VIEW ARTICLE

Disease Control and Pest Management

Determination and Significance of the Mutation Rate of Colletotrichum coffeanum From Benomyl Sensitivity to Benomyl Tolerance. A. H. Ramos, Senior plant pathologist, National Agricultural Laboratories, Ministry of Agriculture, P.O. Box 14733, Nairobi, Kenya; R. E. Kamidi, biometrician, National Agricultural Laboratories, Ministry of Agriculture, P.O. Box 14733, Nairobi, Kenya. Phytopathology 72:181-185. Accepted for publication 5 May 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-181.

The fluctuation test of Luria and Delbrück applied to spore suspensions of Colletotrichum coffeanum made from sporulating lesions on green coffee berries suffering from coffee berry disease (CBD), showed that conidia tolerant to benomyl arose in a coffee field as a result of preadaptive, spontaneous mutations. The mutation rate from benomyl sensitivity to benomyl tolerance was 2.4 × 10^-6 per spore per division. Selection for tolerant mutants due to repeated spray applications of benzimidazole fungicides was simulated by using a mathematical model constructed by Kable and Jeffery. At good spray coverage, the 29 to 34 theoretical spray applications required for the tolerant subpopulation frequency to increase from 0.000042% (2.4 ×10 ^–6 × 100) to 95%, compared well with published data from several benzimidazole fungicide trials on CBD control. The prospect of delaying the buildup of tolerant mutants to unacceptable levels by using these potent fungicides in mixtures with other fungicides to which no tolerance has developed is discussed.