Diversity and distribution of secondary fungi inhabiting chestnut blight cankers
Communities of microorganisms are ubiquitous and represent most of the biomass, individuals, and species in every ecosystem, yet their influence on the ecology and evolution of the flora with which they exist is only now becoming clear. Every terrestrial plant supports a diverse array of microorganisms living within tissues without causing apparent harm, but some have the potential to antagonize plant pathogens and limit their effectiveness at causing disease. We have observed that chestnut blight cankers, caused by Cryphonectria parasitica on American chestnut trees (Castanea dentata), are complex microbial communities that support a diversity of fungi (non-CP) that may influence fitness of infected chestnut and by extension host population structure. The interactions between Cr. parasitica and other fungi within cankers may be further complicated when the pathogen is infected with mycoviruses that can reduce growth and reproduction of the pathogen. We hypothesize that the composition and distribution of both fungi and mycoviruses within a canker may determine the fate of an infected tree, and that mycovirus infection at the edge of cankers slows the pathogen advancing through host tissue and allows non-CP to invade the interior of cankers and antagonize the pathogen. Thus, the interaction between Cr. parasitica, mycoviruses, and non-CP influence the probability that an infected chestnut branch is girdled by the pathogen. Bark plugs were collected along the margins and throughout the inner area of cankers within recovering and non-recovering chestnut populations to identify non-CP using cultural characteristics and ITS sequences.
The frequency and distribution of Cr. parasitica (CP), mycoviruses (HCP), and non-CP within cankers was variable at two recovering chestnut populations (County Line [Manistee Co., Michigan] and West Salem, Wisconsin). At West Salem, 38% of fungi isolated from non-girdling cankers were non-CP compared to 18% from girdling cankers. There were no differences in the ratio of CP, HCP, and non-CP between non-girdling and girdling cankers at County Line, and non-CP was isolated from all areas of the canker. HCP was more commonly isolated from the inner area (52%) than at the margin (40%) of non-girdling cankers. We expected high frequencies of HCP at the margin of non-girdling cankers because HCP is known to slow canker expansion and decrease the probability that an infected chestnut stem will be girdled. Trichoderma spp., well-documented biological control agents, were commonly found within cankers and warrant investigation into their inhibition of CP in vitro and in planta.