INTERACTIONS OF PESTICIDES AND MYCOFLORA OF PEACH TWIGS

The effects of pesticides applied singly (copper oxychloride, thiram) or in combination (captan, dinocap, benomyl and methomyl) on peach twig mycoflora were studied under orchard, glasshouse and laboratory conditions in 1986 and 1987. Fungal populations, which were determined by two indirect methods using agar media and a direct method (scanning electron microscopy), were lowest on untreated twigs in the orchard in April and June, increasing as the season progressed to reach maxima in September and February. Following copper oxychloride and thiram applications, the fungal population in both years, was either slightly reduced or not reduced, but with captan + dinocap applications in both, the flora was appreciably reduced. Other applications (captan + benomyl + methomyl and dinocap + benomyl + methomyl) also significantly reduced fungal populations. Hyphomycetes were the most abundant fungi on both untreated and treated twigs, especially Alternaria spp., Cladosporium spp. and Penicillium spp. Populations of Cladosporium spp. and Alternaria spp. were considerably depressed by the pesticides. However, numbers of Penicillium spp. (especially P. frequentans, a powerful antagonist of the peach pathogen Monilinia laxa), were not reduced in 1986, and reductions in 1987 were less than for Alternaria spp. and Cladosporium spp. Penicillium spp. were more frequent on treated twigs than Alternaria spp. and Cladosporium spp. from the end of May to August. The saprotrophic colonisation of peach twigs by selected fungi in the glasshouse was also found to be reduced by the application of fungicides. The results which can be partially explained by the in vitro sensitivity of the isolates to the fungicides have potential practical implications in the development of an integrated control programme against M. laxa, combining biological and chemical methods. Although natural populations of P. frequentans found in this study during the infection period were insufficient to control the disease, P. frequentans is a good candidate for biological control since it readily colonizes twigs and is highly competitive with other components of the mycoflora.