The American chestnut (Castanea dentata) and the related Allegheny and Ozark chinkapins (which are both varieties of Castanea pumila) probably began declining approximately 200 years ago with the introduction of the soil-borne algal fungus, Phytophthora cinnamomi. Throughout the 1800s, the disease gradually killed chestnut and chinkapin populations on relatively wet sites (Crandall et al., 1945).
Episodes of oak decline have sometimes been blamed on P. cinnamomi but this has not been investigated conclusively. According to Jung et al. 2018, P. cinnamomi is common in eastern American oak forests below 40° northern latitude. While several other Phytophthora species are also pathogenic to oaks, their scattered distributions lead scientist to conclude that they play a minor role in oak decline. P. cinnamomi mainly infects the root system (canker development on stems is limited by cold winter temperatures). Several recent studies have confirmed that fine root losses – consistent with P. cinnamomi infection – are driving oak decline events in moist low-elevation stands where inoculum levels of P. cinnamomi are higher. In colder areas – e.g., plant hardiness zones 6 and 7 – fine root regeneration is limited due to climatic constraints (See Jung et al. 2018).
[P. cinnamomi is most famous for causing severe mortality in jarrah and other forests of Western Australia. Also causing widespread declines in oak forests of Europe. See Jung et al. 2018.]
Crandall, R. S., G. F. Gravatt, and M. M. Ryan. 1945. Root disease of Castanea species and some coniferous and broadleaf nursery stocks caused by Phytophthora cinnamomi. Phytopath. 35: 162-180.
Jung, T., A. Pérez-Sierra, A. Durán, M. Horta Jung, Y. Balci, B. Scanu. 2018. Canker and decline diseases caused by soil- and airborne Phytophthora species in forests and woodlands. Persoonia 40, 2018: 182–220
Robin, C., F. Dupuis, and M. L. Desprezloustau. 1994. Seasonal changes in northern red oak susceptibility to Phytophthora cinnamomi. Plant Dis. 78: 369-374.
Robin, C., M. L. Desprez-Loustau, G. Capron, and C. Delaour. 1998. First record of Phytophthora cinnamomi on cork and holm oaks in France and evidence of pathogenicity. Ann. Sci. Forest. 55: 869-883.