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CROP BREEDING & GENETICS

Advancement toward New Spot Blotch Resistant Wheats in South Asia

CIMMYT, South Asia Regional Office, P.O. Box 5186, Kathmandu, Nepal

^* Corresponding author (rsharma{at}ecomail.com.np; rsharmac{at}gmail.com).

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Spot blotch, caused by Cochliobolus sativus, is a constraint to wheat (Triticum aestivum L.) production in South Asia. A set of genotypes was grown in Bangladesh, India, and Nepal to assess the current status of genetic resistance across locations. This study examined spot blotch resistance and agronomic performance of 24 wheat genotypes through regional trials in 2003, 2004, and 2005 and the estimated reduction in yield caused by spot blotch. We analyzed the area under the disease progress curve (AUDPC) and AUDPC d^–1 to assess spot blotch severity, and recorded grain yield, 1000-kernel weight (TKW), days to heading, and plant height. Disease severity differed in the 3 yr. The highest AUDPC d^–1 in 2005 was associated with the lowest grain yield, with an average 14.8% disease-induced yield reduction. Several genotypes showed low disease severity. A few genotypes had high grain yield. A few resistant genotypes such as Milan/Shanghai #7, Chirya.1, and Chirya.7 had grain yield reductions of about 5%. The genotype BL1473 showed high disease severity but low (5%) disease-induced reduction in grain yield. The genotype Milan/Shanghai #7, with the lowest disease severity and highest grain yield, was also the most stable for spot blotch resistance and grain yield. The results indicated that wheat genotypes with improved spot blotch resistance, high grain yield, acceptable TKW, and plant height are available as a result of the international collaboration in South Asia.

Abbreviations: AUDPC, area under the disease progress curve • GGE, genotype and genotype x environment • TKW, thousand-kernel weight.

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
TWENTY YEARS AGO, spot blotch, also called Helminthosporium leaf blight or foliar blight, caused by Cochliobolus sativus (Ito & Kurib.) Drechsler ex Dastur [anamorph, Bipolaris sorokiniana (Sacc.) Shoem.], was recognized as an important disease of wheat (Triticum aestivum L.) in nontraditional, warm cropping areas of South Asia (Saari, 1985). The region corresponds to CIMMYT Wheat Mega-environment 5 (van Ginkel and Rajaram, 1998) and encompasses the eastern part of the Indian subcontinent. It is characterized by average temperatures >17°C during the coolest month and high relative humidity. Several studies from the 1990s substantiated the significant economic losses from spot blotch on wheat in the region (Dubin and van Ginkel, 1991, Duveiller and Gilchrist 1994; Saari, 1998), and wheat genotypes from the region were reported to be susceptible (Alam et al., 1994; Devkota, 1994). Several wheat genotypes from Brazil, Zambia, and the Yangtze River Valley in China identified as resistant to foliar blight elsewhere (Kohli et al., 1991; Dubin and van Ginkel, 1991) were introduced into the region for use in crosses with susceptible commercial cultivars. Dubin et al. (1998) first reported on the level of resistance in a set of exotic and regional wheat genotypes. The locally adapted genotypes had low levels of resistance; exotic genotypes showed good tolerance, but were mostly late maturing and tall.

In the early 1990s, CIMMYT and national wheat research programs in South Asia initiated a Helminthosporium monitoring nursery (HMN) to monitor disease severity continuously at different locations and to evaluate the stability of host-plant resistance in the best regionally and internationally available resistance sources. Progress in identifying usable sources of resistance was reported a few years later (Bhatta et al., 1998; Dubin et al., 1998) and the need for further regional collaboration through the HMN was recognized (Duveiller et al., 1998). A few studies from the region in the past 5 yr have reported low to moderate success in breeding for foliar blight resistance (Bhandari et al., 2003; Sharma et al., 2004a; Siddique et al., 2006). Nevertheless, recent studies in the region have shown grain yield reductions still >20% (Sharma et al., 2004b; Duveiller et al., 2005; Mahto et al., 2006; Sharma and Duveiller, 2006). As a result, there is still a need to find additional resistant germplasm that can be crossed with commercial cultivars possessing low to moderate resistance. The objectives of this study were to assess foliar blight resistance and agronomic performance of the best exotic sources of resistance and of the most recently released wheat genotypes bred for the region, as well as to estimate the extent of spot blotch induced yield reductions in the region.

