Adaptation of mancozeb by Bipolaris oryzae and B. sorokiniana , the causal organisms of brown spot of wild rice.

Isolates of B. oryzae and B. sorokiniana from wild rice (Zizania aquatica ) initially did not grow on potato-dextrose agar (PDA) amended with mancozeb higher than 100 mu b/ml. Resistance of both species ot mancozeb was obtained by transferring repeatedly to media containing increasing concentrations of the fungicide. After eight to nine passages on media amended with increasing concentrations of mancozeb, four isolates of B. oryzae and B. sorokiniana produced colonies that grew at 8,000 mu g/ml of mancozeb. Growth and sporulation of mancozeb-resistant strains of both species on unamended PDA were reduced compared with the wild types. Resistance to mancozeb was lost after subculturing once on fungicide-free media. The similarity of sensitivity of a limited number of isolates of these fungi from fungicide-treated or untreated fields to mancozeb suggested that resistance to this fungicide did not occur in the field.