Heat stress tolerance is one of major breeding objective in tropical environments. Selection for spot blotch has been suggested to be combined with heat tolerance. In many parts of the Asian subcontinent, wheat is followed by rice causes delays in planting of wheat consequently resulting higher heat stress.
Under field conditions, late planting (resulting in terminal heat stress) was found to increase spot blotch severity as well as severe yield reductions. Delayed seeding was found to increase disease severity even in resistant genotypes and caused higher yield losses. Both of these stresses, spot blotch and terminal heat stress are severe at the end of season affecting grain filling.
Stay green trait is considered very important trait that allow plants to retain their leaves in the active photosynthetic state under stressed conditions. A positive phenotypic correlation between stay green trait and HLB severity has been found. In addition to heat stress, spot blotch also causes post-anthesis chlorophyll depression, measured as Area Under SPAD progress curve (AUSPC) and resistant genotypes show high AUSPC. Canopy temperature depression has been associated with spot blotch resistance and combined spot blotch and post-anthesis heat stress tolerance. Similarly chlorophyll fluroscence (Fv/Fm) was found associated with both spot blotch resistance and heat stress tolerance in a recent study.
There have been considerable progress in development of resistant genotypes, still the challenge lies ahead because of an apparent tendency of increasing spot blotch overall severity in the region and decreasing TKW in recent years. 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 warm south Asian plains and along with increased heat stress tolerance.
Studies that evaluate spot blotch along with terminal heat stress tolerance are limited. Most breeding nurseries are screened in optimal planting conditions, which do not evaluate high terminal heat stress due late planting. In a study that screening 729 lines in lowlands of south Asia, EGPYT 67 ( pedigree- Kauz//Kauz/Star/3/Prinia/4/Milan/Kauz), EGPYT 84 (pedigree - Mrng/Buc//Blo/Pvn/3/Pjb 81) and EGPYT 69 (pedigree- Chirya3/Pastor) were found the best for spot blotch resistance, yield, days to maturity, and thousand kernel weight (TKW) indicating their spot blotch resistance combined with heat stress tolerance. Raj 3765 and Raj 4027 has been found stable heat stressed environments of South Asia and has been suggested for planting under late planting conditions.
Suitable genotype for spot blotch stress warm environments. Being a hemibiotropic pathogen, maintain of good health condition is very important to defense against the disease, as any type of abiotic stresses can increase susceptibility to the disease. Thus in addition to heat stress but other type of abiotic stress, mineral nutrient stress (due to poor fertilization) and moisture stress also affect spot blotch severity. Recent observations in long-term fertility trials in Nepal indicate that soil fertility may be related to higher foliar blight severity and potash may have an important effect in reducing epidemics. To achieve significant progress in development of genotypes with high performance in south Asian context, breeders need to develop genotypes combining multiple stress tolerance along with spot blotch resistance. It seems more useful to consider all traits together, rather than in isolation.
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Sharma RC, Ortiz-Ferrara G, Bhatta MR (2007) Regional trial results show wheat yield declining in the eastern Gangetic plains of south Asia. Asian Journal of Plant Sciences 6: 638-642.
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Rosyara UR, Sharma RC, Shrestha SM, Duveiller E (2005) Yield and yield components response to defoliation of spring wheat genotypes with different level of resistance to Helminthosporium leaf blight. Journal of Institute of Agriculture and Animal Science 26:43-50.
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