Chestnut blight is caused by Cryphonectria parasitica, a member of the largest group of fungi, the ascomycetes. C. parasitica enters through cracks in chestnut bark and through wounds, causing dead areas on bark called cankers. Once introduced, the fungus grows rapidly, producing a network of filaments called mycelial fans that quickly girdle the tree and grow down into the wood, where they destroy the vascular systems that carry sap. The leaves on the stem then die, showing the symptoms that give rise to the name “chestnut blight.”
When the cankers grow into the wood, the fungus forms bright orange, rounded structures on the surface of the bark. These stromata produce two types of spores—ascospores and conidia. Ascospores appear whenever conditions are right, and are forcibly expelled from the stromata to be carried away on the wind. The second spore types, conidia, ooze out after rain and can be carried by waterdrops or on the feet of insects, birds, squirrels, and other creatures.
Adapted from: Hebard, F.V.; Double, M.L.; MacDonald, W.L. 2007. A pathogen without rival. In: Bolgiano, Chris, ed. Mighty giants: an American chestnut anthology. Bennington, VT: The American Chestnut Foundation: 172–174.