www.elsevier.com/locate/fcr

Field Crops Research 103 (2007) 109–118

Progress and challenge towards reducing wheat spot blotch

threat in the Eastern Gangetic Plains of South Asia:

Is climate change already taking its toll?

R.C. Sharma a,*, E. Duveiller b, G. Ortiz-Ferrara a

a International Maize and Wheat Improvement Center (CIMMYT), Kathmandu, Nepalb CIMMYT, Wheat Program, Mexico, D.F., Mexico

Received 21 February 2007; received in revised form 14 May 2007; accepted 14 May 2007

Abstract

Spot blotch, caused by Cochliobolus sativus, is a serious constraint to wheat production (Triticum aestivum L.) in the warmer plains of South

Asia. Fourteen genotypes were tested in Bangladesh, India and Nepal during 6 years (2000–2005) to determine disease severity and resistance

stability over these years. We analyzed the area under the disease progress curve per day (AUDPC/day) to assess spot blotch severity, and examined

grain yield, 1000-kernel weight (TKW), days to heading, and plant height. Disease severity differed in the 6 years and several genotypes showed

low disease severity in all years. A few genotypes had high grain yield and low disease severity in all years. Genotype Altar-84/Ae. Sq. (224)//Yaco,

with the lowest disease severity and highest 1000-kernel weight (TKW) and grain yield was also the most stable for spot blotch resistance, TKW

and grain yield. Results demonstrated that wheat genotypes with improved spot blotch resistance, high grain yield, acceptable TKW, and plant

height are available as a result of the regional and international collaboration in South Asia. Despite this progress, the challenge lies ahead because

of an apparent tendency of increasing spot blotch overall severity in the region and decreasing TKW over 6 years (2000–2005). Higher, average

night time temperature during the month of March is an indicator showing that wheat crop performance is challenged in two ways—increasing spot

blotch severity and decreasing TKW causing lower yield. The results underline the possible effect of changing climatic factors on disease pressure

and the continuous need to identify new resistance sources to develop more spot blotch resistant wheat for the warmer Eastern Gangetic Plains of

South Asia.

# 2007 Elsevier B.V. All rights reserved.

Keywords: Cochliobolus sativus; Eastern Gangetic plains; Genetic stability; Helminthosporium leaf blight; Resistance; Spot blotch; Wheat

1. Introduction

More than 20 years ago, Saari (1985) recognized that spot

blotch caused by the fungal pathogen Cochliobolus sativus (Ito

& Kurib.) Drechsler ex Dastur [anamorph, Bipolaris sorokini-

ana (Sacc.) Shoem.] was a major constraint to wheat (Triticum

aestivum L.) production in non-traditional, warmer cropping

areas particularly in the Eastern Gangetic Plains (EGP) of

South Asia. The importance of spot blotch also known as foliar

blight or Helminthosporium leaf blight in South Asia was

highlighted early in reports from India (Nema and Joshi, 1973),

Bangladesh (Badaruddin et al., 1994), and Nepal (Devkota,

* Corresponding author at: CIMMYT, Singh Durbar Plaza, P.O. Box 5186,

Kathmandu, Nepal. Tel.: +977 1 4219262; fax: +977 1 4229804.

E-mail address: rsharma@ecomail.com.np (R.C. Sharma).

0378-4290/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.fcr.2007.05.004

1994). A Helminthosporium Monitoring Nursery (HMN) was

initiated by CIMMYT (International Maize and Wheat

Improvement Center) in the early 1990s to monitor the stability

of resistance to spot blotch and detect if there were significant

differences in epidemics across warmer wheat growing areas of

the world. In the late 1990s this nursery was sown exclusively at

key sites in South Asia.

