Phytophthora cinnamomi is a virulent pathogen of the American chestnut tree that causes the disease Phytophthora root rot (PRR).

Phytophthora cinnamomi, introduced into North America by European colonists in the 1700’s, is thought to have eradicated American chestnut from low elevation forests in the Southeastern U.S. prior to the introduction of chestnut blight. Combining resistance to the chestnut blight fungus (Cryphonectria parasitica) and P. cinnamomi became an objective of The American Chestnut Foundation’s breeding program after Phytophthora root rot caused mass mortality in backcross orchards planted in the piedmont of the Southeastern U.S. Fortunately, Chinese chestnut is resistant to the disease and some American chestnut backcross hybrids inherited major effect resistance genes from their Chinese chestnut parent. The American Chestnut Foundation is collaborating with Clemson University, North Carolina State University, and the U.S. Forest Service to inoculate advanced generation backcross hybrid families with P. cinnamomi to identify resistant parents. Parents resistant to P. cinnamomi will be bred with those most resistant to the chestnut blight fungus. Selection for ensuing generations will combine resistance to both pathogens.