	MATERIALS AND METHODS

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Twenty-four wheat genotypes tested in the HMN in the 2003, 2004, and 2005 wheat-growing seasons in South Asia were analyzed in this study (Table 1). ‘Sonalika’, ‘Kanchan’, and ‘Ciano’ 79—long-time popular cultivars and checks that are highly susceptible to spot blotch—were included to monitor disease pressure and to compare with the response of the newer genotypes to spot blotch, thereby serving as a measure of progress in resistance breeding. Exotic genotypes included in the study were chosen for their reportedly high levels of resistance to spot blotch. The materials were tested in the HMN in three wheat-growing seasons in the eastern plains of South Asia.

View larger version (27K): [in this window] [in a new window]   Figure 1. Partial map of South Asia showing sites where the Helminthosporium monitoring nursery (HMN) was grown.

For minimizing variation at different sites, a field book with planting layout and instructions for trial management and data recording was provided to each collaborator. Seed packages for planting were prepared by the Wheat Research Program in Bhairahawa, Nepal, and distributed to each collaborator each year.

Days to heading was recorded when spikes of approximately 50% of the plants in a plot were fully emerged from the boot. In each location, spot blotch was visually scored three times for each plot not treated with fungicide between growth stages Z69 and Z83 (Zadoks et al., 1974; Tottman and Hilary, 1987) 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, disease severity percentage was based on the following formula:

The area under the disease progress curve (AUDPC) was calculated using the severity percentage estimates corresponding to the three ratings as outlined by Das et al. (1992):

where x_i = spot blotch severity on the ith date, t_i = ith day, and n = number of dates on which spot blotch was recorded. 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 d^–1) to directly compare among epidemics of different lengths for the three wheat-growing seasons. At maturity, plant height in each plot was measured from ground level to the tip of the spikes. After maturity, plots were individually harvested and threshed. One thousand kernels, taken randomly from each plot, were weighed to measure 1000-kernel weight (TKW).

The experiment was conducted at 11, 12, and 12 sites in 2003, 2004, and 2005, respectively. There were 11 common locations in each of the 3 yr. Each year–location combination was considered a unique and random environment, while genotypic effect was analyzed as fixed. The statistical analysis included an analysis of variance for each environment and a combined analysis across environments using SAS software (SAS Institute, 2003). The significance of F ratios was tested according to the procedure outlined by McIntosh (1983) for analysis of combined experiments.

Genotype and genotype x environment (GGE) biplot analyses for the inverse of AUDPC d^–1 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. The GGE biplot is a method of graphical analysis of multienvironment data (Yan et al., 2000). It is different from a 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 x environment interaction of multienvironment 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 x 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. The inverse of AUDPC d^–1 was used in the analysis, however; hence the direction of the arrow on the performance line represents a lower value, i.e., more resistant genotype. A stability line perpendicular to the performance line also passes through the origin of the biplot; the two arrows in opposite directions represent a 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.

	RESULTS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Spot blotch severity, grain yield, and disease-induced reductions in grain yield differed during the 3 yr (Table 2). The AUDPC was highest in 2003, but did not differ significantly between 2004 and 2005. In contrast, AUDPC d^–1 was highest in 2005, followed by 2003 and 2004. Grain yield was highest in 2004, but did not differ significantly between 2003 and 2005. Disease-induced reduction in grain yield was highest in 2005, followed by 2003 and 2004.

The 24 genotypes showed significant variation for AUDPC, AUDPC d^–1, grain yield, TKW, days to heading, and plant height (Table 3). The spot blotch susceptible cultivars Sonalika, Kanchan, and Ciano 79 had the earliest heading and the highest disease severity. Milan/Shanghai #7 had the lowest disease severity and the highest grain yield, while BL2029 had the highest TKW.

View larger version (36K): [in this window] [in a new window]   Figure 2. Mean area under disease progress curve per day (AUDPC/day) of the 24 wheat genotypes tested in the Helminthosporium monitoring nursery, South Asia, 2003 to 2005. (Refer to Table 1 for names of the genotypes.)