While the first set of exotic foliar blight resistant lines tested

in South Asia showed that resistant lines had a less desirable

agronomic type (Dubin et al., 1998), recent reports from the

South Asia indicated substantial improvement in spot blotch

resistance on key cultivars commercially grown in India

(Bhushan et al., 2002), Bangladesh (Siddique et al., 2006), and

Nepal (Sharma and Duveiller, 2006). Besides, a number of

genotypes were recently identified to carry high and stable

foliar blight resistance which is presently used in the wheat

breeding programs in the Eastern Gangetic Plains (EGP)

R.C. Sharma et al. / Field Crops Research 103 (2007) 109–118110

(Sharma et al., 2004a). However, recent yield trials conducted

by breeding centers show that wheat yields in the EGP region

are decreasing (Sharma et al., 2007). This apparently parallels

wheat production statistics of the regional yields which are

reported to be either stagnant or slightly decreasing, except in

Nepal (FAO, 2006). Further evidence supporting this observa-

tion is that due to lower production, India imported three

million tonnes wheat in 2006 (Reuters, 10 May 2006) which

contrasts with the optimism that followed the Green Revolution

and raises concerns among agricultural policy makers

regarding prospects of self-sufficient staple food production

in the near future (Nagarajan, 2005). Stagnant or lower wheat

yields in recent years in the plains of South Asia is considered

partly due to climatic factors including heat stress characterized

by an increasing trend in average temperature during winter

months (Rane et al., 2000; Nagarajan, 2005; Black, 2006;

Sharma et al., 2007). Another factor causing decline in wheat

productivity in rice–wheat system, the most dominant cropping

system in the region, is related to soil fertility and fertilizer use

(Nagarajan, 2005; Tirol-Padre and Ladha, 2006). Nagarajan

(2005) also listed socio-economic and policy related factors

being responsible for recent decline in wheat production in the

Indo-Gangetic plain. A steadily increase of mean March

temperature was observed at five sites in the EGP region in the

past 6 years (Sharma et al., 2007). Since high temperatures also

aggravate spot blotch severity (Nema and Joshi, 1973; de

Lespinay, 2004; Sharma and Duveiller, 2004; Duveiller et al.,

2005) wheat yield losses in the EGP region could be due to

increase in spot blotch epidemic over the recent years, which

could curtail the benefits from progress recently achieved by

local breeders toward incorporating resistance in locally

adapted high yielding varieties. Hence, increased spot blotch

severity due to steadily changing climatic conditions would

worsen the damage already caused by heat with the reduction of

assimilates translocated to the grain. Likewise, climatic change

in South Asia has been observed in terms of a significantly

increasing number in cloudy and foggy days during winter

months (Debi, 2003), which not only reduces solar radiation use

Table 1

Name and origin of wheat genotypes tested in the Helminthosporium Monitoring

Genotype

No. Name/cross

1 SW89-5422

2 Chirya.7 (CS/Th cu//Vee/3/Ald/Pvn/4/Ningmai No. 4/Oleson//A

3 Yangmai-6

4 Chirya.1 (CS/Th cu//Vee/3/Ald/Pvn/4/Ningmai No. 4/Oleson//A

5 K-7

6 Ning 8201

7 K8027

8 Achyut

9 Coq/F61.70/Cndr/3/Oln/4/Pho

10 Prl/Toni

11 Altar-84/Ae. Sq. (224)//Yaco

12 Barkat/Kavkaz

13 Kan/6/Coq/F61.701//Cndr/3/Oln/4/Phos/Mrng/5/Alan/Cno

14 Gourab (=Turaco/Chil)

a Cross identification number.

but also contributes to prolonged hours of 100% relative

humidity in crop canopy, an ideal situation for early

establishment of spot blotch in wheat. Limited data recorded

with the help of a data logger installed in a wheat field confirms

this prolonged periods of high relative humidity in the canopy

(de Lespinay, 2004). Another phenomenon resulting of

anthropic activities and known as the ‘Asian Brown Cloud’

may also have a direct impact on agriculture through reduction

in total solar radiation due to air pollution leading to lower

photosynthetic activity (UNEP and C4, 2002).

The objective of this study was to examine 6-year data from

the regional HMN in South Asia to determine if resistance to

spot blotch had been stable over time and locations in the key

genotypes tested in all 6 years. One specific objective was to

examine if there was a systematic trend in spot blotch severity

and in turn in 1000-kernel weight (TKW) over the past 6 years

and their relationship with temperature. A recent report has

suggested that grain yield loss in the region was associated with

changes in average night time temperature in the month of

March (Sharma et al., 2007) which is also critical for spot

blotch development (Duveiller et al., 2005). Previous studies

suggested that TKW was negatively correlated with spot blotch

severity (Sharma et al., 1997; Dubin et al., 1998) and

temperature during grain filling period (Sharma, 1993).

