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Sensitivity of Penicillium digitatum and P. italicum to Postharvest Citrus Fungicides in California

September 1999 , Volume 89 , Number  9
Pages  716 - 721

Gerald J. Holmes and Joseph W. Eckert

First author: Department of Plant Pathology, North Carolina State University, Raleigh 27695; and second author: Department of Plant Pathology, University of California, Riverside 92521

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Accepted for publication 24 May 1999.

Penicillium digitatum isolates (326), collected in California citrus groves and packinghouses, were assayed qualitatively for their sensitivity to imazalil, thiabendazole, and o-phenylphenol. Eighteen typical triple-resistant isolates, acquired in each of 3 years (1988, 1990, and 1994), were assayed quantitatively for their sensitivity to each of the three fungicides. No significant differences were found in the mean sensitivity of the isolates collected in different years. However, the proportion of isolates that were resistant to all three fungicides increased from 43% in 1988 to 77% in 1990 and 74% in 1994. Imazalil-resistant biotypes of P. digitatum were isolated frequently in California packinghouses, while resistant P. italicum was rare. No fungicide-resistant biotypes of either species were collected from citrus groves. Wild-type P. italicum was slightly less sensitive than wild-type P. digitatum to all three fungicides. The concentration of imazalil producing 50% growth inhibition (EC50) was three times greater when the age of the P. digitatum assay inoculum was increased from 12 to 24 h. Activity of imazalil increased with pH of the assay medium in the range pH 5.1 to 5.9, reflecting the greater concentration of dissociated imazalil at the higher pH value.

Additional keyword: demethylation inhibitor, fungicide resistance, sterol inhibitor.

© 1999 The American Phytopathological Society