV-B radiation exerts various effects on both host plants and their pathogens. The resulting interaction complicates the understanding of the mode of action of UV on the host-pathogen system. Genetic variability within the populations involved in the pathogenic response further obscures the picture. Although many plant responses to UV are regarded as defense mechanisms that might inhibit fungal proliferation, no conclusive evidence is at hand to support this hypothesis. In fact, in one case UV-B exposure had opposite effects on the plant depending on whether its associated fungal pathogen was exposed to UV-B radiation or protected from it (35).

A better understanding of this complex system might be achieved using near-isogenic lines of both hosts and pathogens. It is recommended that such lines be produced for specific traits such as UV-B-induced flavonoid production or UV-B-produced conidial mortality. If such lines can be produced for a host- pathogen system, it will enable scientists to distinguish between the relative contributions of the host and the pathogen to plant pathogenicity.

Another way to improve understanding of host-pathogen interactions is by applying UV radiation using fiber optics. This approach would restrict UV exposure of the host plant and minimize the defense response while allowing full UV exposure of the fungal pathogen. The fact that P . cubensis Rostovtsev (59) and F. oxysporum (35) display opposite responses to UV-B exclusion demonstrates the need for a precise determination of the effects of additional wavebands on different fungal functions. Future research under polychromatic conditions should lead to an improved understanding of the relative roles of spectral composition during all stages of the life cycle of pathogenic fungi.

Ideally, growers could buy customized greenhouse covers with features that enhance growth and survival of a particular crop, are adapted to a wide range of climatic conditions, and are also effective in preventing or reducing the incidence of plant diseases and pests. The influence of UV radiation on insect orientation, navigation and communication has been frequently noted; however, little is known about the precise mechanisms behind the described effects. The routine experimental approach of using monochromatic light sources in flight chambers to study insect reactions has serious limitations. The greatest shortcoming of this approach is that it fails to simulate the complete natural environment where insects are simultaneously exposed to light signals of varying intensities and wavelengths. In their natural habitat insects are also affected by olfactory and climatic factors, which are lacking in laboratory situations. Because we assume that


an insect’s reaction to UV in nature is the outcome of complex interactions of different parameters, more realistic laboratory conditions are needed, including polychromatic radiation sources. Close cooperation among entomologists, photobiologists and physicists may lead to a breakthrough in understanding insect vision control.

Acknowledpments-This article was an outgrowth of work presented at the 28th Annual Meeting of the American Society of Photobiology and the International Photobiology Congress Symposium, entitled “Solar UV Radiation Effects on Plants: Interactions with Abiotic and Biotic Stress Factors,” held in San Francisco, CA, on 1-6 July 2000. We would like to thank Linda Chalker-Scott and Donald Krizek for their editonal sug- gestions and their efforts to organize this symposium.

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UV Radiation Effects on Pathogens and Insect Pests of Greenhouse-Grown Crops

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