2. Materials and methods

Fourteen genotypes of spring wheat tested during 6 years

(2000–2005) in the Helminthosporium Monitoring Nursery

sown in South Asia, were analyzed in the present study

(Table 1). These genotypes have diverse origins and many of

them are reported to possess high level of resistance to spot

blotch (Sharma et al., 2004a,b; Dubin et al., 1998). A few

genotypes are advanced breeding lines derived from crosses

involving spot blotch resistant parents. For example, Altar-84/

Ae. sq. (224)//Yaco (#11), Chirya.1 and Chirya.7 are a

synthetic-derived hexaploid wheat using tetraploid durum

(Triticum turgidum (Desf.) Husnot) and the D genome from

Nursery, South Asia, 2000–2005

Origin CIMMYT CIDa

China 72,403

ld/Yangmai No. 4) CIMMYT 66,176

China 239,288

ld/Yangmai No. 4) CIMMYT 52,605

Zambia 231,080

China 95,659

India 141,864

Nepal 355,572

CIMMYT

CIMMYT 8,064

CIMMYT

Bangladesh 459,077

Bangladesh

Bangladesh

R.C. Sharma et al. / Field Crops Research 103 (2007) 109–118 111

diploid Triticum tauschii (Coss.) Schmal. (syn. A. squarrosa)

(Mujeeb-Kazi et al., 1996) that is considered to possess

resistance to spot blotch (Mujeeb-Kazi, 1998). Barkat/Kavkaz

inherits resistance to spot blotch from Kavkaz that possess 1B/

1R translocation.

The experimental sites included three locations in Bangla-

desh (Dinajpur, Jamalpur, and Jessore), six in India (Coochbe-

har, Faizabad, Karnal, Pantnagar, Shillongani and Varanasi),

and three in Nepal (Bhairahawa, Rampur, and Tarahara). All

sites except Pantnagar and Karnal are part of the vast Eastern

Gangetic Plains. However, spot blotch is a serious disease of

wheat at all sites included in this study (Dubin et al., 1998;

Sharma et al., 2004a). Rice–wheat cropping system, the most

dominant rotation in the EGP, was practiced at each site in all

years. The trials were conducted at 8, 7, 7, 11, 12, and 12 sites in

2000, 2001, 2002, 2003, 2004, and 2005, respectively.

At each site, the field trial was sown in a randomized

complete block with two replicates. Plot size was 2 m � 0.5 m,

seeded with two rows using 0.25-m spacing. Spot blotch

severity was visually scored three times for each plot in each

location, using the double-digit scale (00–99) developed as a

modification of Saari and Prescott’s severity scale to assess

wheat foliar diseases (Saari and Prescott, 1975; Eyal et al.,

1987). The first digit (D1) indicates vertical disease progress on

the plant and the second digit (D2) refers to severity measured

as diseased leaf area. For each score, percentage disease

severity was based on the following formula:

Severity percentage ¼�

D1

9

��D2

9

�� 100:

The area under the disease progress curve (AUDPC) was

calculated using the percentage severity estimates correspond-

ing to the three ratings as outlined by Das et al. (1992). The

AUDPC gives a quantitative measure of epidemic development

and disease intensity (Reynolds and Neher, 1997). As outlined

by Reynolds and Neher (1997), the AUDPC was standardized

by dividing by the total number of days in the evaluation period

(AUDPC/day) to directly compare among epidemics of

different lengths for the six wheat growing seasons. After

maturity, plots were individually harvested and threshed. One

thousand randomly taken kernels from each plot were weighed

to measure TKW.

Each year–site combination was considered a unique and

random environment, while genotypic effect was analyzed as

fixed. The test of significance using F-ratios was conducted

according to the procedure outlined by McIntosh (1983) for

analysis of combined experiments. The statistical analysis

included an analysis of variance for each environment using

SAS (2003) software. After confirming the homogeneity of

variance (Gomez and Gomez, 1984, p. 322), a combined

analysis of variance was also conducted. Genotype and

genotype � environment (GGE) biplot analyses for inverse

of AUDPC/day, TKW and grain yield were conducted using

GGE biplot software (Yan and Kang, 2002) to determine

stability and to identify genotypes of interest for disease

resistance and grain yield. GGE biplot is a method of graphical

analysis of multi-environment data (Yan et al., 2000). It is

different from regular biplot that simultaneously displays both

genotypes and environments (Gabriel, 1971). The GGE biplot

is a biplot that displays the main genotype effect (G) and the

genotype � environment interaction (GE) of multi-environ-

ment tests. It is constructed by plotting the first two principal

components (PC1 and PC2, also referred to as primary and

secondary effects, respectively) derived from singular value

decomposition of the environment-centered data. A specific

option in GGE biplot analysis allows comparison among a set

of genotypes with a reference genotype. This method defines

the position of an ‘‘ideal’’ genotype, which will have the

highest average value of all genotypes and be absolutely stable;

