Once SOD infections have arrived, what can be done?
- If SOD is present in a portion of the property remember to:
- Schedule all landscaping and construction operations to occur first in the SOD-free area.
- Ensure that equipment is cleaned after work in the SOD infested area.
- Minimize all activities and operations in the Spring. Fall is the best work to avoid spreading infection through disturbance. Pruning of large branches and stems in multi-stemmed oaks should occur possibly in late November, and never in February-June.
- If the property is downwind and down slope from a dense mixed forest with significant infestation, ensuring that water runoff is properly channeled may be beneficial to avoid spread of the disease by water.
- Oaks that have a buffer area of at least 10 yards (10m) around the main trunk, devoid of any bay laurels or rhododendrons, are less likely to become infected. Identify valuable oaks and clear a 10 yard area around them by removing all small and medium bays, or by pruning large bay laurel branches of large trees that may come into the “buffer”. Bay laurels need to be treated with systemic herbicides at least a couple of weeks before being cut down, to minimize re-sprouting. Avoid all overhead irrigation and summer irrigation near oaks. For oaks over 32″ in diameter (80cm), 50-65 ft (15-20m) may be appropriate.
- If oaks are not infected they can be protected with a single yearly treatment of phosphonate in November and early December. If treating in the Spring for the first time, repeat in the Fall and switch to a single yearly Fall treatment. Treatments are not a cure but will increase resistance in about three quarters of treated plants.
- Phosphonate treatments have no known damaging side-effects on the environment and their action enhances the natural defense mechanisms in the plant. If injecting the product, we recommend alternating with bark applications every other year, to minimize wounding.
- Ensure you or your tree-care specialist has attended a workshop on how to properly apply the product. Topical bark applications require:
- Topical bark applications require removal of thick moss and tarping of shrubs or other small plants nearby.
- Injections require knowledge of appropriate timing, number of injections per tree, and injection technique, injections should not be performed in the Spring.
- No chemical treatment of bay trees is known to be effective, and other alleged treatments of oaks have been shown to be ineffective in controlled experiments.
- Additional treatments such as carving or scribing the infection out of an oak, or amending the soil near oaks with a thin layer of compost are still untested and we cannot recommend them. In addition, wounds may be detrimental to the tree and facilitate further SOD infection. To minimize such risk do not attempt to remove lesions in the Spring when infection potential is high. Lesion removal from oaks is still completely untested and could have unknown side effects.
Research behind the Recommendation:
The arrival of SOD in an area may be very gradual over time or may occur in a single year. Knowledge of SOD biology and disease distribution may help you prepare for the arrival of SOD and may allow time to protect valuable trees. The SODmap Mobile app, available for iPhone and Android, is an invaluable tool for visualizing SOD infections on the ground, in real time.
Dead oaks represent a significant hazard to people and property and increased fire hazard, however oaks that are infected by SOD and are apparently green may be equally hazardous. In SOD-infested areas, monitor closely oaks and tanoaks that are near homes, access roads, paths, yards, and play areas, and remove trees that show significant SOD symptoms and may be hazardous due to their location. Dead standing oaks, whether still green or brown represent a serious hazard for homeowners and are documented to fuel hotter fires that can kill medium sized redwoods.
UC Berkeley researchers have surveyed a large number of Coast Live Oaks, Tanoaks, and California Bay Laurels and determined that significant natural resistance to SOD is absent or very rare. However all three species have individual plants that may be more tolerant than others. Currently resistant plants are not available on the market, but several oak species (Valley oaks Blue oaks, Oregon oaks) are not susceptible to SOD.