LAUSR

The genus Bipolaris causes two important cereal diseases: brown spot of rice (Bipolaris oryzae) and leaf spot of corn (Bipolaris maydis). This study examined the morphology of the two Bipolaris species and its pathogenicity and cytology on resistant and susceptible rice and corn cultivars. Resistant (CH-45) and susceptible (IR-72) rice cultivars were inoculated with B. oryzae (adapted) and B. maydis (non-adapted) isolates. B. oryzae was pathogenic on both rice cultivars producing higher lesion density and size on the susceptible cultivar at 3 to 5 days after inoculation which is significantly different from the resistant cultivar (P=0.05). B. maydis was non-pathogenic on both rice cultivars. Resistant (Sweet Grande) and susceptible (SuperSweet) corn cultivars were also inoculated with two Bipolaris spp. B. maydis was pathogenic on both corn cultivars producing higher lesion density and size on the susceptible cultivar at 7 days after inoculation which is significantly different from the resistant cultivar (P=0.05). B. oryzae was non-pathogenic on both corn cultivars. At the cytological level, conidia of both Bipolaris species germinated on two hosts but only B. maydis produced appressoria on resistant and susceptible corn cultivars. H2O2 accumualtion around the hyphal tips is due to the turgor pressure exerted by the hyphae which forces the DAB stain to acumulate in the immediate surrounding of the swollen hyphae at 24 h after inocualtion. In contrast, death cells, also a form of defense response were formed around the attempted penetration site on resistant rice cultivar inoculated with B. maydis at 24 h after inoculation. This study suggests that in rice-corn cropping system, B. oryzae is not a potential source of inoculum for leaf spot of corn. Likewise, B. maydis is not a potential source of inoculum for brown spot of rice.