Evaluation and standardization of growth stage of wheat against spot blotch caused by Bipolaris sorokiniana

Aim: Spot blotch caused by Bipolaris sorokiniana (Sacc.) Shoem being a major disease of wheat in warm and humid regions of the world with a worldwide distribution pose a thrust area for sustaining wheat productivity. The pathogen is most aggressive under high relative humidity and temperature associated with the low fertility of soils leading to reduction in grain weight and yield. It is the major biotic constraint in wheat mainly in the Gangetic plains, especially in the rice-wheat cropping system as rice serves as a host for the pathogen and provides favourable environment for its survival and multiplication. Losses due to spot blotch disease depends on the level of resistance in a cultivar and prevailing weather conditions. The present study aimed to determine the crop growth stage of wheat at which recording of disease was most strongly related to the severity of spot blotch for efficient selection of resistant genotypes. Methodology: Field trials were conducted to study the response of twenty one wheat genotypes against spot blotch under epiphytotic conditions during 2006-07 to 2011-12 crop seasons at Norman E. Borlaug Crop Research Centre of G. B. Pant University of Agriculture and Technology, Pantnagar. The disease severity was assessed at three different growth stages (GS) viz. GS 55 (half of inflorescences emerged), GS 75 (medium milk) and GS 87 (hard dough) on Zadoks scale (Zadoks et al., 1974) following the method proposed by Singh and Kumar (2005) on Double digit scale (0-9) Results: The results indicated that the best stage to record the disease was medium milk (GS 75) and not GS 55 (half of inflorescences emerged), or dough stage (GS 87) for accurate assessment of resistance level. Interpretation: Varieties like HD2888, HS375, PDW291, VL804, VL829, WH1021 and WH1105 were highly resistant to spot blotch.