that is, it expresses no genotype by environment interaction. A

set of concentric circles are generated using the ideal genotype

as the concentric center. The ideal genotype is used as a

reference to rank the other genotypes. A performance line

passing through the origin of the biplot is used to determine

mean performance of a genotype. The arrow on the

performance line represents increasing mean performance.

However, inverse of AUDPC/day was used in the analysis;

hence the direction of arrow on performance line represents

lower value, i.e. more resistant genotype. A stability line

perpendicular to the performance line is also passing through

the origin of the biplot; the two arrows in opposite directions

represent decrease in stability. A genotype farther form the

biplot origin on either side on the stability line represents

relatively lower stability. A genotype closer to the performance

line is considered more stable than the one placed farther.

Regression analysis was used to detect trends over years in

grain yield, disease severity, TKW, and mean minimum (or

night time) temperature in March. TKW was also regressed

over spot blotch severity in the 6 years to determine disease

effects on this trait. The effects of mean minimum temperature in

March on TKW and disease severity were also estimated using

regression analysis. Linear regression coefficient (b) was used to

infer tendency for a trait; the positive and negative values

representing increasing and decreasing trends, respectively.

3. Results

3.1. Genotype overall performance

There was a significant effect of environment, genotype, and

genotype � environment interaction on days to heading and

maturity, plant height, grain yield, TKW, and spot blotch

measured as severity percentage and AUDPC/day (ANOVA not

presented). The mean disease severity, AUDPC/day, grain yield

and TKW greatly varied in different years underscoring the

effect of environmental conditions. The 14 wheat genotypes

differed significantly for days to heading and maturity, plant

height, grain yield, spot blotch severity, AUDPC/day and TKW

(Table 2).

Mean spot blotch severity ranged from 22.5 to 38.3%

(Table 2). None of the genotypes was highly susceptible to spot

blotch considering that highly susceptible genotypes could

score 100% at the end of the season (Duveiller et al., 2005).

Table 2

Mean values for various traits of the 14 wheat genotypes tested across 57 environments in South Asia

Genotype number Days to Plant

height (cm)

Grain yield

(kg/ha)

Spot blotch 1000-Kernel

weight (g)Heading Maturity Severity (%) AUDPC/day

1 81 117 85 3456 25.8 13.8 38.4

2 79 118 90 3481 22.5 14.4 37.1

3 80 116 94 3173 23.4 15.5 39.0

4 78 115 87 3547 24.5 14.9 38.4

5 76 115 106 3776 31.7 25.8 40.8

6 78 115 85 3177 29.4 19.4 40.5

7 84 120 104 3320 26.9 19.0 39.7

8 80 118 100 3234 27.9 19.7 36.2

9 72 112 92 3609 37.7 26.0 42.7

10 74 114 96 3543 35.5 25.5 39.7

11 78 116 99 3930 23.2 14.2 45.9

12 72 113 88 3621 34.5 26.8 37.9

13 72 112 97 3865 36.7 25.3 37.4

14 71 112 93 3500 38.3 27.2 41.3

LSD0.05 1.4 1.3 2.2 87.4 1.1 0.6 0.7

CV (%) 6.8 4.3 8.8 9.4 13.6 11.7 7.0

R.C. Sharma et al. / Field Crops Research 103 (2007) 109–118112

The ranges of values for grain yield and TKW ranged from

intermediate to high. Most of the genotypes had acceptable

days to heading (<80 days) and maturity (<120 days) and plant

height (<100 cm). Genotype #11 (Altar-84/Ae. sq. (224)//

Yaco) had the highest grain yield and TKW and was among the

most resistant genotypes.

