Cause Cryphonectria parasitica (formerly Endothia parasitica), a fungus. Chestnut blight has been reported from all Pacific Coast States and provinces but is not commonly found. Latest report for Oregon was in 1934 in an orchard near Gunter. Oregon continues to quarantine all Castanea (Chestnut) and Castanopsis (Chinquapins) spp. from Eastern States, which must be certified free of blight before entering the state.
American and European chestnuts, including chinquapin (Castabiosus sp.), are susceptible. Chinese (Castanea mollissima) and Japanese chestnuts have some resistance to the disease. Breeders have used two approaches to developing new blight-resistant chestnuts. In one case, Chinese chestnut is hybridized to American chestnut followed by repeated backcrossing to American chestnut using pollen from rare, flowering, wild trees. In another case, genetically modified trees have been developed that express a wheat oxalate oxidase transgene, which confers blight resistance comparable to that of Chinese chestnut. The goal is to use these resistant trees as restoration tools for North American forests.
This disease was introduced into North America and Europe from East Asia. First reported in New York in 1904 it spread quickly taking the American chestnut (Castanea dentata) from a dominant forest tree species to a small component of the understory. The European chestnut (C. sativa) was not quite as susceptible but it was nonetheless equally devastated. In Europe it was noticed that in some cases this perennial canker that killed trees was not perennial, began to heal over and did not kill trees. It was discovered that the fungus was not as virulent because it was infected by any of several viruses, a phenomenon called hypovirulence.
Inoculation of these hypovirulent fungi into regular cankers would transmit the virus(es) to the "healthy" fungi and cankers would start to heal. In some situations the virus would spread locally slowing the progress of the disease through the trees. Hypovirulence has controlled chestnut blight well in some locations in Europe, Wisconsin, and in Michigan but has failed almost completely in eastern North America. Where effective, the incidence of the disease remains high but the severity is much less.
Symptoms Slightly sunken or swollen cankers on branches or trunk. Cankers are yellow-brown and oval to irregular in shape. Leaves wilt, turn yellow or brown, and remain attached when cankers girdle the tree. Water sprouts may develop below the cankered area. The canker may contain many orange, pimple-like fruiting bodies (pycnidia) that produce yellow to orange masses of spores during wet weather.
Cultural control Quickly remove infected trees after a positive diagnosis from a reputable laboratory.
Biological control Although hypovirulence is interesting, there is so little disease in the PNW that if it occurs the trees should be removed rather than letting the disease spread.
References Double, M.L., Jarosz, A.M., Fulbright, D.W., Davelos Baines, A., and MacDonald, W.L. 2018. Evaluation of two decades of Cryphonectria parasitica hypovirus introduction in an American chestnut stand in Wisconsin. Phytopathology 108:702-710.
Milgroom, M.G. and Cortesi, P. 2004. Biological control of chestnut blight with hypovirulence: A critical analysis. Annual Review of Phytopathology. 42: 311-338.
Sandercock, A.M., Westbrook, J.W., Zhang, Q., Johnson, H.A., Saielli, T.M., Scrivani, J.A., Fitzsimmons, S.F., Collins, K., Perkins, M.T., Craddock, J.H. and Schmutz, J. 2022. Frozen in time: Rangewide genomic diversity, structure, and demographic history of relict American chestnut populations. Molecular Ecology. 31:4640-4655.