Chemical ecology of a pollinator-vectored plant pathogen

Background/Question/Methods

At least 26 plant pathogens are transmitted at flowers by pollinators.  Despite the ecological and economic importance of pollinator-vectored plant pathogens, we know surprisingly little regarding how floral traits influence disease transmission.  Furthermore, there is an almost complete lack of knowledge regarding plant chemical traits that mediate the attraction of vectors and disease establishment in hosts.  This lack of information is surprising given the importance of volatile chemistry in attracting pollinators to flowers, as well as non-volatile chemistry influencing pathogen establishment in vegetative plant tissues.  Here, we investigate how volatile and non-volatile chemistry of blueberry (Vaccinium corymbosum) influences patterns of host plant resistance and transmission of mummy berry disease (Monilinia vaccinii-corymbosi), an economically important pollinator-vectored fungal pathogen.  We focus on phenolic oxidation as a non-volatile chemical resistance mechanism due its importance in defense against similar necrotrophic fungal pathogens and the abundance of phenolics in blueberry tissues.  In addition, we examine how volatile chemistry of infected leaves mimic floral scent.  Because pollinators acquire conidia (the infectious phase of the pathogen) at infected leaves and vector this phase to flowers, attraction to infected leaves is an integral component of disease transmission.

Results/Conclusions

Across fourteen blueberry cultivars, we found no relationship between plant phenology or growth rate and susceptibility to mummy berry disease.  However, we found a negative relationship between condensed tannin oxidation and susceptibility to mummyberry disease.  Phenolic oxidation may mediate resistance through either the production of harmful hydroxyl radicals or covalent complexation with fungal proteins.  Regarding volatile chemistry, we found evidence of floral mimicry in the volatiles produced by leaves infected with mummy berry disease.  Three volatiles unique to blueberry flowers were produced by infected leaves, and two unique fungal odors (octenone isomers) were produced by infected leaves.  The three ‘floral’ volatiles produced by infected leaves are known to be attractive to blueberry pollinators such as bumble bees, and octenones are known to be attractive to insects in other fungal systems.  Overall, our results suggest that blueberry possesses phenolic-based chemical defenses against mummy berry disease, but is also chemically manipulated by mummy berry in order increase disease transmission.