3.2. Stability analysis

The GGE biplot analysis of the 14 wheat genotypes for

inverse of AUDPC/day revealed that Altar-84/Ae. sq. (224)//

Yaco (#11) was the most stable for low spot blotch severity

among the resistant genotypes since it was closest to the point

Fig. 1. GGE biplot showing a comparison of 14 wheat genotypes with an ideal

genotype for inverse of area under disease progress curve per day across 57

environments, South Asia. The environments are hidden (for clarity of the

graph) and treated as random samples of the target environments (refer to

Table 2 for name of the genotypes corresponding to number 1–14).

of ideal genotype (Fig. 1). Several other resistant genotypes that

were relatively more stable included Yangmai-6 (#3),

SW89.5422 (#1), Chirya.7 (#2), Chirya.1 (#4) and K8027 (#7).

The GGE biplot analysis for TKW revealed that all

genotypes except K8027 (#7) were positioned along the mean

line (Fig. 2). However, among the genotypes with high TKW,

Altar-84/Ae. sq. (224)//Yaco (#11) was the most stable by being

closest to the point of ideal genotype. Other two genotypes with

higher and stable TKW were Coq/F61.70//Cndr/3/Oln/4/Pho/

Mrng (#9) and Gourab (#14).

The GGE biplot analysis for grain yield showed that Altar-

84/Ae. sq. (224)//Yaco (#11) was the most stable among higher

yielding lines by being closest to the point of ideal genotype

Fig. 2. GGE biplot showing a comparison of 14 wheat genotypes with an ideal

genotype for 1000-kernel weight 57 environments, South Asia. The environ-

ments are hidden (for clarity of the graph) and treated as random samples of the

target environments (refer to Table 2 for name of the genotypes corresponding

to number 1–14).

Fig. 3. GGE biplot showing a comparison of 14 wheat genotypes with an ideal

genotype for grain yield across 57 environments, South Asia. The environments

are hidden (for clarity of the graph) and treated as random samples of the target

environments (refer to Table 2 for name of the genotypes corresponding to

number 1–14).

Fig. 4. Trend in mean 1000-kernel weight (TKW), area under spot blotch

progress curve per day (AUDPC/day), grain yield and mean minimum tem-

perature in March over 6 years (A), effect of temperature on AUDPC/day, TKW

and grain yield (B), and effect of spot blotch on TKW and grain yield recorded

in the Eastern Gangetic Plains of South Asia in 6 years, 2000–2005.

R.C. Sharma et al. / Field Crops Research 103 (2007) 109–118 113

(Fig. 3). Other high yielding genotypes that were relatively

more stable included Kan/6/Coq/F61.701//Cndr/3/Oln/4/Phos/

Mrng/5/Alan/Cno (#13), K-7 (#5) and Barkat/Kavkaz (#12).

3.3. Yield and disease trend analysis

Regression analysis revealed an increasing trend for AUDPC/

day (b = 2.4, r = 0.96**) over 6 years (Fig. 4A). Thus over years,

average spot blotch severity records increased in all genotypes,

particularly in genotypes #10 and #12–14 from Bangladesh.

There was a decreasing trend for TKW (b = �1.02, r = �0.92*)

and grain yield (b = �139, r = �0.45) in the same period

(Fig. 4A). Mean minimum temperature in the month of March

showed an increasing trend (b = 0.60, r = 0.93**) (Fig. 4A).

Mean maximum temperature in March and minimum and

maximum temperatures in January and February showed a weak

fit in the regression model (values not shown). This increasing

trend in the mean minimum temperature in March over 6 years,

with exception in 2003, was evident at all five key sites in the

region from where the temperature data were analyzed (Fig. 5).

These five sites represent the entire transect of the EGP region

where the HMN trials were evaluated. Even if the mean

maximum temperature showed in general a tendency to increase

(Fig. 5), the regression analysis was better explained by the mean

minimum temperature in March (Fig. 4A). The mean minimum

temperature in March showed a positive relationship with spot

blotch severity (b = 3.22, r = 0.84**), and a negative association

with TKW (b = �1.53, r = �0.88**) and grain yield (b = �197,

r = �0.43) (Fig. 4B). Spot blotch severity showed a negative

association with TKW (b = �0.42, r = �0.93**) and grain yield

(b = �80, r = �0.67) (Fig. 4C).

All 14 genotypes showed an increasing trend in spot blotch

severity and a decreasing trend in TKW over 6 years (Fig. 6).

