Phytophthora ramorum was not discovered as the causal agent of sudden oak death until 2000-2001. Prior to then, it was not understood to be a nursery issue; therefore, there was no concern about spread from nurseries into the wildlands. Since then, DNA analysis has confirmed that the pathogen was unknowingly introduced to both California and southern Oregon via infected nursery stock from multiple nurseries. Prior to 2000 and our knowledge of the pathogen, it appears P. ramorum was moving around via the sale and trade of infected ornamental plants and possibly through movement of cuttings or rootstock for production. While we do not know where the pathogen originates from or how ornamental plant nurseries originally were introduced to it, we believe the nursery to wildland introduction in California began around 1987 in Marin and Santa Cruz Counties.
Is it okay to prune coast live oaks in the winter if there is no SOD inoculum in the area? Are there other pests that are active in the winter that are of concern?
Although there may be few insects flying in the winter, the wet winter season (actually fall through spring) is a prime time for production of fruiting bodies by many wood decay fungi. The combination of fresh pruning wounds (especially large wounds), high spore densities of wood decay fungi, and moist conditions (favoring spore germination) can favor new infections by these fungi. From the standpoint of reducing risk of infection by wood decay fungi, pruning in late spring or early summer is probably preferable.
In general, pruning live branches from mature oaks should be kept to a minimum. However, if it is necessary to prune coast live oak branches, as long as they are less than 4″ in diameter, the presence of SOD is not a concern. Though, it is important to prune the oak correctly.
• Make sure you prune right where the tapering of the branch ends from the main stem or even slightly closer to the stem (1/2 in. from the end of the tapering towards the stem).
• Make sure the cut is clean and allows for water to drip without accumulating.
Once down, spores from bay foliage will not travel nearly as far as they do in the canopy. The concern on the ground is that infested leaves, while still fresh, could have spores picked up during a wet event from soil splash onto the trunk of the tree. So, spread bay foliage in a thin layer in a sunny area that will promote drying. Be sure to not have foliage near the base of oak or tanoak trunks where soil splash/infection could occur.
The lab can, upon request, analyze wood chips at cost ($90 US dollars per sample). Testing would confirm the presence or absence of any known dangerous pathogens.
Tree Health Answers and Questions. Ask the experts any questions about tree health, diseases, and management.
Does the sudden oak death pathogen sporulate on madrone like it does on California bay laurel? Sudden oak death can kill small madrones and cause significant dieback of larger madrones. While Phytophthora ramorum (the pathogen that causes sudden oak death) does sporulate on madrone, making in an infectious host, it is not a very effective at transmtting the pathogen as infected leaves whither rapidly, eliminating pathogen viability.
Cleaning tools in SOD-infested areas.
Can tools become infectious when used on trees infected by SOD? Yes, in particular when dealing with infected foliage, infested soil, and marginally with infected wood. The risk of spreading SOD is higher if soil or infected leaves are present on tires, shoes or tools. Wood is generally, not as infectious
Differently from other pathogens which produce invisible microscopic and often sticky spores that can remain viable even on apparently clean tools, the SOD pathogen produces spores that are normally lodged in plant debris, organic matter, and plant or soil residue.
- Clean all visible organic residue, plant tissue or soil from the tool. A wire or plastic brush is generally effective. Removal of all visible debris will eliminate 99% of all infectious propagules.
- Washing with a disinfectant that will kill any remaining spores. We have found that 70% ethanol, diluted bleach, and Lysol brand disinfectant are all effective. Lysol, which contains benzalkonium chloride, a powerful and effective antiseptic, is the product we use most often as ethanol tends to be the most expensive of the three, and bleach can cause corrosion of metal tools.
Note: it is important to ensure all parts that came in contact with infected plant material are visibly clean and that no organic matter remains. This is particularly crucial for tools like chainsaws or machinery like chippers. The physical cleaning step (above) is critical, and can be effective even without washing although the application of a disinfectant may help). Washing first and then cleaning will be ineffective. Always wear safety glasses and protective equipment as recommended by the manufacturer and always follow the manufacturer’s directions, restrictions, and precautions on the product label.
Research behind the Recommendation:
To test the effectiveness of tool cleaning on the transmission of P. ramorum, a chainsaw and handsaws were “contaminated” with SOD-infected wood chips and saw dust. The saws were treated by different methods in an attempt to “decontaminate” them including, removing the visible organic matter with a wire brush or brushing followed by a disinfectant rinse. The disinfectants used were 10% ethanol, 5% bleach, and lysol (diluted as instructed on the label).
After cleaning, the saws were examined under magnification, any remaining bits of plant or organic matter plated on selective agar Petri dishes. The percentage of infectious propagules that grew on the agar plates is graphed below. Click to rebigulate the pdf.
Links and References:
- Protecting trees from SOD before infection
- SOD Sanitation Guide
- What to do after SOD arrives in your area.
Disclaimer: Mention of any company, trade name, or commercial product does not constitute endorsement by the University of California or recommendation for use. Always follow the manufacturer’s directions, restrictions, and precautions on the product label.
- SOD is an exotic disease caused by the microscopic pathogen Phytophthora ramorum, estimated to have been introduced into California 20-25 years ago from unknown region of the world.
- P. ramorum was unwittingly introduced into California’s natural landscape when infected ornamental plants, such as Rhododendrons and Camellias (which carry the disease), were outplanted into the environment. On many ornamental plants, as well as many native forest plants (including California bay laurels and tanoak), the pathogen causes a disease called ramorum blight. Ramorum blight is a foliar disease that often supports pathogen sporulation and spread, as the spores build up on the leaves and twigs of these hosts which can then be transferred to nearby oaks and tanoaks, causing sudden oak death. Ramorum blight is rarely lethal.
- In California, foliar infections on Bay Laurel leaves are mostly responsible for spreading the disease. Oaks and tanoaks are infected when in proximity of bay laurels. Oaks only get a stem infection; tanoaks can develop both stem and a foliar infection. Tanoaks are the only tree species that can spread the the disease and die from it as well. When P. ramorum infects oaks and tanoaks it destroys the cambium under the bark and effectively girdles the tree. Girdled trees are doomed, but can survive for 1 to 5+ years thanks to stored resources and their natural tolerance to drought.
- Besides Tanoaks, California Coast Live Oak, Black Oak, Shreve’s Oak, and Canyon Live Oak are known to be infected by P. ramorum. Oaks are not infectious in nature and do not spread the disease further, but oak wood may be infectious if carried to an uninfested area.
- P. ramorum can infect leaves and trunks of it’s hoasts without the need for wounds, but it does require rainfall and temperatures between 60 and 80F (25 to 27C). As a result most infections occur in the rainy season and in particular when spring temperatures rise (mid April to June).
- Yearly infection levels will depend on the presence of rainfall in the Spring; while tanoaks and bay laurels are infected yearly, oaks are normally infected only in years with abundant Spring percipitation.
- P. ramorum is dispersed aerially usually at short distances 100 yrds (100m) or less, but occasionally up to 1-2 miles (1.7-3.5km).
- Infectious airborne microscopic structures known as sporangia are produced during rain events on plant surfaces, primarily leaves, and can also accumulate in soil and stream water.
- Besides the natural spread of the pathogen, movement of infected plants or plant parts, soil, and water may lead to new infestations. Soil and plant material on tools and equipment may vector the disease.
- During dry periods, the pathogen survives within infected plant tissues, and by producing a thick walled resting structure called a chlamydospores. As a result SOD remains persistent in any site, but oak infection varies as it requires high infection levels of bay laurels.
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.