Relative Fitness of Imazalil-Resistant and -Sensitive Biotypes of Penicillium digitatum. G. J. Holmes, Former Graduate Research Assistant; University of California, Riverside 92521. J. W. Eckert, Professor, Department of Plant Pathology, University of California, Riverside 92521. Plant Dis. 79:1068-1073. Accepted for publication 13 July 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-1068.

Imazalil-resistant (R) and wild (S) biotypes of Penicillium digitatum collected in California citrus packinghouses and groves were evaluated alone and in R/S combinations in lemons and on culture medium for phenotype stability and relative fitness. R and S phenotypes were stable in the presence or absence of imazalil over several disease cycles in fruit and several spore generations on culture medium. The proportion of R spores in R/S mixtures (1:1) declined more rapidly on fruit than on culture medium. Competitiveness was studied with 11 random combinations of 11 S and 11 R isolates over two disease cycles in nontreated lemons. A significant reduction in percent R spores was observed in eight of 11 mixtures. However, percent R spores increased in three of the mixtures, suggesting that some R biotypes are more competitive than some S biotypes, at least during two disease cycles. In seven of nine possible combinations of three R and three S biotypes, R biotype spore populations declined to almost zero by the end of the second disease cycle in lemons. With the same R/S combinations on culture medium, R biotypes accounted for greater than 15% of the spore populations al the end of the third spore generation. There was no correlation between isolate competitiveness in R/S mixtures and spore production, radial growth in vitro, or latent period in fruit when isolates were evaluated alone.

Keyword(s): Citrus limon, fungicide resistance, green mold, sterol biosynthesis inhibitor