Persistence of Benomyl-Tolerant Strains ofCercospora beticolain the Absence of Benomyl

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Abstract
In 1976 and 1977, 98-100% of C. beticola isolates obtained from diseased sugar beets near Willcox, Arizona [USA] growing in benomyl-,triphenyltin-treated, or nonsprayed fields grew in PDA [potato dextrose agar] containing 5 .mu.g a.i. [active ingredient] benomyl/ml. Benomyl-sensitive isolates from Colorado were inhibited completely by 0.1 .mu.g benomyl/ml. In 1978, 100% of the isolates from a triphenyltin-sprayed field also were tolerant to 100 .mu.g benomyl/ml. The level of tolerance declined considerably between 1976 and 1977. In 1976, all isolates from benomyl-sprayed and nonsprayed fields grew in PDA containing 1000 .mu.g benomyl/ml, 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 1000 .mu.g benomyl/ml. All of the isolates from 1978 grew in PDA cultures containing benomyl at 10 .mu.g/ml, but non grew in those containing 100 or 1000 .mu.g/ml. Most Arizona isolates of C. beticola, whether from sprayed or nonsprayed fields, were 100-1000 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.

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