In 2021, completely brown tanoaks (Notholithocarpus densiflorus) and tanoaks with flagging branches started to show up prominently along stretches of Highway 101 in southern Humboldt and Mendocino Counties. Local observers wondered if these tanoaks were evidence of the sudden oak death pathogen (Phytophthora ramorum) expanding its known range in these counties. To answer this question, cooperators from several organizations conducted a survey of these symptomatic and dying tanoaks. The survey included observations of oaks from counties as far-flung as San Luis Obispo, Santa Cruz, Mendocino, Humboldt, and Del Norte Counties and included surveyors and diagnosticians from UC Cooperative Extension, Cal Fire, and the California Department of Food and Agriculture.
Rather than confirming the expansion of sudden oak death, the survey linked most of the collected symptoms to other tree pests. Not surprisingly, most of these pests are ones with a strong relationship to drought conditions—unlike P. ramorum, which spreads in high-precipitation years. Of the 70 samples processed in the survey, over a quarter were infected by Tubakia californica and around a fifth were infected by “bot canker” pathogens, particularly Diplodia corticola. Other pathogens isolated more than once included species of Diaporthe and Biscogniauxia. These pathogens have also been linked to similar symptoms and damage in coast live (Quercus agrifolia), canyon live (Quercus chrysolepis), and California black oak (Quercus kelloggii). Additionally, in several locations the symptoms of these pathogens have been accompanied by a variety of sucking and foliar feeding insects such as oak pit scale (Asterodiapsis spp), oak lecanium scale (Parthenolecanium quercifex), oak treehopper (Platycotis vittata) and California oakworm (Phryganidia californica). Apart from this survey, many of the isolated pathogens have also been previously reported causing damage throughout other parts of California.
All these pathogenic fungi have been identified in the past as “latent” pathogens - ones that can produce spores, infect trees, and spread widely in mild and wet conditions but that usually reside in infected trees for long periods of time without causing obvious symptoms or damage. When drought conditions cause water stress to the tree, these fungi turn pathogenic, spreading in the tree wood and causing twig, branch, or even whole-tree death. These observations of latent pathogens causing widespread symptoms in coastal California align with the results of several other investigations of tree decline during 2020 and 2021, including decline of blackwood acacia (Acacia melanoxylon) and eucalyptus trees in the East Bay (Alameda and Contra Costa Counties), yellowing and dieback of incense-cedar in north-central and northwestern California, and decline of some populations of bishop (Pinus muricata) and Monterey pine (Pinus radiata) in the Bay Area and the north coast.
More surveys of symptomatic tanoaks and true oaks are planned for 2022. For more information and further reading about these pathogens, go to the California Forest Pest Council website at https://www.caforestpestcouncil.org/stem-diseases and scroll down to “Tanoak and True Oak Blight/Canker.”