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Persistence of Benomyl-Tolerant Strains of Cercospora beticola in the Absence of Benomyl. E. G. Ruppel, Research plant pathologist, Agricultural Research, Science and Education Administration, U.S. Department of Agriculture, Crops Research Laboratory, Colorado State University, Fort Collins, CO 80523; A. D. Jenkins(2), and L. M. Burtch(3). (2)(3)Agronomist and chief agronomist, Spreckels Sugar Division, Amstar Corporation, Chandler, AZ 85224 and Mendota, CA 93640. Phytopathology 70:25-26. Accepted for publication 23 July 1979. 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, 1980. DOI: 10.1094/Phyto-70-25.

In 1976 and 1977, 98100% of Cercospora beticola isolates obtained from diseased sugarbeets near Willcox, AZ, growing in benomyl-,triphenyltin-treated, or nonsprayed fields grew in PDA containing 5 μg a.i. benomyl per milliliter. Benomyl-sensitive isolates from Colorado were inhibited completely by 0.1 μg benomyl per milliliter. In 1978, 100% of the isolates from a triphenyltin-sprayed field also were tolerant to 100 μg benomyl per milliliter. The level of tolerance declined considerably between 1976 and 1977. In 1976, all isolates from benomyl-sprayed and nonsprayed fields grew in PDA containing 1,000 μg benomyl per milliliter, whereas only 71% of the isolates from the triphenyltin-sprayed field grew at that concentration. In 1977, only 1, 1, and 0% of the isolates from benomyl-sprayed, nonsprayed, and triphenyltin-sprayed fields, respectively, grew in PDA with 1,000 μg benomyl per milliliter. All of the isolates from 1978 grew in PDA cultures containing benomyl at 10 μg/ml, but none grew in those containing 100 or 1,000 μg/ml. Most Arizona isolates of C. beticola, whether from sprayed or nonsprayed fields, were 1001,000 times more tolerant to benomyl in vitro than were sensitive control isolates from Colorado over the 3-yr study. Thus, benomyl-tolerant strains of C. beticola showed a high degree of persistence in the absence of benomyl, even in fields where triphenyltin was used for leaf spot control.

Additional keywords: Beta vulgaris.