Development, Persistence, Survival, and Strategies for Control of Thiabendazole-Resistant Strains ofPenicillium expansumon Pome Fruits

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Abstract
Both the proportion of isolates of P. expansum resistant to thiabendazole (TBZ) at 40 .mu.g ml and the persistence of this characteristic after a single transfer on apples increased during 3 consecutive years of postharvest treatment with TBZ. Resistance to TBZ was maintained both when the isolates were grown for 20 transfers on a fungicide-free medium or when they were inoculated into and recovered from untreated apple fruits for 5 transfers. Fruit decay and initiation of sporulation always developed more slowly after inoculation with resistant strains than with sensitive ones. In the absence of fungicides, fruit decay caused by mixtures of sensitive and resistant strains developed at a rate similar to that caused by the sensitive strain alone. The proportion of resistant spores collected from decayed fruit inoculated with a mixture of strains, with no TBZ selection pressure, showed a rapid decline. Three fungus control strategies were compared for the control of a highly resistant population of P. expansum: uniform treatment with a mixture of fungicides, heterogeneous treatment in which 2 fungicides were applied separately to different lots of fruit placed in the same storeroom, and a combination of heterogeneous and mixed treatments in which a mixture of fungicides and a single fungicide were applied to different lots of fruits placed in one storeroom. All 3 strategies significantly reduced decay incidence in relation to the control; however, the last of these 3 strategies caused the most rapid decline in the proportion of resistant strains.