There were variations in the slope of the regression line and r

value for both spot blotch severity and TKW among the

genotypes. A greater average annual increase in spot blotch

severity was evident on less resistant genotypes like #12

(Barkat/Kavkaz) and #14 (Gourab) (3.0%).

4. Discussion

4.1. Progress towards improving spot blotch resistance

The results from 57 environments confirm the low disease

severity of a few genotypes showing their values in improving

spot blotch resistance of the commercial cultivars in South

Asia. Though not a part of this study, another recent study

conducted at most of these sites in the same years, commercial

Fig. 5. Mean minimum and maximum temperature over 6 years at five sites in

the Eastern Gangetic Plains where the trials were conducted.

R.C. Sharma et al. / Field Crops Research 103 (2007) 109–118114

cultivars had shown much higher disease severity with above

80% on the susceptible check (Sharma et al., 2007). These

results demonstrate that newer wheat genotypes in the region

possess improved resistance over old commercial cultivars, and

confirm previous findings based on data from limited

environments that included several resistant genotypes reported

in this study (Sharma et al., 2004a,b; Joshi et al., 2004). A few

genotypes with lower disease severity (Chirya.7, Yangmai-6

and Chirya.1) had relatively lower grain yield and TKW, and

later maturity compared to genotype #13 which has been

developed in the region. However, the genotype with the

highest grain yield and TKW (#11: Altar-84/Ae. sq. (224)//

Yaco) also had low disease severity. This shows progress in

combining spot blotch resistance and high grain yield, which

was not possible earlier. Dubin et al. (1998) had reported that

high yielding wheat cultivars during early 1990s in India, Nepal

and Bangladesh were susceptible to spot blotch. There are still

high yielding recently developed commercial wheat cultivars in

the region with low to intermediate levels of resistance that

show up to 20% yield loss due to spot blotch (Sharma and

Duveiller, 2006; Siddique et al., 2006). Identification of high

yielding and spot blotch resistant genotypes in this study offers

opportunity to further increase the yield of the commercial

cultivars by improving resistance through selective breeding.

One genotype, K8027 (#7), included in this study, had also

been studied in an earlier study in 1993 and 1994 by Dubin et al.

(1998). They reported that the leading commercial wheat

cultivars of South Asia in early 1990s had much higher spot

blotch severity than K8027, which showed high level of

resistance in their study. This study shows that a few of the

recently developed wheat genotypes have resistance level equal

to or better than K8027, besides showing higher yield and TKW

and earlier maturity. This finding demonstrates that improve-

ment achieved in the region is due to international effort in

improving spot blotch resistance of the wheat cultivars in the

Eastern Gangetic Plains of South Asia. Also, it shows that the

best sources of spot blotch resistance are coming not only from

the wide crosses alone, but also from the traditional hexaploid

wheats.

This study confirms that spot blotch resistant wheat

genotypes are becoming more acceptable agronomically, and

are available for direct use in breeding programs to develop

commercial cultivars. The results further substantiate the

usefulness of the regional spot blotch resistance monitoring

nursery as a vehicle for introducing new sources of spot blotch

resistance that are agronomically acceptable and provide high

and stable yields (Sharma et al., 2004a,b). Even though our

study includes only one of the climatic factors emphasized by

Garrett et al. (2006), nevertheless, our observation on

temperature effect on spot blotch epidemic is factual.

4.2. Challenges

One specific objective of the present work was to examine

any possible drift in spot blotch severity that might have

occurred in the region in the past 6 years. The results confirm a

tendency towards increasing spot blotch severity in the region.

In the same period, there has been a decreasing trend in TKW

and grain yield. TKW is often used as an indicator of resistance/

tolerance by being negatively correlated with spot blotch

severity (Dubin et al., 1998; Sharma et al., 1997, 2004b).

Fig. 6. Trend in mean 1000-kernel weight and mean spot blotch severity of 14 wheat genotypes over 6 years in the Eastern Gangetic Plains of South Asia (refer to

Table 2 for name of the genotypes corresponding to number 1–14).

