Reaction of rice (Oryza sativa) cultivars to penetration and infection by Curvularia tuberculata and C. oryzae

Isolates of Curvularia species were collected from weedy Cyperaceae species and are being evaluated as possible biocontrol agents of sedge weeds in rice (Oryza sativa). Curvularia species have been reported from rice; thus cultivars of rice were tested to determine rice seedling responses to these potential biocontrol agents. All 13 rice cultivars were resistant to Curvularia tuberculata isolate 93-022, 12 were resistant to C. tuberculata isolate 93-020, and 7 were resistant to C. oryzae isolate 93-061. In the resistant cultivars, lesions on the leaf laminae were small, light to dark brown, with a dry appearance. Spots on the leaf margins and leaf tips were light brown to cream and dry. In the susceptible cultivars, the brown lesions coalesced with necrotic centers. Sporulation was observed in the lesions on susceptible cultivars but not on the resistant cultivars. The histopathology of C. tuberculata and C. oryzae was studied in two resistant rice cultivars, IR 64 (IRRI Acc. no. 66970) and Norin 21 (IRRI Acc. no. 7686), by light microscopy. C. tuberculata exhibited polar germination beginning at 4 h postinoculation (HPI); whereas C. oryzae was characterized by bipolar germination starting at 2 HPI. Simple terminal or intercalary appressoria were initiated at 24 HPI over stomatal apertures, or rarely, on the epidermal cell walls and bulliform cells. No infection cushions were formed. Penetration occurred by the formation of a fine penetration peg beneath the appressorium. A chlorotic reaction was observed in areas beneath and adjacent to the appressoria and germ tubes and in the infected cells. Resistance of IR 64 and Norin 21 to C. tuberculata and C. oryzae infection was mainly expressed after penetration as a slow and restricted mycelial growth and no sporulation. C. tuberculata isolate 93-022 is the preferred isolate for further study as a biological control agent against Cyperus difformis, C. iria, and Fimbristylis miliacea.