VARIATION IN RESISTANCE TO SPOT BLOTCH AND THE AGGRESSIVENESS OF BIPOLARIS SOROKINIANA ON BARLEY AND WHEAT CULTIVARS
Bipolaris sorokiniana is the causal agent of spot blotch and other diseases in barley and wheat in different parts of the world. This study investigated the reaction of nine barley cultivars and four wheat cultivars to infection by 11 B. sorokiniana isolates. All B. sorokiniana isolates produced necrotic lesions on the inoculated barley and wheat cultivars, with no significant differences among most isolates in the necrotic lesion size. However, B. sorokiniana isolates induced significantly varying levels of chlorotic lesions and spores eight days following inoculation (P < 0.05). Isolates BsJ53 and BsJ51 were the most aggressive on barley cultivars, while isolates BsJ53, BsJ51, BsWM and BsWS were the most aggressive on wheat cultivars. The aggressive isolates resulted in large chlorotic lesions (> 7 mm) and produced thousands of conidia per cm2 of leaf area. A significant and positive correlation was found between the number of conidia produced on leaves and the size of the chlorotic lesions (P < 0.05), which suggests that the chlorotic weakened tissues provide a favorable site for spore production. Screening cultivars for resistance to spot blotch disease showed that barley cultivars ’Omani’ and ‘Beecher’ and wheat cultivars ’Cooley’ and ‘K65’ were the least susceptible to B. sorokiniana. The study shows the lack of host specialization of B. sorokiniana on wheat and barley. The apparently resistant barley and wheat cultivars should be used in future breeding programs for resistance to spot blotch.