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Tolerance of Mycosphaerella citri to Benomyl in Florida Citrus Groves. J. O. Whiteside, Plant Pathologist, University of Florida, Agricultural Research and Education Center, Lake Alfred, FL 33850. Plant Dis. 64:300-302. Accepted for publication 26 November 1979. Copyright 1980 American Phytopathological Society. DOI: 10.1094/PD-64-300.

Benomyl-tolerant isolates of the citrus greasy spot fungus (Mycosphaerella citri) were defined as those showing little or no reduction in growth on agar media containing 100 µg/ml of benomyl. Sensitive isolates were completely inhibited by 1 µg/ml of benomyl. Postinoculation foliar sprays of benomyl at 150 µg/ml reduced greasy spot severity only on leaves inoculated with sensitive isolates. Tolerance in the population of M. citri in citrus groves was estimated by ejecting ascospores from perithecia-bearing fallen citrus leaves onto water agar containing 100 µg/ml of benomyl and observing the ascospores for distorted or normal germ tubes. Eleven of 12 groves never treated with benomyl showed no tolerance among the 13 × 104 ascospores examined from each sample; in the remaining grove, about 0.01% of ascospores were tolerant. In 21 of 37 treated groves sampled, more than 0.1% of the ascospores were benomyl-tolerant, but only where more than two sprays had been applied. Thirteen groves had received one benomyl treatment per year for 5 yr; tolerant ascospores exceeded 1% in 11, ranging between 10 and 20% in four and exceeding 80% in three.