Role of citrus volatiles in host recognition, germination and growth of Penicillium digitatum and Penicillium italicum

Volatiles emitted from wounded peel tissue of various citrus cultivars had a pronounced stimulatory effect on germination and germ tube elongation of both Penicillium digitatum and P. italicum; however, P. digitatum appeared to be more sensitive to the stimulatory action of citrus peel volatiles. When exposed to volatiles from grapefruit peel discs, the percentage of germinated spores of P digitatum and P. italicum was 75.1% and 37.5%, respectively, whereas germination of controls was 6.8% and 14.7%, respectively. In contrast, Botrytis cinerea and P expansum were either not affected or inhibited by the peel volatiles. GS-MS analysis of volatiles present in the peel of various citrus fruit cultivars revealed that limonene is the major fruit peel volatile. Its percentage ranged from 89% to 95% at the early stages of fruit development throughout the harvest season. Myrcene and alpha-pinene made up the second and third greatest amounts among the volatiles found in these oils, ranging from 2.12% to 2.33% and from 0.71% to 1.25%, respectively. All four monoterpenes, limonene, alpha-pinene, beta-pinene and myrcene were stimulatory to R digitatum and P. italicum but inhibitory to or had no effect on P. expansum and B. cinerea. Germ tube elongation in P digitatum responded most strongly to limonene and less strongly to alpha-pinene and beta-pinene while myrcene had little effect. In contrast in R italicum, myrcene stimulated germ tube elongation the most followed by limonene, with alpha-pinene, and beta-pinene being about equal. Germination of P. italicum conidia was highest in response to myrecene with the effect of the other compounds being about equal at concentrations of 5 mu L or more per plate. (C) 2008 Elsevier B.V. All rights reserved.