View larger version (38K): [in this window] [in a new window]   Figure 3. Mean grain yield under fungicide-treated and untreated treatments of the 24 wheat genotypes tested in 35 environments across South Asia. (Refer to Table 1 for names of the genotypes.)

View larger version (40K): [in this window] [in a new window]   Figure 4. Spot blotch induced reduction in grain yield of the 24 wheat genotypes averaged across 35 sites in South Asia. (Refer to Table 1 for names of the genotypes.)

View larger version (26K): [in this window] [in a new window]   Figure 5. Genotype and genotype x environment biplot showing a comparison of 24 wheat genotypes with an ideal genotype for the inverse of the area under the disease progress curve per day across 35 environments in South Asia. The environments are hidden (for clarity of the graph) and treated as random samples of the target environments. (Refer to Table 1 for names of the genotypes.)

View larger version (29K): [in this window] [in a new window]   Figure 6. Genotype and genotype x environment biplot showing a comparison of 24 wheat genotypes with an ideal genotype for grain yield across 35 environments in South Asia. The environments are hidden (for clarity of the graph) and treated as random samples of the target environments. (Refer to Table 1 for name of the genotypes.)

	DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

The relative magnitudes of disease severity across years as measured by AUDPC vs. AUDPC d^–1 differed, despite a high correlation coefficient (0.89, P < 0.01) between the two. This raises the issue of how disease parameters should be reported from different years and locations. For comparing epidemics of different lengths, Reynolds and Neher (1997) suggested that AUDPC should be standardized by dividing it by the total number of days in the evaluation period. The year of the highest reduction in grain yield coincided with the highest AUDPC d^–1, but not with the highest AUDPC (Table 2), indicating the differing usefulness of the parameters for predicting yield losses. Comparing disease severity measured in different assessment periods, Duveiller et al. (2005) also found the standardized value of AUDPC d^–1 more precise for predicting yield reductions.

The 24 genotypes represented a range of variability for disease and agronomic characters (Table 3), with opportunities for selecting wheat genotypes for spot blotch resistance and acceptable agronomic characters. Mean AUDPC d^–1 values for several genotypes (no. 14: Milan/Shanghai #7; no. 1: SW89.5422; no. 2: Chirya.7; no. 6: Yangmai #6; no. 7: Chirya.1; and no. 20: Altar-84/Ae. sq. (224)//Yaco) were consistently low in all 3 yr (Fig. 2), which suggests the possibility of selecting for stable resistance to spot blotch, and confirms the previous observation by Sharma et al. (2004b), who reported wheat genotypes with stable resistance across diverse environments.

There were 10 wheat genotypes in the innermost circle that could be considered stable across environments for low AUDPC d^–1 (Fig. 5). All these genotypes could be of value for regional wheat breeding programs attempting to improve spot blotch resistance. Several genotypes appeared desirable for AUDPC d^–1, but only one genotype (Milan/Shanghai #7) was highly stable for grain yield across environments (Fig. 6), although there were other genotypes with high yield performance. Interestingly, Milan/Shanghai #7 is also a promising high-yielding, disease-resistant genotype identified in the disease-prone environment bordering the Caspian Sea in Iran where Fusarium head scab [Gibberella zeae (Schwein.) Petch)], and foliar diseases including Septoria tritici blotch (Septoria tritici Roberge in Desmaz.), and tan spot [Pyrenophora tritici-repentis (Died.) Drechs.] are endemic (E. Duveiller, unpublished data, 2006). Another genotype with acceptable grain yield stability was Kan/6/Coq/F61.701//Cndr/3/Oln/4/Phos/Mrng/5/Alan/Cno (no. 22), which has been developed in the region (Bangladesh). The genotypes K 7 (no. 10) and Altar-84/Ae. sq. (224)//Yaco (no. 20) also had high and stable grain yield. The genotype Altar-84/Ae. sq. (224)//Yaco (no. 20), a synthetic derived wheat, had high grain yield (Fig. 5) and low disease severity (Fig. 3), suggesting its suitability as a suitable parent for increasing both grain yield and resistance to spot blotch. Milan/Shanghai #7 was developed in CIMMYT, Mexico, from a cross between the tan spot resistant Milan and spot blotch resistant Shanghai #7. The high level of useful resistance in Milan/Shanghai #7 was further confirmed by it having the lowest disease-induced reduction in grain yield (Fig. 4) in this study. Another genotype with relatively high disease severity but low disease-induced grain yield reduction was BL1473. The low grain yield reduction in BL1473 confirms its genetic tolerance to spot blotch, as reported by Sharma et al. (2004b); BL1473 has a high TKW and is among the earliest maturing genotypes. A cross between BL1473 and Milan/Shanghai #7 might be useful to combine higher resistance with background tolerance, resulting in lines with possibly more durable resistance to spot blotch, a concept proposed by Zuckerman et al. (1997) for Septoria tritici blotch in wheat.

