Spatial and temporal dynamics of the fungal community of chestnut blight cankers on American chestnut (Castanea dentata) in Michigan and Wisconsin

Abstract Chestnut blight cankers caused by the fungal pathogen Cryphonectria parasitica on infected American chestnut trees can be invaded by hypoviruses that infect C. parasitica mycelia. Hypoviruses lower pathogen virulence (i.e. hypovirulence) and decrease the probability of stem girdling. Cankers also are susceptible to invasion by non-C. parasitica fungi; however, the influence of invading fungi on disease severity is unknown. Fungi may antagonize C. parasitica growth and combine with hypovirulence to further reduce the probability of stem girdling. We conducted a survey to investigate how spatial and temporal dynamics of the fungal community within cankers correlate with canker severity and the likelihood of girdling. Cankers from six American chestnut populations were monitored from 2012 to 2016. We found that the spatial distribution of the fungal community within a canker resembled a mosaic that increased in fungal diversity over time. Cankers with increased fungal diversity were not associated with higher stem survivorship over time, compared to cankers with abundant hypovirulence in recovering chestnut populations. Fungal communities also were unstable, and cankers on surviving trees containing hypovirulence consistently were invaded by non-C. parasitica fungi. Our results suggest that canker communities are not stable, and volatility of the fungal community indicates that cankers can change quickly from less severe to more severe cankers through loss of hypovirulence. Non-C. parasitica fungi may facilitate canker expansion through greater inhibition of hypovirulent C. parasitica relative to virulent C. parasitica, which may permit virulent C. parasitica to escape hypovirus infection and resume rapid canker expansion.