Compatible and incompatible Xanthomonas infections differentially affect herbivore-induced volatile emission by pepper plants

Recent studies have alerted us to the potential for conflicts between pathogen- and herbivore-induced plant defenses. The lack of studies on the induced chemical changes that simultaneous insect and pathogen attacks have on the host plant has become apparent. In the present study, we found that pepper plant volatile profiles can be differentially induced by compatible and incompatible bacterial infection and beet armyworm (BAW) damage when applied alone or in combination upon the same host. We also found that plants under simultaneous compatible bacterial and BAW attack are able to produce volatiles in quantities greater than those produced by healthy plants in response to BAW feeding. In contrast, plants exposed to the incompatible pathogen challenge showed a total volatile release below the level of healthy plants exposed to BAW damage. This suppression of BAW-induced volatiles coincided with increased methyl salicylate production from incompatible bacteria-infected plants. Feeding choice experiments revealed that, when given a choice, BAW larvae fed significantly more on leaves of plants infected with the incompatible bacteria as soon as 2 d after inoculation, while a significant increase in insect feeding on the plants infected with the compatible bacterial strain was not seen until day 4 after inoculation. Additionally, survival for third instars to pupation was significantly higher when feeding on infected plants than on healthy plants, regardless of compatibility. These results are indicative of lowered herbivore defenses due to disease progression on the plants.