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Competition Between Benomyl-Resistant and Sensitive Strains of Venturia inaequalis on Apple Seedlings. D. C. McGee, Associate professor, Department of Botany and Plant Pathology, University of Maine, Orono 04475, Present address of senior author: Department of Plant Pathology, Seed and Weed Sciences, Iowa State University, Ames 50011; M. G. Zuck, assistant scientist, Department of Botany and Plant Pathology, University of Maine, Orono 04475. Phytopathology 71:529-532. Accepted for publication 28 October 1980. Copyright 1981 The American Phytopathological Society. DOI: 10.1094/Phyto-71-529.

The fitness of benomyl-resistant and sensitive populations of Venturia inaequalis was compared in the absence of benomyl selection pressure by passing mixed (1:1 in initial spore suspension inoculum) populations through several sporulation cycles on apple seedlings under greenhouse conditions. After eight sporulation cycles there was no major change in the original 1:1 ratio of resistant:sensitive components of a population initially composed of 11 isolates of each type taken from the same orchard. Similar results were obtained following four sporulation cycles after inoculation with a 1:1 mixture of spores of 16 sensitive and 16 resistant isolates obtained from different orchards. Mixtures of the same resistant and sensitive populations, at original ratios of 3:1, 1:1 or 1:3, exhibited similar trends during three sporulation cycles. Differences in the fitness of three separate resistant populations were observed when they were separately mixed at a 1:1 ratio with the same sensitive population; in one of the mixtures, a significantly greater decline in the resistant component was observed during five sporulation cycles as compared to the other two. An increase in the benomyl-resistant component of a 1:9 (resistant:sensitive) population occurred over five cycles when it was exposed to sublethal doses of benomyl, benomyl mixed with captan, or benomyl alternated with captan. The original ratio did not change significantly, however, when the population was exposed to captan alone or was not exposed to a fungicide.

Additional keywords: fungicide tolerance.