VIEW ARTICLE

Disease Control and Pest Management

Relative Fitness of Benzimidazole- and Cadmium-tolerant Populations of Sclerotinia homoeocarpa in the Absence and Presence of Fungicides. C. G. Warren, Graduate Student and Research Aide, The Pennsylvania State University, University Park, PA 16802; P. L. Sanders(2), H. Cole, Jr.(3), and J. M. Duich(4). (2)(3)(4)Research Assistant, Professor, and Professor, respectively, The Pennsylvania State University, University Park, PA 16802. Phytopathology 67:704-708. Accepted for publication 7 December 1976. Copyright © 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-704.

To evaluate the relative potential for survival of fungicide-tolerant isolates of Sclerotinia homoeocarpa in nature, equal populations of tolerant and sensitive isolates were introduced into a disease-free, geographically isolated turf plot area in the fall of 1973 and the populations were allowed to increase naturally. Dollar spot leaf lesions were sampled from infection centers monthly from the following July through November, and 1,411 isolates obtained subsequently were tested for fungicide tolerance on cadmium succinate- and benomyl-amended potato-dextrose agar. In the absence of fungicides, benomyl-tolerant isolates dropped to a small percentage of the total S. homoeocarpa population, whereas sensitive and cadmium succinate tolerant isolates increased to approximately equal high frequencies. In 1975, the plot again was sampled on a regular basis. In August 1975, when infection centers were very numerous, the area was sprayed with the fungicides, cadmium succinate and benomyl, in an alternating criss-cross pattern to exert a selection pressure on the extant fungus population. In the presence of selection pressure from fungicide, the population was predominantly benomyl-tolerant in areas where benomyl was applied, and predominantly cadmium-tolerant where cadmium was applied. Six weeks after the fungicide applications were stopped, the benomyl-tolerant population rapidly decreased, whereas the cadmium-tolerant population remained high. These results indicate that even though benomyl-tolerant populations were low in the absence of benomyl, the application of benomyl resulted in a rapid resurgence of benomyl-tolerant population. With cadmium-tolerant isolates, the use of cadmium fungicides resulted in an almost complete predominance of the cadmium-tolerant population, which persisted even when cadmium sprays were curtailed.