Fungi Associated with Hypovirulent Cankers of Differing Ages on American Chestnut

A research project to introduce hypoviruses and monitor their spread was initiated 20 years ago in an American chestnut (Castanea dentata) stand growing 900 km west of the natural range of the species in western Wisconsin (USA). During this period, CHV1 and CHV3 hypoviruses were deployed by treating cankers with the resident Cryphonectria parasitica strains that were hypovirus infected. Hypovirus spread has been assessed annually by removing and culturing small bark plugs from cankers and evaluating whether the resulting colonies were morphologically virulent or hypovirulent. A variety of fungi other than C. parasitica have been recovered from cankers during this study. In order to better understand the role of recovered fungi and their relationship over time, fungal isolates recovered in 2012 from 500 cankers were categorized by the age of the canker (based on date of discovery), appearance/morphology and location within the stand. Cankers were divided into four age categories: (1) one-year-old cankers; (2) two-to-four years-old; (3) five-to-ten years-old; and (4) eleven-to-twenty years-old. Virulent C. parasitica isolates were most frequently recovered from one-year old cankers, but recovery decreased from a high of 60% in one-year-old cankers to 43, 30 and 19%, respectively, for the three increasingly older categories. In contrast, the recovery of hypovirulent C. parasitica isolates increased from 17% in one-year-old cankers to 35% in cankers representing the oldest canker category. Isolation of Trichoderma atroviride and T. aureoviride increased as cankers aged and accounted for 42% of all the colonies recovered from the oldest cankers. Most species of Trichoderma are aggressive saprophytes; some are proven biocontrol agents. Other fungi that were isolated included Botryosphaeria sp., Didymostilbe sp., Dothiorella sp., Epicoccum nigrum, Paraconiothyrium sp., Pestalotia sp., Penicillium spinulosum, Penicillium glabrum and Umbellopsis isabellina but these represented only 4-7% of isolates among all age categories. A decrease in the recovery of virulent C. parasitica strains and a corresponding increase in recovery of hypovirulent C. parasitica strains would be expected if biological control is active. The frequency with which Trichoderma spp. have been recovered may implicate this organism as a contributor to chestnut blight biological control.