R.C. Sharma et al. / Field Crops Research 103 (2007) 109–118 115

R.C. Sharma et al. / Field Crops Research 103 (2007) 109–118116

During the period, 2000–2005, there are indications of steadily

slightly increasing trend for average night time temperature in

the month of March in the region. These facts have no doubt

effect on plant physiology as dark respiration would increase,

causing premature plant senescence. Thus confounding effects

may occur between an increase in disease susceptibility and

negative abiotic effects on crop physiology. These effects are

intrinsically linked and very difficult to unravel through the

present study. However, the cumulative and related effects of

crop susceptibility to disease, crop physiology and stresses has

been shown through systematically designed studies conducted

at representative locations in the same region that are in

agreement with this observation (Sharma and Duveiller, 2004;

Duveiller et al., 2005; Rosyara et al., 2007). This supports

general speculations about global warming and its specific

relevance to wheat production environments for the South Asia

region (Black, 2006; Hodson and White, 2007). Temperature

data from five sites in the region shows increasing trend in

average night time temperature at each location in the 6 years

with exception in 2003. Similar trend was present for average

maximum temperature but not at each site. Higher temperature

favors spot blotch severity (Duveiller et al., 1998; de Lespinay,

2004). Also, high temperature during March that coincides with

wheat grain filling period in the Eastern Gangetic Plains of

South Asia reduces TKW (Sharma, 1993). Hence, a same cause

where higher temperature is an indicator, results in a complex

effect operating in opposite directions on spot blotch severity

and TKW in the region. Garrett et al. (2006) has underlined the

intricate interrelationships between plant disease and climate.

Also, because anthropogenic climate change is characterized

by greater increases in Tmin. (minimum temperature) than

Tmax. (maximum temperature), improved understanding of the

temperature response is needed to better quantify and reduce

uncertainties in climate change impact assessments (Lobell

and Ortiz-Monasterio, 2007). CERES (Crop Environment

Resource Synthesis) projections of response to increased

temperature in North America suggested that the apparent

historical importance of Tmin. mainly results from covariation

between temperatures and solar radiation (Lobell and Ortiz-

Monasterio, 2007). In South Asia the ‘Asian Brown Cloud’

effect, signifying hazy sky, has been identified, causing

reduction in solar radiation reaching the crop canopy. As a

result, biomass and yield of irrigated wheat in the January–

March 1999 decreased by an estimated 1–2% depending on

regions using wheat growth simulator (WTGROWS), with

higher effect in Eastern and Western Uttar Pradesh of India

(UNEP and C4, 2002). However, the effect was found to be

limited and differences among wheat growing regions in India

were statistically non-significant. A weak reaction of wheat

yield to 20% radiation reduction from the normal level means

the radiation requirement by the crop is satisfied even under

radiation reduction scenario (UNEP and C4, 2002). Thus, in

South Asia, the effect of average temperature per se (i.e. heat)

combined with the higher relative air humidity during foggy

days in winter are likely to be more significant on wheat spot

blotch severity at regional level than changes in solar

radiation.

A critical analysis shows that a linear increase in spot blotch

severity could in part have caused decrease in TKW,

temperature being another factor for such reduction. This

result confirms a recent observation by Sharma et al. (2007)

who reported increase in spot blotch severity recorded during

six seasons in South Asia on a set of local commercial cultivars.

This finding is also in agreement with the recent reports that the

disease continues to cause substantial grain yield losses on

recently developed moderately resistant wheat cultivars in

Bangladesh (Siddique et al., 2006) and Nepal (Sharma and

Duveiller, 2006). Considering the importance of crop

physiology and the abiotic stresses such as heat in particular

on disease development underlined in earlier studies (Sharma

and Duveiller, 2004; Duveiller et al., 2005), the results of the

present study raise a special concern. They suggest that spot

botch will remain a disease of resource poor farmers who

cannot afford early sowing, live in marginal warmer wheat

growing areas and are in the frontline against higher threats as a

consequence of climate change. Recently, Black (2006)

concluded that heat is going to be a daunting factor to higher

wheat yields in warm regions of the world including South

Asia. The present study supports that increasing night time

temperature in March is associated both with loss in TKW and

increase in spot blotch severity. This demonstrates that

increase in temperature during wheat grain filling could prove

double jeopardy to wheat yield in South Asia—hastening

wheat physiological maturity by reducing the grain filling

period (Sharma, 1993), and increasing spot blotch severity

(Nema and Joshi, 1973; de Lespinay, 2004; Sharma and

Duveiller, 2004; Duveiller et al., 2005). Thus, yield

performance is threatened, should the overall phenomenon

of increasing temperature and disease severity intensify

(Duveiller et al., in press). If crop profitability declines and

heat tolerance in wheat is not satisfactory, biotic constraint like

spot blotch may lead to less acreage grown to wheat in future in

warmer areas, prompting farmers toward crop diversification

(Black, 2006). In the EGP, this may soon translate in a

comparative economic advantage of winter (rabi) maize and

potato over bread wheat, which is already being experienced in

Bangladesh (S. Waddington, CIMMYT, Bangladesh, personal

communication).

