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Negatively Correlated Cross-Resistance to Diphenylamine in Benomyl-Resistant Penicillium expansum. D. A. Rosenberger, Plant pathologist, New York State (Geneva) Agricultural Experiment Station, Hudson Valley Laboratory, P.O. Box 727, Highland 12528; F. W. Meyer, research technician, New York State (Geneva) Agricultural Experiment Station, Hudson Valley Laboratory, P.O. Box 727, Highland 12528. Phytopathology 75:74-79. Accepted for publication 11 July 1984. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-74.

Combinations of diphenylamine (DPA) at 1,000- 2,000 μg/ml and benomyl, thiabendazole, or thiophanate-methyl provided better control of benomyl-resistant Penicillium expansum in inoculated apples stored at 2.2- 4.4 C than either DPA or the fungicides used alone. The addition of DPA did not significantly affect control achieved with the fungicides when fruit were stored at 16- 22 C. Comparisons of mycelial growth rates for three benomyl-resistant and four benomyl-sensitive isolates in the presence of varying concentrations of benomyl, DPA, or benomyl plus DPA showed benomyl-resistant isolates were more sensitive to DPA than benomyl-sensitive isolates. Testing of 20 additional, randomly-selected isolates of P. expansum from apple packinghouses showed that DPA concentrations required to reduce mycelial growth by 50% were 24.4, 16.9, and 3.1 μg/ml for the three benomyl-sensitive, eight moderately benomyl-resistant (EC50 = 14.2 μg benomyl per milliliter), and six highly benomyl-resistant (EC50 >100 μg benomyl per milliliter) isolates, respectively. Three of the 20 isolates were resistant to both benomyl and DPA. Control of benomyl-resistant P. expansum by DPA may represent the first commercially significant application of negatively correlated cross-resistance for reducing losses to fungicide-resistant pathogens.

Additional keywords: apple storage scald, ethoxyquin, iprodione, postharvest fungicides.