Microbial interactions influence plant-pathogen interactions in American chestnut blight
Chestnut blight cankers caused by the introduced fungus Cryphonectria parasitica are dynamic microbial communities that influence the fate of diseased American chestnut trees. Hypovirulence via mycovirus infection of the pathogen can cause reduced growth and reproduction of the pathogen and occurs in both Europe and North America. Mycoviruses have been successfully introduced throughout Europe, leading to a general recovery of European chestnut populations. In contrast, there are only a few examples where mycoviruses are prolonging the survival of the highly susceptible American chestnut and most populations are not recovering from blight. Survival of an infected tree is based on cankers that do not expand around the stem to kill everything distal to the infection (i.e. girdling). The probability that a canker will girdle a branch is influenced by tree susceptibility, pathogen virulence, the mycovirus’s ability to reduce pathogen virulence and potentially other antagonistic microbes inhabiting the canker. We sampled girdling and non-girdling cankers at American chestnut stands in Michigan and Wisconsin to determine if non-girdling cankers were inhabited by microbes that differ from those found in girdling cankers. We hypothesize that specific combinations of mycoviruses AND microbial antagonists within cankers may act synergistically to slow canker expansion and ultimately reduce the probability of girdling. To test this hypothesis, we first conducted a dual-culture plating assay in vitro to screen for potential fungal antagonists of the pathogen, then created cankers using a strain of C. parasitica to test different treatment combinations of a mycovirus and microbes isolated from the extant cankers to determine their ability to inhibit canker expansion.
Based on results of the dual-culture plating assay, Trichoderma isolates were able to outcompete C. parasitica for resources in culture. Other fungi tested (e.g. Umbelopsis, Penicillium) were slow growing compared to Trichoderma isolates and did not significantly inhibit the pathogen. Experimentally induced cankers on American chestnut stems were treated with the GH2 mycovirus and/or one of three Trichoderma isolates collected from extant cankers. Canker expansion was monitored throughout the summer and fall of 2013. Cankers treated with GH2 along the canker margin had slower canker expansion rates than cankers treated with GH2 within the interior of a canker. Cankers treated with Trichoderma within the inner portion of a canker did not reduce canker expansion. Treating cankers with both GH2 along the canker margin and Trichoderma within the interior did not further reduce canker expansion rates below that found with GH2 alone.