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Spot blotch is an important disease of wheat (Triticum aestivum L.) in South Asia. Division of test sites for this disease into homogenous subregions is expected to contribute to more efficient evaluation and better differentiation of cultivars. Data from a collaborative regional program of South Asia conducted by CIMMYT were analyzed to group testing sites into relatively homogenous subregions for spot blotch area under the disease progress curve (AUDPC). Five-year data of eight locations from Eastern Gangetic Plains Nursery (EGPSN) and five locations of the Eastern Gangetic Plains Yield Trial (EGPYT) conducted in three countries (India, Nepal, and Bangladesh) of South Asia were used. A hierarchical cluster analysis was used to group locations on the basis of genotype x location interaction effects for spot blotch AUDPC. Cluster analysis divided South Asia into two broad regions and four subregions. This classification was not entirely consistent with the geographic distribution of locations, but clusters mostly followed general geographicclimatic locations. The locations Varanasi (India) and Bhairahawa (Nepal) were identified as the most suitable sites for evaluation of spot blotch, followed by Rampur (Nepal). The major determinant for the clustering was mean temperature. The results suggest that the major wheat region of South Asia can be divided into subregions, which may reduce the cost of resistance evaluation and aid in developing wheat with resistance to this disease.

Spot blotch, caused by Cochliobolus sativus, is a constraint to wheat (Trificum 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.sup.-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.sup.-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.

Helminthosporium leaf blight [HLB, caused by spot blotch, Cochliobolus sativus (Ito & Kuribayashi) Drechs. ex Dastur, and/or tan spot, Pyrenophora tritici-repentis (Died.) Drechs.], is the most serious disease constraint to wheat (Triticum aestivum L.) yields in the warmer plain areas of South Asia. A selection strategy is needed to identify early maturing, HLB-resistant genotypes, given that most early maturing wheat cultivars in the region are either susceptible or have low levels of HLB resistance. A study was conducted to determine whether three traits could be simultaneously improved with a selection index ([I.sub.s]) combining the area under disease progress curve (AUDPC) as an assessment of disease severity, days to heading (DHD), and thousand-kernel weight (TKW). Results from replicated field tests at two sites in Nepal in 2002 showed that selection in the [F.sub.3] generation with the low and high [I.sub.s] was effective in identifying [F.sub.4] lines with low and high AUDPC, respectively. The use of low [I.sub.s] was associated with higher grain yield and higher TKW, without significantly affecting DHD and plant height. The AUDPC was reduced by 579 to 837, depending on location and population, while TKW was increased by 7.8 to 12.7 g, and grain yield by 786 to 1491 kg [ha.sup.-1]. The use of [I.sub.s] also produced positive response in biomass and grain yields. There was an average 43% increase in grain yield of the low [I.sub.s] group compared with the high [I.sub.s] group. The results suggested that selection for early maturing, HLB-resistant wheat lines with high grain yield and kernel weight is possible with a [I.sub.s]., FOLIAR BLIGHT, also known as HLB, is a serious disease of wheat in the warmer areas of South Asia where spring wheat is grown during the winter season (November to [...]

Worldwide Puccinia striiformis f. sp. tritici (Pst) epidemics have been reported to be driven by few genetic lineages, while a high diversity is evident at the Pst Himalayan centre of diversity. This study investigated the relationship between pathotype diversity and genetic structure in Nepal, the eastern Himalayan region, which has been largely unexplored. Despite the high genetic diversity and recombinant structure detected through microsatellite genotyping, characterization of virulence phenotypes for 62 isolates identified only eight pathotypes, with two pathotypes predominant over all the populations. This is in contrast to the Pakistani and Chinese recombinant populations, where high pathotype diversity is associated with genetic diversity. The most prevalent Nepali pathotype was not a unique clonal lineage, but was represented by seven multilocus genotypes from four distinct genetic subgroups, suggesting strong directional selection on virulence genes, resulting in convergent pathotypes in distinct genetic groups. This convergent selection is discussed in comparison with clonal French and recombinant Pakistani populations. Additionally, the Nepali Pst population carried virulence to 17 out of 24 tested yellow rust resistance genes (Yr), with the absence of virulence to Victo and Early Premium and resistance genes Yr5, Yr10, Yr15, Yr24 and Yr26. Virulence to Yr2, Yr7, Yr27 and YrSu were fixed in all isolates, in line with the deployment of these resistance genes in Nepal. The results reflect the influence of resistance gene deployment on selection of virulence and pathotypes in a recombinant pathogen population, which must be considered in the context of durable resistance gene deployment. [ABSTRACT FROM AUTHOR]