What fungal pathogen causes chestnut blight?

Cryphonectria parasitica causes chestnut blight. This ascomycete fungus attacks the inner bark and vascular cambium of chestnut trees, producing expanding cankers that eventually girdle branches and trunks.

The pathogen is native to East and Southeast Asia, where some chestnut species have greater resistance. It was introduced to North America in the early 20th century, probably through imported Japanese chestnut trees. The disease devastated American chestnut populations: by around 1940, most mature trees in the eastern United States had been killed.

The fungus spreads through airborne sexual spores, rain-splashed asexual spores, and contact with infected bark. It produces oxalic acid, which sharply lowers the pH of infected tissue and helps kill cambium cells. Although the tree above a girdled section dies, roots can survive and produce new sprouts.

A related biological-control story involves CHV1, a mycovirus that weakens C. parasitica. This hypovirulence has helped limit damage in parts of Europe, though it has been less effective across the genetically diverse fungal populations of North America.