Double-stranded RNA (dsRNA) mycovirus hyperparasites can infect the chestnut blight pathogen, Cryphonectria parasitica, causing a reduction in virulence and inhibition of sexual reproduction in C. parasitica, which can lead to recovery of the blight pathogen's host, American chestnut (Castanea dentata). Studies to determine the level at which mycoviruses can influence population-level and genomic level diversity of C. parasitica have previously not been done. The main objectives of this dissertation are: 1) determine how mycovirus hyperparasites can alter vegetative compatibility group (VCG) diversity over long time periods, 2) evaluate the affect of mycoviruses on genome level diversity in C. parasitica, 3) test a hypothesis that mycovirus biological control agents would be most successful on American chestnut trees in the 1--10cm diameter at breast height (DBH) size class. VCG studies suggest that the presence of mycoviruses influence C. parasitica population structure in Michigan. In C. parasitica populations where mycoviruses are present VCG diversity is low and VCGs generally unique to individual sites and where mycoviruses are absent VCG diversity is higher and VCGs are shared across these sexually reproducing populations. Similarly, genome-wide diversity in C. parasitica also appears to be structured according to mycovirus presence and absence. Population genetic differentiation values (Φ PT) are higher for mycovirus-infected C. parasitica populations while lower values are found at mycovirus-free sites. The pattern of differentiation does not suggest any isolation by distance. The overall pattern suggests significant migration among epidemic populations of C. parasitica where mycoviruses are absent, while pathogen populations with mycoviruses are largely isolated. Thus, C. parasitica in Michigan is composed of a patchwork of sites whose dynamics appears to be governed by the presence/absence of mycoviruses. Preliminary results of a long-term experiment using mycoviruses as biological control mechanism in C. parasitica is showing promising results. Trees in the 1--10cm DBH size class were tested for response to mycovirus infected strains of C. parasitica. Annual survivorship was 73.6% across three tree populations. Persistence of mycovirus in treated cankers however, was only around 50% from year to year suggesting that repeated introductions of mycoviruses as biological control agents may be necessary to achieve success. Further, local environmental conditions, tree genotype, and amount of competition from large overstory trees may play a role in American chestnut recovery. This study investigates both phenotypic and genome diversity in C. parasitica relative to the presence and absence of naturally occurring mycovirus hyperparasites that are useful for biological control of the chestnut blight pathogen.
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