None of the wheat genotypes included in the study is highly

susceptible to spot blotch as shown by their low to intermediate

severity scores. Earlier reports have shown that SW89.5422,

Chirya.1, Chirya.7, Yangmai-6, and K-7 show <10% spot

blotch induced losses in grain yield and TKW compared to

>30% in susceptible wheat cultivars from South Asia region

(Sharma et al., 2004a,b; Sharma and Duveiller, 2006). Since all

14 genotypes showed a tendency towards higher spot blotch

severity and lower TKW over the 6-year period, factors other

than wheat genotypes appear to posing challenge towards

future improvement of resistance. All trials reported in this

study were seeded around optimal time (between 20 November

and 10 December) and received standard wheat crop manage-

ment practices. Hence variations in climatic factors, heat in

particular, and their effect on increased disease severity and loss

in TKW are suspected to have major implications. Differences

R.C. Sharma et al. / Field Crops Research 103 (2007) 109–118 117

in virulence in the spot botch-wheat pathosystem are unlikely to

occur since C. sativus isolates represent a continuum of strains

differing in aggressiveness and no race speciation has been

found in this non-specific pathogen as confirmed in several

studies on pathogen diversity (Di Zinno et al., 1998; Maraite

et al., 1998; Ruckstuhl, 1998). Thus, environmental factors

unrelated to pathogen specialization in virulence could have a

major effect. Also, the direct effect of increasing temperature

on multiplication and sporulation rate of the prolific C. sativus

fungus is very well known in vitro and in vivo (Duveiller et al.,

1998). In research in Mexico, Duveiller et al. (1998) showed

that disease severity increased in controlled conditions with

higher temperature, which is supported by a recent study on the

effect of temperature on spot blotch severity (de Lespinay,

2004). This study demonstrates that wheat genotypes with

stable resistance to spot blotch combined with stable grain yield

and TKW and acceptable maturity and plant height are

available in South Asia as a result of international collaboration

over the past 15 years. These genotypes could prove valuable

for wheat breeding programs in the region to improve wheat

yield and spot blotch resistance. The key superior genotypes

could also be valuable for other warm wheat growing areas

where spot blotch is a problem.

The findings also demonstrate that grain yield in the Eastern

Gangetic Plains of South Asia is showing a decreasing trend

and there is an increasing tendency for spot blotch severity in

the past 6 years. The conclusion based on the 6-year data

underlines a potential threat due to climatic change: the

increase in temperature could destabilize wheat productivity in

the heat prone wheat growing environments in South Asia. In

agreement with the conclusion on interaction among climatic

variables and plant disease by Garrett et al. (2006), our results

show that a close monitoring of climatic changes and disease

development, i.e. spot blotch severity observed through a

regional effort like the Helminthosporium Monitoring Nursery,

is fundamental to achieving stability in wheat production in the

region.

Acknowledgments

Financial support for this regional collaborative study in

South Asia was made available to CIMMYT through the

Directorate General for Cooperation and Development (Bel-

gium Government). The authors appreciate the assistance of

Mr. S. Pradhan in preparing the figures and tables. The authors

acknowledge the collaboration of wheat programs in India

(Directorate of Wheat Research, Karnal; Banaras Hindu

University, Varanasi; Narendra Dev University of Agriculture

and Technology, Kumarganj; Uttar Banga Krishi Viswavidya-

laya, Coochbehar; G.B. Pant University of Agriculture and

Technology, Pantnagar; Assam Agricultural University, Shil-

longani), Nepal (National Wheat Research Program, Bhair-

ahawa; Institute of Agriculture and Animal Science, Rampur;

Regional Agricultural Research Station, Tarahara), and

Bangladesh (Regional Agricultural Research Station, Jessore;

Wheat Research Center, Dinajpur; Wheat Research Center,

Jamalpur).

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