Many of the most ecologically and economically important diseases of plants are vectored by insects, and the spread of these diseases ultimately depends on complex interactions among plants, pathogens, and insect vectors.  Yet, empirical evidence regarding the mechanisms by which plant-insect interactions mediate pathogen spread is sparse.  Induced changes in the emission of plant volatiles—airborne chemicals released by plants that serve as key foraging cues for insects—is well-documented in response to herbivore feeding, but less so for pathogen infection.  Here, we investigate changes in volatile emissions from leaves and flowers of the wild gourd Cucurbita pepo ssp. texana induced by the bacterial wilt pathogen Erwinia tracheiphila and subsequent effects on the on the behavior of striped cucumber beetle (Acalymma vittatum), which vector the pathogen.  Floral and leaf volatile collections from healthy and wilt-infected gourd plants were conducted both in a controlled greenhouse setting, where plants were hand-inoculated, and under more natural field conditions at Russell E. Larson Experimental Station in Central PA, where plants were insect-inoculated through naturally occurring herbivory.  Lab and field preference assays were employed to compare the relative attraction of striped cucumber beetles to male flowers and leaves of healthy and pathogen-infected plants. 

Results/Conclusions

Our results indicate that wilting leaves on Erwinia-infected gourd plants exhibit elevated volatile emissions, including the release of unique compounds that are absent from the volatile profiles of mock-inoculated plants.  Furthermore, field volatile collections indicate that only symptomatic leaves on infected plants release these pathogen-induced volatiles—the volatile profiles of asymptomatic leaves on infected plants are indistinguishable from those of leaves on healthy plants.  In contrast, greenhouse and field collections show that male flowers on wilt-infected plants exhibit reduced volatile emissions, releasing lower amounts of a volatile blend otherwise similar to that of male flowers on healthy plants.  Furthermore, laboratory behavioral assays and field sampling indicate that beetle vectors are preferentially attracted to wilting leaves (compared to healthy leaves) and to flowers on healthy plants (compared to those on wilting plants). Taken together, our results suggest a pattern of pathogen-induced effects consistent with disease transmission, as cucumber beetles are known to aggregate/feed on wilting leaves, where they presumably pick up the pathogen, and previous work from our group indicates that the pathogen can be introduced through floral nectarines.