Submitted to: Postharvest Biology and Technology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/30/2007
Publication Date: 4/1/2008
Citation: Droby, S., Eick, A., Macarisin, D., Cohen, L., Rafael, G., Stange Jr, R.R., Mccollum, T.G., Dudai, N., Nasser, A., Wisniewski, M.E., Shapira, R. 2008. The role of citrus volatiles in germination and growth of Penicillium digitatum and Penicillium italicum. Postharvest Biology and Technology. 49:386-396. Interpretive Summary: Some fruits are highly susceptible to postharvest rots and appear to have very little resistance to specific fungal pathogens. The application of chemical fungicides is required to prevent the economic losses that would occur as a result of these postharvest infections; however, consumers prefer a reduction in the use of chemical pesticides. In order to develop new approaches to disease control, a better understanding of the interactions between the host fruit and the fungal pathogen are required. The present study examined the effect of volatile compounds emitted from citrus fruit on germination of citrus-fruit pathogens and non-pathogens. Results indicated that the main volatile emitted from grapefruit was limonene, representing up to 95% of the detected volatiles. The second, most abundant compound was myrcene. Both compounds had a significant, stimulatory effect on the germination of pathogen spores but had either no effect or an inhibitory effect on spores of non-pathogens. Limonene was most stimulatory to the pathogen, Penicillium digitatum, while myrcene had the greatest stimulatory effect on the germination of spores of the pathogen, Penicillium italicum. This information provides additional information that can be used to better understand how postharvest pathogens adapt to specific fruit species and to better understand the underlying mechanisms of innate resistance and host susceptibility.
Technical Abstract: Volatiles emitted from wounded peel tissue of various citrus cultivars had a pronounced stimulatory effect on germination and germ tube elongation of both P. 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, B. 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 – 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 P. 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 P. italicum, myrcene stimulated germ tube elongation the most followed by limonene, with alpha-pinene, and beta-pinene being about equal. Germination of P. italicum condia was highest in response to myrecene with the effect of the other compounds being about equal at concentrations of 5 ul or more per plate.