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Ascospore Tolerance to Dodine in Relation to Orchard Control of Apple Scab. J. D. Gilpatrick, Associate Professor, New York State Agricultural Experiment Station, Cornell University, Geneva 14456; D. R. Blowers, former Temporary Research Assistant, New York State Agricultural Experiment Station, Cornell University, Geneva 14456. Phytopathology 64:649-652. Accepted for publication 20 November 1973. DOI: 10.1094/Phyto-64-649.

Ascospores of Venturia inaequalis from orchards with varying histories of fungicide use and performance were tested for sensitivity to dodine in spore germination tests on glass slides. LD₅₀‘s for inhibition of germination were 0.3 - 0.65 µg/ml of dodine for spores obtained from an experimental orchard in which dodine controlled apple scab satisfactorily. LD₅₀‘s were 0.75 - 1.25 µg/ml for spores from an orchard where dodine gave inadequate scab control in 1967-69. In each test, ascospores from the dodine-tolerant source were 2-3 times less sensitive to dodine than those from the nontolerant source. Ascospores from other orchards where dodine failed to control scab in 1969 were also found to be dodine-tolerant, but spores from wild trees or abandoned orchards with no previous dodine exposure were not tolerant to dodine. Spores from orchards with less exposure to dodine than tolerant sources were semitolerant. These studies support the hypothesis that failure of dodine to control apple scab in many orchards of western New York in 1969 was due to a developed tolerance of V. inaequalis to this fungicide, and that this is related to the intensive and almost exclusive use of dodine in problem orchards in the previous decade.