Coast live oak resistance to Phytophthora ramorum

The oomycete Phytophthora ramorum is a plant pathogen with an unusually broad host range. Recognized in 2000 as a previously unknown and likely introduced species, this pathogen has become established in central and northern coastal California, southwestern Oregon, and Western Europe. Tree species that may be killed by stem cankers include true oaks (Quercus agrifolia Nee, Q. kelloggii Newb., Q. parvula var. shrevei (C.H. Muller) Nixon, and Q. chrysolepis Liebm.), and tanoaks (Notholithocarpus densiflorus Hook. and Arn.), in North America; and Japanese larch (Larix kaempferi (Lam.) Carriere) in the United Kingdom. The disease, referred to as sudden oak death (SOD), is changing the composition of forests in California and potentially threatens forests worldwide. Coast live oak has been heavily-impacted by the epidemic, with infection and mortality rates in infested areas averaging 5 percent and 3 percent, per year, respectively, in forests of Marin County, California. Since this species is one of the dominant mast-producing trees in much of coastal California, the species composition and ecological integrity of many of these forests are threatened. Median survival of naturally infected coast live oaks in long-term disease progression plots established in 2000 was estimated as 9.7 years, using Weibull survival models. Beetle attacks in cankers reduced survival times by 65 to 80 percent. While coast live oak mortality has steadily increased since 2000, the infection rate in these sites has been declining. In heavily-affected areas, asymptomatic coast live oaks persist that have never exhibited the bleeding symptom of infection or that are in apparent remission from previously recorded bleeding.