Fruit Crops News

Effects of Orchard Fungicides on Biological Control of Mites

Two recent studies emphasize the importance of carefully choosing fungicides as well as insecticides in apple IPM programs. One three-year study conducted in Nova Scotia, Canada, compared four orchard IPM programs, including two using captan as the fungicide, and two using metiram (a dithiocarbamate) as the fungicide. Twelve or 13 fungicide applications were made each year. The results of the study indicated that predator and pest mite populations were influenced by the miticide used. Captan was considered "neutral" while metiram tended to upset biological control, by reducing numbers of predatory mites.

The second study was conducted in Vermont in an orchard of scab-resistant apple cultivars, to evaluate the impact of two fungicides on mite populations. Even though scab-control fungicides are not necessary in such orchards, fungicides may need to be applied to control other pathogen-caused diseases. In both years of the study, 1988 and 1989, six applications, mostly of a tank mix of benomyl and mancozeb, were used to control cedar apple rust, powdery mildew, and frog-eye leaf spot. On 54 of 56 sampling dates, the number of predacious mites was significantly reduced in the fungicide-treated trees compared to the untreated controls. Also, more pest mites were found in fungicide-treated trees than in the untreated.

The results of these studies confirm earlier studies that show some fungicides can be detrimental to mite IPM programs by reducing predatory mite numbers. Growers should at least consider the known impact of various fungicides on predator mites and, where possible, choose those products that are least disruptive to IPM programs.

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