Sensitivity of Penicillium digitatum and P-italicum to postharvest citrus fungicides in California

Penicillium digitatum isolates (326), collected in California citrus groves and packinghouses, were assayed qualitatively for their sensitivity to ima zalil, 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 s ignificant differences were found in the mean sensitivity of the isolates c ollected 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 ra re. No fungicide-resistant biotypes of either species were collected from c itrus groves. Wild-type I! italicum was slightly less sensitive than wild-t ype P. digitatum to all three fungicides. The concentration of imazalil pro ducing 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.