The biological characteristics of dicarboximide‐resistant isolates of Monilinia fructicola from New Zealand stone‐fruit orchards

Dicarboximide‐resistant isolates of Monilinia fructicola were detected in New Zealand stone fruit orchards in 1985. The EC50 values of resistant isolates ranged from 3 to 217 mg a.i./l iprodione, compared with 0‐3 to 0‐7 mg a.i./l for sensitive isolates. The degree of dicarboximide resistance was maintained over nine generations in fruit tissues in most isolates, but in four isolates there was a significant decline. Three resistant and two sensitive isolates were selected for further study on agar and host tissues. Resistant isolates caused disease, grew and sporulated as well as sensitive isolates on flowers not amended with dicarboximide fungicides, but some were less fit on fruit. The pathogenicity and spore production of a resistant strain on flowers was reduced significantly by dicarboximide applied prior to pathogen inoculation. Fitness of isolates on flowers and on fruit was poorly correlated. Resistant isolates were significantly less competitive than sensitive isolates when mixed inocula were applied to flowers and fruit. The relative performance on flowers and fruit was not a reliable indicator of competitive ability.