Generalist vs. specialist pathogens: Consequences of insect vectoring of fungal pathogens in wild chili peppers

Background/Question/Methods

In natural populations, interactions between plants and fungal pathogens are often mediated by insect activity. Insect foraging creates wounds in plant tissue that can provide an entry point for wind- or water-dispersed pathogens. Insects can also actively transmit pathogens between plants. In this study, we examine the relationship between fungal seed pathogens, the heteropteran insect Acroleucus coaxalis, and their impact on the seeds of the wild chili pepper, Capsicum chacoense. We conducted a reciprocal transplant using two populations of C. chacoense, one from a wet (high pathogen pressure) site and one from a dry (low pathogen pressure) site.  To decouple the potential impact of plant wounding and insect vectoring on pathogen transmission, we subjected fruits to one of two treatments: wounding with a sterile acupuncture needle or wounding with locally caught Acroleucus. We used the fruits from this experiment to address the following questions: 1) is Acroleucus actively vectoring fungal pathogens to seeds? 2) do pathogens transmitted by Acroleucus have a greater impact on seed survival and germination? 3) are pathogens transmitted by Acroleucus more virulent?

Results/Conclusions

To address question 1, we quantified fruit and seed infection in fruits and found that both needle and insect treatments caused fungal infections. However, at our dry site where pathogen pressure is low,  holes created by Acroleucus led to significantly more fungal infection than needle treatments. To address question 2, we conducted a seed germination study and found that fungal pathogens from the insect treatment are more likely to kill seeds than those from the needle treatment. To address fungal virulence (question 3), we used growth rate as a proxy for virulence and conducted fungal growth assays on media that mimicked the nutritional content (and spiciness) of chili peppers. Fungal pathogens isolated from insect treated seeds have faster growth rates than those isolated from the needle treatment. Our results suggest that Acroleucus is indeed an active vector of fungal propagules and is likely moving specialist pathogens that have greater virulence and are more detrimental to plant fitness than generalist pathogens that gain entry into fruits via needle wounds.