Resistance to Phytophthora cinnamomi among seedlings from backcross families of hybrid american chestnut

American chestnut (Castanea dentata (Marsh.) Borkh.) once was a primary hardwood species in forests of the eastern United States. Sometime during the late 18th century, it is speculated that Phytophthora cinnamomi, which causes Phytophthora root rot (PRR) on many woody plant species, was introduced to the southeast region of the United States, and this pathogen spread as people moved inland from the coast (Zentmyer 1980). In the 1800s, PRR (also known as ink disease) caused extensive mortality to American chestnut trees in the southern portion of its range (Freinkel 2007, Zentmyer 1980). Then, in the early 1900s, chestnut blight, caused by Cryphonectria parasitica, almost eliminated American chestnut trees from eastern forests (Freinkel 2007). Since 1989, The American Chestnut Foundation (TACF) has been producing hybrid chestnut seedlings by crossing Chinese chestnut (C. mollissima), which is resistant to C. parasitica, with American chestnut and then backcrossing progeny to C. dentata to produce resistant American-type chestnut trees. Since 2000, hybrid seedlings planted in some locations in southeastern states have died from PRR before they could be challenged by naturally-occurring populations of C. parasitica. Therefore, we wanted to determine if any of the backcross trees selected for resistance to C. parasitica were resistant to P. cinnamomi as well because Chinese chestnut also is resistant to this pathogen.