Effects of Mixtures of Benomyl and Mancozeb on Buildup of Benomyl-ResistantVenturia inaequalis

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Abstract
In 1983 and 1984, a block of 72 Rome Beauty apple trees [malus domestica] was inoculated with benomyl-sensitive and benomyl-resistant strains of Venturia inaequalis. Fungicide mixtures consisting of different concentrations of benomyl and mancozeb then were applied. In both years, populations of the resistant strains increased rapidly as the concentration of benomyl in the mixture was increased. An increase in the mancozeb concentration tended to delay the buildup of benomyl-resistant strains. In 1983, 98.7% of the initial population was sensitive and the mixture containing the highest benomyl rate (225 mg/L) provided adequate control. In 1984, only 69% of the initial population was sensitive and none of the mixtures gave adequate control. In a second experiment, sprays of benomyl plus mancozeb, each at one-half the recommended rate, were applied to four blocks of 25 trees, each with initial proportions of resistant lesions of 0.006, 0.037, 0.313, and 0,803. The rate of increase in the proportion of resistant lesions was most rapid when neither the sensitive nor the resistant subpopulation dominated in the early stages of selection. The rate was lower when the initial population was composed of mostly sensitive or resistant strains.