First detected in the mid-1990’s, the plant disease known as Sudden Oak Death (SOD) is responsible for widespread tree mortality in the central coast region of California. Phytophthora ramorum, a fungus-like organism, has been identified as the pathogen now known to infect approximately 120 ornamental and wildland plant species and genera – and that number is increasing every year.
SOD is usually recognized as a forest phenomenon. It is typically not seen in true residential landscape settings, although occasional outbreaks at retail nurseries and wholesale growing grounds may alter that picture. And while the term “sudden” refers to the relatively rapid browning of the foliage, a tree showing these symptoms has in actuality already been infected for months or years with the pathogen.
It must also be emphasized that the vast majority of oak mortality seen in Napa County is due to causes other than SOD. Drought stress, oak root fungus, crown rot, various insects, and such construction-related factors as soil compaction, grade changes, and root injury contribute significantly to the decline and eventual death of numerous trees. In Napa County, SOD mainly affects the following trees: Coast Live Oak, California Black Oak, Tanoak, and California Bay Laurel.
Valley Oak, Blue Oak, Oregon Oak, scrub oaks, and other members of the so-called “white oak” group are not susceptible to SOD. While certain oaks may die from the disease, most other host plants display only leaf spots and/or branch/twig dieback – mortality occurring only under extreme conditions. The Bay Laurel is the primary culprit in our area responsible for fostering the germination of P. ramorum spores that eventually spread to the oaks.
Fourteen counties in California – stretching from Monterey to Humboldt – and one in Oregon are currently known to be infested with SOD in natural settings. Because the pathogen requires a moist environment to germinate and disperse, most infestations are found in fog-belt or densely wooded, riparian areas. Natural spread usually occurs by wind-driven rain, soil erosion, and streams. In Napa County, with a few exceptions, SOD has been confirmed mostly on the eastern side of the county – in the Mayacamas Mountains – and in the S.F. Bay-influenced southern portion. The disease is not expected to survive in hot, dry climatic conditions that exist in such areas as Pope Valley and Lake Berryessa.
Comprehensive state, federal, and international quarantine measures have been instituted to minimize the likelihood of the artificial (i.e., human) spread of SOD. The movement of host plant material – for example, nursery stock, firewood, green waste – to areas outside of the infested counties is restricted, so please contact the Agricultural Commissioner’s Office concerning current regulations. Helpful information is also available for property owners who want to try to control SOD; for instance, an effective, preventative treatment now exists to help protect valuable landscape oaks. For those who enter infested areas and participate in recreational activities – hiking, mountain biking, horse riding, fishing, hunting, etc. – guidelines have been formulated to help reduce the chances of spreading the disease. Contact the Agricultural Commissioner's Office for details and recommendations, and visit the California Oak Mortality Task Force for the latest news on the subject.