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Three F1 progenies and their families in the segregating generations (F3, F4, F5 and F6), obtained after crossing resistant × susceptible wheat genotypes were studied in the field to determine the genetics of resistance to spot blotch caused by Bipolaris sorokiniana. Spot blotch scores in the F1 generation showed absence of dominance. Individually threshed F2 plants were used to advance the generations. Progenies (200‐250) of resistant genotypes Acc. No. 8226, Mon/Ald, Suzhoe#8 crossed with susceptible ‘Sonalika’ were evaluated in the F3, F4, F5 and F6 generations under induced epiphytotic conditions. Based on disease score distribution in individual progeny rows, F3 progenies were grouped into four classes: homozygous resistant, homozygous susceptible, segregating resistant and segregating susceptible. Resistance appeared to be under the control of three additive genes. The presence of three genes was also noted in the distribution of F4 and F5 lines. In the case of F6 progeny rows, both quantitative and qualitative models were used to estimate the number of segregating genes based on a 2‐year trial. It appeared that resistance to spot blotch was controlled by the additive interaction of more than two genes, possibly only three.

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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 × 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 geographic‐climatic 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.

Heat is an important abiotic stress during wheat (Triticum aestivum L.) grain-filling in South Asia. A study was undertaken to determine effectiveness of selection for reduction in 1000-kernel weight (TKWR) under heat stress to increase grain yield. Selection was made for low and high TKWR and selected progenies were evaluated in timely and late seeded trials at two locations in Nepal in 2003. One thousand kernel weight (TKW), biomass yield, grain yield, harvest index (HI), grain-filling duration (GFD) and area under spot blotch progress curve per day (AUDPC/day) were examined. The low and high TKWR groups did not differ significantly for TKW, biomass yield, grain yield, HI, days to heading, GFD and AUDPC/day under timely seeding. However, low TKWR lines showed higher TKW, biomass yield, grain yield, HI, and GFD and lower AUDPC/day than the lines with high TKWR under late seeding. Realized heritability estimates for TKWR ranged from 0.68 to 0.85. The findings show that selection for low TKWR could be used as an indirect selection criterion to identify high grain yielding lines under terminal heat stress.

An adaptation analysis was conducted in an attempt to identify adapted germplasm and potential indicator or probe varieties that could be used for more efficient germplasm introduction and evaluation. A set of 39 advanced wheat breeding lines and named varieties from Australian breeding programs and 10 from the CIMMYT/ICARDA programs were tested using 20 environments across the Australian wheatbelt during a period of 3 years. AMMI analysis and classification analysis were performed on grain yield data. Five groups of genotypes with similar patterns in performance within each group were identified, mostly reflecting their origin and pedigree. Most of the genotypes from the CIMMYT/ICARDA programs clustered together as did most of those from the University of Adelaide and Agriculture Western Australia breeding programs. Four groups of environments with similar trends in discriminating genotypes within each group were identified. There was a clear discrimination between subtropical and Mediterranean environments. Subtropical environments with supplementary irrigation showed similar patterns to Mediterranean environments. Basic differences in adaptation and phenotypic stability among genotypes from the CIMMYT/ICARDA programs in relation to genotypes from several breeding programs in Australia were identified. CIMMYT/ICARDA genotypes such as Attila, Nesser, Pfau/Seri//Bow, Genaro 81, and Maya/Nac performed well, especially in subtropical environments. The Australian varieties Hartog and Vulcan showed similar performance and could be used as indicator varieties for assessing introduced germplasm for subtropical regions. University of Adelaide developed genotypes Trident, Spear, Excalibur, and RAC 655, along with the Agriculture Western Australia genotypes Tammin and 82Y:1186, showed wide adaptation to all environments and could be used as indicator varieties for wide adaptation. Similarly, genotypes such as BT Schomburgk, Pelsart, and Sunvale could be used as indicator varieties for the other genotype groups. The results of this study can serve as a basis for identification and introduction of germplasm from the CIMMYT/ICARDA programs for various Australian production environments. It has also provided an understanding of the pattern of discrimination of genotypes across each region of the Australian wheatbelt.

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An understanding of how environment controls the initiation and development of the leaf is required to construct dynamic crop simulation models. The aim of this study was to determine the effect of vernalization and photoperiod on total number of leaves at anthesis, leaf emergence rate, and phyllochron in 20 spring wheat genotypes (Triticum aestivum L.). An experiment was conducted under greenhouse conditions during spring 1992 at ICARDA, Tel Hadya, Syria, with all combinations of three photoperiods (8-, 12-, and 16-h daylength) and two vernalization treatments (vernalized and nonvernalized). Total number of leaves on the main stem at anthesis decreased with increasing photoperiod. Vernalization reduced the total number of leaves on main stem at anthesis in the eight vernalization-sensitive genotypes. Leaf number on the main stem was linearly (r = 0.99) related to accumulated growing degree days (°C d). Genotypes differed in leaf emergence rates. Leaf-emergence rate increased with increasing photoperiod. Phyllochron decreased with increased daylength from 124 °C d leaf" 1 at 8 h to 97 °C d leaf" 1 at 16-h photoperiod. These results suggests that, to model leaf appearance and canopy development in wheat, genotypic coefficients of phyllochron need to be determined in relation to photoperiod. Additionally, the effect of vernalization at inductive photoperiods on the phyllochron in genotypes adapted to heat-prone tropical environments needs further study. W HEAT is grown throughout the major agroclimatic zones of the world. Its adaptation to different regions is controlled by vernalization, temperature, and photoperiod. An understanding of adaptation allows a better targeting of germplasm to specific environments,