An invasive fungal pathogen, white pine blister rust (Cronartium ribicola) infects and kills whitebark pine (Pinus albicaulis) throughout the Northwest USA. Blister rust has decreased whitebark pine populations by over 90% in some areas. Whitebark pine, a keystone species, has been proposed for listing under the Endangered Species Act in the U.S., and the loss of this conifer is predicted to have severe impacts on forest composition. Hundreds of asymptomatic fungi live inside whitebark pine tissue, and recent studies suggest that these fungi can influence the frequency and severity of pathogens such as white pine blister rust.
We used molecular methods to characterize the foliar fungal community in whitebark pine seedlings from 21 half-sibling seed families, sourced throughout the Pacific Northwest, in a common garden. We investigated endophyte communities before and after experimental inoculation with blister rust and compared community composition in susceptible vs. resistant seedlings to determine endophytes most likely to influence disease severity. We also explored the defensive chemistry in needle tissue of these same seedlings in relation to the fungal community and overall disease severity. Specifically, we addressed the following questions: (1) Does infection by the white pine blister rust pathogen influence the foliar fungal endophyte community of whitebark pine? (2) Do endophyte communities vary between resistant and susceptible phenotypes? (3) How does tree defensive chemistry relate to foliar endophyte community composition and disease severity in whitebark pine?
Seed family was the biggest driver of endophyte community composition in our common garden, but we also saw shifts in fungal communities in response to blister rust infection. Seed family identity also influenced defensive chemistry, with terpenes showing varying concentrations in resistant and susceptible seedlings. In addition, both defensive chemistry and individual endophytes correlated with characteristics of disease severity. Endophyte communities and defensive chemistry in whitebark pine that can predict disease severity may act as biomarkers of disease resistance for future breeding programs. These results suggest that the resistance seen in natural whitebark pine populations may be a combination of genetics, endophytes and the terpenes composition in needle tissue, where initial interactions between endophytes, the host and blister rust take place.