California’s coast is defined by cliffs, beaches, mountains, and a doomed lush forest. Already coastal forests have brown patches of dead trees scattered throughout and in 14 years this will cover 8,700 square miles. A modeling study projected that the disease killing millions of California trees is impossible to eradicate and our once lush forests will be a mosaic of dead, damaged, and survivor trees. The mathematical modeling used can analyze other infestations of diseases and invasive species, for example when Nile crocodiles show up in the Florida Everglades or the next aquatic pest is released into the Great Lakes along with ship ballast water. Knowing when to act and how to allocate resources to the problem requires rapid evaluation. We waited too long to save these trees and “too long” was only a few years.
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Related to a disease that caused the 1840’s potato famine in Ireland, sudden oak death (SOD) was first noticed in Marin County in 2000 when residents asked for help with tanoak trees that suddenly died. Tanoaks are crooked trees with porous wood grain and unsuitable for timber, unprofitable so their deaths didn’t receive much attention.
Sudden oak death is a water mold1, a fungus-like disease that blocks the movement of water, nutrients, and carbohydrates in the tree. Initially it was thought to affect only oak2, and tanoak3. As its name implies, trees don’t die slowly. Diseased trees show symptoms throughout the entire tree all at once — growing tips shrivel, leaves and twigs turn brown, and dark red sap oozes from cankers on the lower trunk.
Considered a minor disease of Rhododendrons in the Netherlands, SOD never seemed especially threatening until it hit California, presumably arriving on nursery stock in the 1990’s. In 2002, SOD was found in coast redwoods and Douglas firs. Unlike tanoak, these trees are commercially valuable, so the timber industry and scientists quickly leaped to attention, allocating study funding and developing quarantines to regulate the movement of trees and tree products from diseased areas. Once SOD weakens trees, beetles come in and finish them off. Oaks and tanoak are killed outright, while SOD causes twig and leaf diseases in numerous other plants, including California bay laurel, Douglas fir, coast redwood, rhododendron, and camellia. Tanoak is most susceptible and has the highest death rate.
Consequences of oak tree losses are serious to wildlife like scrub jays, acorn woodpeckers and squirrels dependent on the acorns. As more trees die, forest ecosystem functions alter. Exposed soil surfaces, increased sunlight penetration, and other consequences of dead trees and reduced canopy cover affect water quality, erosion, and understory habitats.
Forests in 15 coastal California counties from Monterey County north into Oregon are affected. Spread from infected trees by spores carried in water and rain, SOD also is found on forest debris, nursery plants, and firewood from infected trees. It also can be dispersed by bicycles, car tires, and shoes. SOD moves quickly through forests, although some individual trees are resistant. California bay laurel is both vector and predictor of SOD presence in the cool moist climate it favors. Because it survives and remains infected, bay laurel is a disease reservoir and when its branches overhang other trees, SOD spores may be transmitted. Nurseries mimic these moist conditions and can harbor SOD, spreading it via the plants they cultivate and sell. Strains of SOD can be traced to nursery stock. In 2012, one of these strains was found on an ornamental rhododendron in Placer County (Sierra foothills). It was the first SOD found in interior California, but there’s no evidence it spread to the forest.
Over 963 square miles in California currently are infested with SOD and as these areas become larger, the infestations grow faster. Rate of SOD increases as infestation area increases so the best way to slow SOD progress is to manage small areas and new infestations immediately. This will be critical in other areas as SOD has been found on nursery stock in Canada, Germany, Spain, France, Poland, Belgium, Sweden, Italy, the Netherlands, and the U.K. It has also been isolated from forest trees in the U.K. and in the Netherlands. Due to SOD’s preference for cool moist climates, it can spread to northern forests in Europe, along the Atlantic, and in Mediterranean climates.
Now, SOD is thought to be unstoppable in California and will affect 8,700 square miles by 2030, although local containment may be possible in some areas.
Scientists used SOD data in an example of how mathematical modeling can help define the most effective management of established epidemics across a mosaic of different habitat types. This situation is likely to be more common with climate change impacts and involves invasive species as well as pathogens. The underlying dynamics of SOD’s spread apply to other epidemics in natural ecosystems.
The study model was designed to address these questions.
- How quickly must management start?
- When is an epidemic too large to prevent further spread effectively?
- How should local treatment be deployed?
- Where should treatment be targeted?
- How should expenditure be balanced on detection and treatment?
The study concluded that slowing the overall spread of SOD isn’t possible now and hasn’t been for years, although control may have been possible if initiated when the disease was first identified in 2002. Controlling it in 2002 would have required an enormous level of cooperative efforts and over 60 million dollars funding per year for many years. Because the disease growth rate correlates to the size of each infestation, attempts to ameliorate the spread must focus on small outbreaks as larger areas are beyond control. Lessons learned from not dealing with SOD right away teach us how to deal with other pathogens and infestations. Immediate response often is the only way to stop infestation progress.
Various programs help landowners and managers to understand SOD risk to their trees and offer guidance on how to respond.
- The California Oak Mortality Task Force website has information on SOD, diagnosis, management, and other help.
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OakMapper tracks known occurrences of SOD and also is a citizen science project where people can report SOD locations.
- A free app for phone and tablet, SODmap, “interfaces with the SODmap database and allows you to visualize both SOD-positive and SOD-negative trees, based on laboratory results. The app automatically updates the reference database when new points are added….”
- The University of California Forest Pathology and Mycology Lab has a video describing three steps in evaluating and addressing SOD risk of your trees and other help.
- In some cases, pruning back or removing adjacent bay laurels can minimize the risk of SOD spreading to other trees. California bay laurel generally isn't killed by SOD, though it is a foliar host, meaning SOD can live and reproduce just fine on its leaves. This is particularly troublesome because bays tend to spread laterally and overhang adjacent oaks making disease spread easier.
- Phosphonate can help reduce cankers and slow tree death, but is most effective used as a preventative rather than a treatment.
Scientific Names
1. Sudden oak death — Phytophthora ramorum
2. Oaks — Quercus species
3. Tanoak — Notholithocarpus densiflorus
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