Chestnut blight, the disease that decimated the American chestnut trees of the eastern U.S. in the early 1900s, is mainly caused by Cryphonectria parasitica, a member of the largest group of fungi, the ascomycetes (sac fungi).
C. parasitica enters through wounds and cracks in chestnut bark, 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 gave 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.
Because the root system of the American chestnut tree has some resistance to infection by the fungus, shoots still form from old stumps. These are the small American chestnut trees you still see, especially in the forests of the Southern Appalachian mountains. The chestnut blight usually kills these shoots before they produce seeds, but just enough have survived through the years to provide the genetic material needed to engineer—either through crossbreeding or biotechnology—American chestnut tree seedlings with possible resistance to the chestnut blight.