Effects of Three Fungicides on Mycelial Growth, Sclerotium Production, and Development of Fungicide-Tolerant Isolates of Sclerotinia minor. D. M. Porter, Supervisory Plant Pathologist, Agricultural Research Service, U.S. Department of Agriculture, Tidewater Research Center, Suffolk, VA 23437. P. M. Phipps, Associate Professor of Plant Pathology, Virginia Polytechnic Institute and State University, Tidewater Research Center, Suffolk 23437. Plant Dis. 69:143-146. Accepted for publication 30 July 1984. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1985. DOI: 10.1094/PD-69-143.

Mycelial growth of Sclerotinia minor was reduced in vitro on media amended with benomyl and dicloran but was much less evident on media amended with procymidone. Very little mycelium was observed on media amended with procymidone at 0.5 μg/ml and none was observed at 1 and 5 μg/ml after incubation for 96 hr. Tolerance to procymidone developed in in vitro bioassays at a frequency of 2.3% on media amended with this fungicide. Development of tolerance to benomyl and dicloran was not observed. Growth of tolerant S. minor isolates on media amended with procymidone at 100 μg/ml was similar to that on unamended media. Mycelial growth rates for tolerant isolates were similar to those of nontolerant isolates at 15, 18, 21, 27, and 30 C. Growth of tolerant isolates on soil plates sprayed with procymidone was similar to growth on untreated plates. Sclerotia produced by procymidone-tolerant isolates were frequently larger and fewer than those produced by isolates on unamended media. Tolerance to procymidone persisted after 10 weekly hyphal-tip transfers on unamended media. Procymidone-tolerant isolates of S. minor were not found in field plots treated with this fungicide.