Chestnut Blight

Cryphonectria parasitica is an ascomycete fungus native to East Asia, introduced to North America in the early 1900s through imported nursery stock. By mid-century it had killed an estimated 3 to 4 billion American chestnut trees (Castanea dentata), effectively eliminating the species as a canopy tree across its native range. The pathogen spreads via airborne conidia and sexual ascospores, entering trees through wounds, bark cracks, or insect galleries. Once inside, it colonizes the cambium and produces oxalic acid, which lowers local pH and kills the surrounding tissue.

Infection progresses as a sunken, orange-brown canker that girdles branches or the main trunk. Wilting foliage above the canker is typically the first visible symptom. During wet weather, orange pustules form on the canker surface as the fungus produces pycnidia and perithecia. Trees rarely die outright; root systems often survive and send up new sprouts, but those sprouts succumb again once they reach canker-susceptible diameter. Warm, humid conditions favor rapid spore dispersal and infection, though the fungus is present wherever susceptible hosts grow throughout the eastern and central United States.

For home growers, the most cost-effective strategy is variety selection from the start. Chinese chestnut (Castanea mollissima) and blight-tolerant hybrids including Dunstan and Sleeping Giant carry strong natural resistance and support productive orchards without ongoing chemical inputs. These varieties are not fully immune but tolerate infection well enough that canker spread rarely threatens the tree's viability. For high-value specimens, mud-pack treatment applied over young cankers and hypovirulent strain biocontrol, where regionally available through state extension programs, have shown moderate efficacy. Neither approach scales to large plantings or substitutes for resistant variety selection.