PROLONGED FIELD PERSISTENCE OF RESISTANCE TO BENOMYL IN MONILINIA-FRUCTICOLA

Benomyl-resistant isolates of M. fructicola were obtained from a stone-fruit orchard where resistance had previously been found in 1976. Only three benomyl sprays had been applied in the period 1978-1988. A programme of six benomyl sprays over one season (1988/89) on peach and nectarine trees resulted in the build-up of a population of M. fructicola that was resistant to the fungicide. Resistant isolates were found to have an ED50 of > 50 μg a.i.ml-1 benomyl. Sensitive isolates failed to grow on 1 μg ml-1 benomyl. No resistance was detected to iprodione; all isolates failed to grow at 10 μg a.i.ml-1 in vitro. It is concluded that, where benomyl-resistant strains of M. fructicola have been detected, it may never be possible to resume the effective use of benzimidazole fungicides. © 1990.