RELATIVE FITNESS OF IMAZALIL-RESISTANT AND IMAZALIL-SENSITIVE BIOTYPES OF PENICILLIUM-DIGITATUM

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 at 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.