- The development of fungicide resistance in plant pathogenic fungi is an evolutionary process that consists of two steps. First, a mutation must occur that allows one or more individuals in the population to survive and reproduce in the presence of the fungicide. Second, there must be selection by the fungicide for this resistant subpopulation. One proposed method for decreasing the rate of selection is to apply a high risk fungicide in combination with a low risk fungicide. Three experiments were designed to examine (1) the effect of fungicide mixtures on selection and control, (2) the effect of the initial proportion of resistance on selection, and (3) differences in fitness between resistant and sensitive strains. In the first experiment, a block of 72 apple trees were inoculated with benomyl-resistant and sensitive strains of Venturia inaequalis. Fungicide mixtures containing different concentrations of benomyl and mancozeb were then applied. Selection for the resistant population and the total amount of disease were minimized with an increase in the mancozeb concentration in the mixture. In the second experiment, 100 dwarf apple trees planted in four blocks of 25 trees each. Each block was inoculated with a conidial suspension having a different initial proportion of benomyl-resistant conidia: 0.001, 0.01, 0.1, and 0.5. Sprays of a benomyl and mancozeb mixture, each at one-half the recommended rate, were applied to all blocks. The rate of selection for a greater proportion of resistance in the population was most rapid when neither subpopulation dominated; the rate decreased as either subpopulation became dominant. In the final experiment, sensitive and resistant strains were isolated from two commercial orchards and inoculated onto 'McIntosh' trees in the greenhouse. Comparison of the means of the sensitive and resistant isolates from the Schultz orchard indicated they had similar latent periods, sporulation capacities, colonization abilities, and infection efficiencies. However, the resistant isolates from the Barnard orchard were less fit than the sensitive isolates for these same four fitness parameters. The subpopulation diversities were less consistent. For some parameters the sensitive subpopulation had a greater diversity, while for other parameters the converse was true.
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- Ph.D. The Pennsylvania State University 1985.
- Source: Dissertation Abstracts International, Volume: 46-06, Section: B, page: 1771.
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