VIEW ARTICLE

Postharvest Pathology and Mycotoxins

Growth, Sporulation, and Virulence of Isolates of Penicillium digitatum Resistant to the Fungicide sec-Butylamine. J. L. Smilanick, Graduate research assistant, Department of Plant Pathology, University of California, Riverside 92521, Present address: Horticultural Crops Research Laboratory, USDA-ARS, 2021 South Peach Avenue, Fresno, CA 93727; J. W. Eckert, professor of plant pathology, Department of Plant Pathology, University of California, Riverside 92521. Phytopathology 76:805-808. Accepted for publication 6 March 1986. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/Phyto-76-805.

Isolates of Penicillium digitatum resistant to the fungicide sec-butylamine (SBA) were obtained by ultraviolet irradiation or collection from natural resistant populations in citrus packinghouses. Resistant isolates were not controlled on inoculated lemons by practical SBA fruit treatments. The EC₅₀ in liquid culture was 1-2 μmol/ml and 10-40 μmol/ml SBA for sensitive and resistant isolates, respectively. The growth rates, sporulation, and virulence of resistant and sensitive isolates were similar both in culture and in untreated lemons. The relative fitness of the isolates was tested by inoculating fungicide-free lemons with a 1:1 mixture of spores of two isolates and measuring the frequency of SBA-resistance in the spores collected from the diseased fruit. In 27 pairs of resistant and sensitive isolates, followed for four spore generations, the proportion of the resistant spores increased in 19, decreased in five, and did not change in three pairs. These results suggest that the acquisition of SBA resistance is not usually accompanied by a decrease in the vigor or parasitic fitness of P. digitatum.