EFFECT OF FREQUENCY OF DICARBOXIMIDE APPLICATIONS ON RESISTANT POPULATIONS OF MONILINIA-FRUCTICOLA AND BROWN ROT IN NEW-ZEALAND ORCHARDS

Dicarboximide-resistant isolates of Monilinia fructicola (Wint.) Honey were detected in New Zealand stone fruit orchards in 1985. In contrast to benzimidazole resistance in M. fructicola, the occurrence of dicarboximide resistance had not led to disease control failures at that time. A mass screening system was used to detect low frequencies of resistant isolates in orchards, based on growth on agar discs amended with a discriminating dose of iprodione (5 mg a.i. l-1). In 1987 the proportion of resistant isolates in six blocks ranged from 0.2 to 5%. The frequency of dicarboximide applications influenced the proportion of resistant isolates from 1987 to 1989. Th decline of resistant isolates in M. fructicola populations when dicarboximide applications were reduced or absent, suggested that resistant strains may not be as virulent and fit as sensitive strains. There were no positive correlations between brown rot incidence and the proportion of resistant isolates at the frequencies tested in 1988 and 1989. These results indicated that dicarboximide fungicides can continue to be used, but that recommendations for restricted frequency and period of application should continue to be observed.