Determination of hypovirulent and virulent isolates of Cryphonectria parasitica collected from the Great Smoky Mountains National Park, and their Vegetative Compatibility

Baird, Richard

The American chestnut was once a dominant overstory species throughout the southern Appalachian Mountains. By 1950, a pandemic outbreak of the chestnut blight fungus (Cryphonectria parasitica) had devastated this once important tree species throughout its native range, although a small number of mature trees persist. Researcher is looking for hypovirulent strains of C. parasitica, possibly caused by a viral infection, with the possibility that these hypovirulent strains could be used as an inoculation for young chestnuts that would protect them from the worst effects of the blight. During 2006, 66 live chestnut trees with cankers were visited in the Smokies and 63 isolates from these trees showed signs of being hypovirulent and are being tested to see if they are infected with the virus. During the summer of 2007, the locations of 50 American chestnut trees were visited in the GSMNP. A total of 14 trees were live and contained one or more cankers caused by C. parasitica. From these live trees, 15 cankers were sampled, and 39 isolates of C. parasitica have been obtained. Of 339 isolates (including those from Nantahala NF), 54 had abnormal cultural morphologies and 3 contained dsRNA. Analysis of vegetative compatibility (VC) divided all isolates into 34 groups, 16 of which only contained one isolate. A total of 19 isolates and 3 controls were inoculated onto healthy American chestnut trees in the Nantahala National Forest and data on canker growth and stromata production were obtained over six months. Results from the field trial indicated that five isolates were potentially hypovirulent. Based on those data, isolate, 236-1C, has the greatest potential for use as a biological control agent for the pathogen in the GRSM, but compatibility is limited to select VC groups. Additional hypovirulent isolates representative of the other VC groups must be identified before large scale biocontrol can succeed.