Negatively Correlated Cross-Resistance to Diphenylamine in Benomyl-ResistantPenicillium expansum

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Abstract
Combinations of diphenylamine (DPA) at 1000-2000 .mu.g/ml and benomyl, thiabendazole or thiophanate-methyl provided better control of benomyl-resistant P. expansum in inoculated apples stored at 2.2.degree.-4.4.degree. 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.degree.-22.degree. C. Comparisons of mycelial growth rates for 3 benomyl-resistant and 4 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 .mu.g/ml for the 3 benomyl-sensitive, 8 moderately benomyl-resistant (EC50 = 14.2 .mu.g benomyl/ml), and 6 highly benomyl-resistant (EC50 > 1000 .mu.g benomyl/ml) 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 1st commercially significant application of negatively correlated-resistance for reducing losses to fungicide-resistant pathogens.

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