This past winter, many coast live oaks (Quercus agrifolia) throughout the East Bay appeared largely bare. Their condition caused many homeowners and land managers to wonder if the trees they care for will survive and what they can do to sustain them.
While these oaks are evergreen, they typically drop leaves year-round. But to withstand exceptionally hot and dry conditions in 2020, many coast live oaks dropped leaves earlier in the summer and more intensively—they appeared to be dead. Encouragingly, starting about February, the oaks flushed, pushing out bright green crowns of leaves. If extreme drought continues, or escalates, this pattern will likely repeat this year. How many years of drought stress—and how extreme a drought—the trees can tolerate remains to be seen.
But will rain bring relief to the oaks? While rain will alleviate much drought stress, it can carry another threat to coast live oaks: the sudden oak death pathogen, Phytophthora ramorum. This invasive microbe requires water to reproduce and infects oaks during years with abundant rainfall. In forests, it spreads via wind-blown rain.
The pathogen, first detected in the Bay Area in the mid-1990s, is now widespread and common in much of the East Bay (scroll to the latest SOD Blitz survey results and map on the SOD Blitz Project page); millions of oaks and tanoaks (Notholithocarpus densiflorus) have been killed. The most recent wave started in 2016–2017, when California received record rainfall. Trees became infected at that time, then in 2020 into 2021, stressed by recent drought conditions, many died across the East Bay hills. Sudden oak death is a wilt disease; trees die when their compromised vascular systems can no longer supply the water needed for physiological function. During drought, while previously infected trees may die in larger numbers, new infection and pathogen spread are limited.
Determining the cause of death or poor health in coast live oaks can be difficult. Trees killed by sudden oak death infection often retain their leaves: the entire crown appears to turn brown rapidly (see the two following photos) with bleeding or oozing on the trunk. However, infection doesn’t always follow this pattern. Some trees will lose their leaves slowly, for as long as a decade post-infection, causing the trees to gradually decline, then die. Other individuals are naturally resistant and will recover.
Pathogen movement on nursery plants. Another concern for the spread of the sudden oak death pathogen is inadvertent transport on infected nursery stock. With a broad host range including rhododendrons, camellias and other popular horticultural and native plants, the pathogen is under state and federal quarantines to prevent human-assisted spread. Nurseries provide a conducive environment for the pathogen, so use of systematic phytosanitary measures is critical to prevent infested nursery stock from contaminating yards or wildlands. (Concerns over infested plant shipments are highlighted in “The Diseased Rhododendrons That Triggered a Federal Plant Hunt” by Ellie Shechet in The New Republic, April 2021.)
Take-home messages available from Bay Area trees. Observing East Bay oak health illustrates the importance of climatic conditions in shaping our forests. As drought intensifies, in addition to the direct effects of insufficient water for tree survival, concerns over fuel ladders and fire risk are increasing. Also, to sustain urban forests under heavy human population pressure, invasive species introductions and spread need to be prevented. For now, despite drought and the sudden oak death pathogen, most coast live oaks in the Bay Area are thriving—their resilience is an inspiration, marvel, and joy.
More information on growing and caring for coast live oaks is at Calscape; growth habits and other information is available from the U.S. Forest Service Fire Effects Information System. For more information on sudden oak death, see suddenoakdeath.org, including a homeowner’s guide.
— Susan Frankel, plant pathologist, U.S. Forest Service, Pacific Southwest Research Station, Albany, and CNPS East Bay Chapter member