The resistant genotypes generally had somewhat later maturity, as reflected in a negative correlation coefficient (–0.73, P < 0.01) between spot blotch severity and days to heading. This suggests that the early maturity of susceptible genotypes needs to be combined with high resistance and grain yield to fit the short wheat-growing season common in the rice–wheat systems in South Asia. The average height of the resistant genotypes in our study was acceptable, that is, not too tall. Heights of up to 1 m are acceptable, and medium-height plant types that yield good amounts of both grain and straw serve the needs of resource-poor farmers, given the value of wheat straw for cattle fodder (Sharma, 1992).

Studies in South Asia during 1993 to 1994 reported that local wheat cultivars were susceptible to spot blotch or possessed only low levels of resistance (Dubin et al., 1998). A later study reported that wheat genotypes showing stable resistance did not give high yields or lacked yield stability (Sharma et al., 2004a). The findings of this study are different from earlier reports by demonstrating that the combination of high levels of stable resistance and high and stable grain yield exist in the newer wheat genotypes. Earlier reports suggested that spot blotch resistance is associated with late maturity (Dubin and Rajaram, 1996; Dubin and Duveiller, 2000), and late-maturing genotypes would be low yielding due to high temperature stress during the wheat grain filling period in the eastern Gangetic Plains of South Asia. In addition, the most resistant and highest yielding genotype, Milan/Shanghai #7, headed 8 d later than the standard check for maturity, Sonalika, thus showing progress toward earlier maturity in a resistant genotype (Dubin et al., 1998; Sharma et al., 2004a). Dubin et al. (1998) had reported that the most resistant genotype headed 20 d later than Sonalika.

These findings confirm that exceptional, 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.

An earlier study (Badaruddin et al., 1994) reported yield losses of up to 41% for Kanchan and 71% for Sonalika. Grain yield in some high-yielding, advanced wheat breeding lines from this study was reduced by <5% by spot blotch infections. While 11 to 15% average reductions in grain yield from the disease are considered acceptable levels in breeding programs, unrelenting efforts to further enhance resistance are needed. Given that C. sativus is a highly aggressive pathogen, evolving fast and characterized by a continuum of strains differing in levels of pathogenicity (Maraite et al., 1998; Duveiller and García Altamirano, 2000), new sources of foliar blight resistant genotypes showing superior agronomic characteristics in regional tests (Duveiller and Sharma, 2004; Sharma et al., 2004a) should be used in breeding programs to sustain current levels of resistance. Any new genotype must possess resistance to this disease to survive in South Asia. The results underline priorities for further research to develop improved wheat germplasm resistant to spot blotch, thereby reducing crop yield losses in the warmer plains in South Asia. A regular stability analysis often does not provide relative ranking of superior entries in reference to an ideal genotype that results in making a subjective judgment in selecting a cultivar. The GGE biplot approach used in this study could help breeders better decide what genotypes should be promoted or released: the visual combined assessment of resistance and its stability is a big advantage, and adds confidence in the decision to promote a superior genotype.

	ACKNOWLEDGMENTS

 
Financial support for this regional collaborative study in South Asia was made available to CIMMYT/South Asia through the Directorate General for Cooperation and Development (Government of Belgium). We appreciate the assistance of Mr. S. Pradhan in preparing the figures and tables and gratefully acknowledge the editing assistance of CIMMYT science writer Mike Listman. We 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 Viswavidyalaya, Coochbehar; G.B. Pant University of Agriculture and Technology, Pantnagar; Assam Agricultural University, Shillongani), Nepal (National Wheat Research Program, Bhairahawa; 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).

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Received for publication April 7, 2006.

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TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

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