Leaf tip necrosis: A phenotypic marker associated with resistance to spot blotch disease in wheat

Abstract

Spot blotch, caused by Bipolaris sorokiniana (Sacc.) Shoem. syn. Drechslera sorokiniana (Sacc.) Subrm and Jain (syn. Helminthosporium sativum, teleomorph Cochliobolus sativus), is an important disease of wheat (Triticum aestivum L.) in warmer and humid regions of the world. To date, no morphological marker is known to be associated with resistance to this disease. The purpose of this study was to find out the association of leaf tip necrosis (Ltn) with resistance to spot blotch disease. A total of 1407 spring wheat genotypes that originated from the Indian and CIMMYT wheat breeding programs were evaluated for Ltn and resistance to spot blotch for three seasons (1994-1995, 1995-1996, and 1996-1997) under field conditions. Disease severity was recorded at six growth stages under artificially created epidemics. About 75% of the genotypes showing Ltn (Ltn+) were resistant or moderately resistant, whereas 82% not showing it (Ltn-) were moderately susceptible or susceptible. Mean spot blotch rating of the Ltn+ genotypes was significantly lower than the Ltn- genotypes at all growth stages and the genotype x environment interaction was nonsignificant. To confirm the association of Ltn with resistance, individual F2-derived F3, F4, F5, and F6 progenies from the cross of the 'HUW234' near-isogenic pair for Ltn were evaluated for spot blotch severity. In each generation, the Ltn+ homozygons progenies had significantly less disease than those homozygous Ltn-. These results confirm that leaf tip necrosis is associated with moderate resistance to spot blotch and can be used as a morphological marker to facilitate selection for resistance.