Resistance of bipolaris-oryzae to fenapanil

Fenapanil-resistant strains of B. oryzae [isolated from wild rice, Zizania aquatica] were obtained by successive transfers on a series of fenapanil-amended potato-dextrose agar media containing increasing concentrations of the fungicide or by selection from a conidial suspension on a medium containing 100 .mu.g/ml of fenapanil. The ability of the fungus to acquire resistance to fenapanil was accompanied by a reduction in its fitness for survival and virulence. Generally, strains with a greater degree of fenapanil resistance grew slower and produced fewer conidia than strains with less resistance to fenapanil. Most of the fenapanil-resistant strains of B. oryzae declined in their resistance to fenapanil after 1 passage on a fungicide-free medium. Both fenapanil-resistant and -sensitive strains were effectively controlled on fenapanil-treated plants in the greenhouse; however, the fenapanil-sensitive strain was more virulent on untreated plants.