Do secondary fungal invaders influence disease severity of chestnut blight on American chestnut?

Background/Question/Methods

In the 1900s, the invasive fungal pathogen, Cryphonectria parasitica, nearly eliminated American chestnut (Castanea dentata) from forests in eastern North America. The pathogen causes a canker that can girdle and kill stems and branches of infected trees. Cryphonectria parasitica also invaded Europe, but disease symptoms were less severe over time and cankers often failed to girdle stems. The reduction in virulence was traced to mycoviruses that invaded the pathogen, which led to recovery of European chestnuts (C. sativa) from blight. Mycovirus-mediated hypovirulence has occurred rarely in North America despite attempts to utilize it as a biocontrol. Mycoviruses have been introduced into a C. parasitica population at West Salem, Wisconsin for over 20 years and some trees are recovering due to reduced pathogen virulence. We noted a temporal pattern where non-pathogenic fungi (secondary invaders) have increased over time in non-girdling cankers, suggesting that secondary invaders may contribute to reducing the impact of infections. To test this hypothesis, cankers were sampled at three locations: a mycovirus-introduced site in Wisconsin, and two sites in Michigan (Manistee and Roscommon) where mycoviruses occurred naturally. Frequencies of C. parasitica, mycovirus-infected C. parasitica, and secondary invaders were determined for all cankers, based on twelve isolates per canker.

Results/Conclusions

The pattern of secondary invaders in cankers differed among populations. At West Salem, Wisconsin, secondary invaders were more commonly obtained from non-girdling, (25.0% of samples) versus girdling (9.8%) cankers, and nearly 60% of the secondary invaders recovered were Trichoderma spp. The frequency of secondary invaders (26.7%) was similar at the Manistee County site, but girdling and non-girdling cankers had similar levels of secondary invaders, 26.9% and 26.5%, respectively. Trichoderma comprised only 4.8% of the secondary invaders found at Manistee. Sixty-eight percent of the secondary invaders at Manistee remain to be identified. At Roscommon, the second site with naturally occurring mycoviruses, secondary invaders were rare, comprising only 8.3% of all canker samples. While Trichoderma was found at Roscommon, it comprised only 16.7% of the secondary fungi isolated.  These data suggest that the potential role of secondary invaders may differ among chestnut populations. Currently, we are characterizing the secondary invaders using ITS sequencing. Preliminary competition experiments also are being conducted in vitro to compare competitive ability of secondary invaders against C. parasitica in the presence and absence of mycoviruses.