California bay laurel trees with dramatic leaf blight and lower crown leaf death have been attracting attention throughout central and northern coastal California this year. Similar symptoms were initially observed several years ago at Armstrong Redwoods
State Park, Sonoma County. At that time, symptomatic leaves were submitted to the California Department of Food and Agriculture (CDFA) for Phytophthora ramorum (the pathogen that causes sudden oak death) testing. Laboratory findings determined the causal agent to be a previously undescribed fungal species in the Cylindrocladium (asexual)/Calonect
ria (sexual) genus. The same species was also isolated from tanoaks at the park. Following this
year's wet winter, similar symptoms were noticed on bay trees along Highways 17 and 152 in Santa Cruz and Santa Clara Counties, as well as in Humboldt County. While the Highway 17 trees yielded the same Cylindrocladium species, the trees from Highway 152 and Humboldt County did not, instead yielding a different fungus. Additional sampling along Highway 152 and in Humboldt is planned to gain more insight into the complex of pathogens causing these symptoms.
Although this Cylindrocladium does not appear to kill entire bay trees, it has been observed killing up to two-thirds of the leaves in the crowns of some trees. It causes scattered leaf spots that coalesce to form larger, dry, brown lesions and eventual leaf death, as well as twig lesions that remain confined to superficial tissue layers. Infected bay laurel twigs apparently do not die and eventually refoliate. Long-term tree health impacts remain unknown.
The genus Cylindrocladium includes many pathogenic species worldwide, including boxwood blight (Calonectria pseudonaviculata). It is unknown whether this undescribed Cylindrocladium is native to California. CDFA pathologists are working with an expert in the Netherlands to formally describe this species and gain insight into its genetics and origin. This disease serves as a good reminder that laboratory analysis is important in distinguishing new disease outbreaks from existing diseases. If you observe unusual foliar symptoms on a tree, contact your county Agriculture Commissioner, UC Cooperative Extension Advisor, or Cal Fire Forest Pest Specialist for help.