Characterization of Nigrospora oryzae Causing Brown Spot Disease in Rice and Emergence of Quinone Outside Inhibitor Fungicide Resistance in Korea

In this study, 24 fungal isolates were obtained from rice leaves exhibiting brown spots in paddy fields of Chungcheongbuk-do, Korea, and were identified as Nigrospora oryzae through ITS sequence analysis. N. oryzae 21OCN1 exhibited vigorous mycelial growth at 25°C on potato dextrose agar, oatmeal agar, and V8 agar media, while conidial production was significantly enhanced on Czapek-Dox agar medium under combined ultraviolet (UV) exposure and aeration. Pathogenicity tests revealed that N. oryzae 21OCN1 was pathogenic to rice leaves and panicles at the heading stage, but no disease symptoms were observed on panicles at the ripening stage. Fungicide sensitivity was evaluated using the agar dilution method, and fungicides belonging to Fungicide Resistance Action Committee (FRAC) mode of action code (MOA) C3, C5, and G1 exhibited strong inhibitory effects on mycelial growth. In particular, for strobilurin fungicides (MOA code; C3), the addition of 100 μg/ml of salicylhydroxamic acid to the medium clearly distinguished between sensitive and resistant isolates. A G143A amino acid substitution in the cyt b gene was confirmed in resistant isolates. These findings enhance our understanding of the mycelial growth, sporulation, pathogenicity, and fungicide response of N. oryzae, and provide a valuable basis for the development of effective management strategies for rice diseases caused by this pathogen.