Biochemical and epidemiological components associated with rice resistance to brown spot

ABSTRACT Brown spot (Bipolaris oryzae) is one of the most important leaf diseases affecting rice. To epidemiologically and biochemically understand the variation in resistance among rice genotypes, in this study some resistance components and some biochemical variables, widely known as plant defense mechanisms against pathogens, were evaluated in rice genotypes classified, according to SOSBAI, as moderately susceptible, moderately resistant and resistant. Seven rice cultivars were inoculated with 1 × 104 conidia mL-1 of B. oryzae and the following resistance components were evaluated: relative infection efficiency (ERI); final lesion number (NFL); final lesion length (CFL); lesion growth rate (r) and final severity (SF). In addition, biochemical variables such as activity of the enzymes superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), chitinase (CHI) and phenylalanine ammonia-lyase (PAL) were analyzed. Results showed significant differences among cultivars for resistance components, as well as for enzyme activities. All tested genotypes were susceptible to B. oryzae, varying for the level of partial resistance to brown spot. The variables r and CFL showed higher correlation to SF, indicating to be the variables that most influenced the final disease intensity in each genotype. Rice genotypes showing higher partial resistance to brown spot had alterations characterized as earlier, potentiated and longer SOD, CAT POX and PAL activities.