Sudden Oak Death and Ramorum Blight

September 2023
Marion Murray, Extension IPM Specialist and Ann Mull, Research Assistant (No longer at USU)

Sudden oak death (SOD) and ramorum blight are caused by Phytophthora ramorum, a non-native water mold (oomycete) that originates from parts of Asia. Sudden oak death was given its name due to the appearance that entire trees were dying in just 2 to 4 weeks (Fig 1). In reality, the disease progresses over an extended period, estimated at more than 2 years after initial infection. Ramorum blight is nonlethal, typically causing foliar browning and wilting.

Phytophthora ramorum is currently only established in forests of California and Oregon, where it has killed tanoak and other oak species. In 1995, the pathogen was first detected in the U.S. in California’s San Francisco Bay Area, where it caused widespread tanoak mortality.

Currently, P. ramorum is established in 18 California counties as well as in Oregon’s Curry and Lincoln counties. The pathogen has also been detected and contained in other U.S. nurseries. Surveys for P. ramorum are ongoing, including in Utah, and state and federal quarantines exist for proven hosts and associated plants to limit human-assisted spread.

Impact

Tanoak and oak (Quercus spp.) are dominant tree species in California’s redwood and mixed conifer/ hardwood forests. Death of these trees caused by SOD impacts forest ecosystems by altering species composition, decomposition rates, and nutrient cycling. To date, SOD has killed 63% of tanoaks in parts of California’s Monterey County, and 90% to 100% in some areas of Point Reyes National Seashore. Although the resulting dead trees can increase fuel for fires, wildfires have been shown to significantly reduce P. ramorum occurrences. In Oregon, more than $22 million has been spent identifying and treating P. ramorum-infested areas, and California property values have decreased from tree losses. Nurseries in California and Oregon growing susceptible host plants are subject to federal quarantines, and many nurseries testing positive for P. ramorum that transport outside state lines have ceased producing plants that are susceptible, such as rhododendron, camellia, pieris, kalmia, and viburnum.

In Utah, the likelihood of this pathogen causing widespread damage is low due to Utah’s arid climate and lack of widespread oak forests, which is counter to that needed by this pathogen. Isolated pockets of P. ramorum may occur where moisture conditions are favorable, such as in higher-humidity nursery or greenhouse settings.

Plant Hosts

Phytophthora ramorum attacks 130 host species from 34 genera. In the U.S., tanoak and coast live oak are the primary trunk hosts, and California bay laurel is the primary reservoir of inoculum (foliar host). Oaks from the red or intermediate groups are affected, but oaks from the white group, which includes Utah’s gambel oak (Quercus gambelii), are typically not susceptible to SOD. In Europe, P. ramorum attacks European larches and plantation Japanese larch (Larix kaempferi), and the primary foliar host is rhododendron. Known hosts are included in Table 1. 

Table 1. Phytophthora ramorum Host Trees and Shrubs

Oak Hosts
tanoak (Notholithocarpus densiflorus)
coast live oak (Quercus agrifolia)
European turkey oak (Q. cerris)
canyon live oak (Q. chrysolepis)
southern red oak (Q. falcata)
holly oak (Q. ilex)
California black oak (Q. kellogii)
Shreve’s oak (Q. parvula var. shrevei)

Non-Oak Hosts
California bay laurel (Umbellularia)
Oregon grape (Berberis aquifolium)*
European beech (Fagus sylvatica)
European ash (Fraxinus excelsior)
witch hazel (Hamamelis virginiana)
Douglas-fir (Pseudotsuga menziesii)*
wood rose (Rosa gymnocarpa)
lilac (Syringa vulgaris)
English yew (Taxus baccata)
whortleberry (Vaccinium myrtillus)*
Various species of the following genera: fir (Abies)*, maple (Acer)*, fern (Adiantum)*, manzanita (Arctostaphylos)*, huckleberry (Vaccinium)*, mountain laurel (Kalmia), rhododendron and azalea (Rhododendron), viburnum (Viburnum), larch (Larix) and others

Damage and Symptoms

Sudden Oak Death
Sudden Death On primary hosts, cankers form on the trunk and lower branches that ooze and bleed dark black to red or amber sap (Fig. 2). These cankers kill the phloem tissue, eventually girdling the tree and turning foliage brown (Fig. 3).

Ramorum Blight
Ramorum Blight symptoms vary depending on the host plant, and confirmation requires laboratory analysis of infected material, as damage from several other diseases or injuries is very similar. They typically include brown lesions on the foliage or brown leaf tips that may be edged with a black line and yellow halo (Fig. 4). In extreme cases, ramorum blight can be lethal to young plants, and on California bay laurel, the tips and edges of leaves older than 1 year are most susceptible. On rhododendron, lesions are identical on the upper and lower leaf surfaces.

Disease Cycle

Phytophthora ramorum requires water for spore dispersal, and hosts can become infected via windblown spores or spores in soil or water. Reproduction is asexual and produces three types of microscopic spores. Sporangia release from infected foliage and are dispersed by splashing or wind-driven rain. Upon landing on the host, they either germinate and cause infection directly, or in free water, they release zoospores that cause infection. Sporangia are short-lived, but can disperse up to 2 miles in extreme, wet winds. Chlamydospores form within dead tissue and wood debris and are important for long-term survival in drought, heat, and cold, as well as long-distance dispersal via infected soil on shoes, cars, and equipment. In favorable conditions, chlamydospores can germinate and infect directly or they can produce a sporangium.

California bay laurel (also known as Oregon myrtle) and tanoak are most susceptible to P. ramorum infections in spring and summer, forming “transmission highways,” where infested trees produce an unusual overabundance of pathogen spores that disperse broadly across the landscape. Spores can survive in potting media and soil in the absence of a host for at least 33 months.

Monitoring and Prevention

Scientists conduct wide-scale monitoring for P. ramorum via aerial surveys and stream water analyses. Individuals can inspect plants by looking for bleeding cankers, brown leaf spotting, and black-margined leaves with halos. Note that these symptoms are shared by many pathogens and do not implicitly indicate the presence of P. ramorum.

To prevent introduction into Utah, be cautious when visiting infested forested areas of California and Oregon. During the wet, rainy, and cool seasons, stay on established trails and avoid collecting plants, firewood, acorns, leaves, soil, or water, especially where symptomatic plants exist. Remove soil and plant material from bikes, shoes, and clothes, then rinse with water and spray with disinfectant before leaving the site. Wash debris from off-road vehicles and campers before leaving the area. As this pathogen is particularly well-adapted to spread from nursery stock, always list declarable items when traveling abroad, including all plant material. Visit the U.S. Customs and Border Protection website to learn what items are prohibited and restricted-entry.

Management

SOD has not been detected in Utah, so there is no need to manage for P. ramorum at this time. Where it occurs, professional applicators have used products containing phosphorous acid (phosphite) to reduce canker growth. When purchasing susceptible host plants, inspect leaves for symptoms, and purchase plant material from reputable nurseries devoid of this pathogen. Greenhouse operations and nurseries can place a 1-inch gravel layer on the soil surface to kill P. ramorum inoculum via soil solarization. In general, exposing soil to 122 °F for 30 minutes, 104 °F for 2 days, or 95 °F for 4 days will kill P. ramorum spores.

References

• Animal and Plant Health Inspection Service (APHIS). (n.d.). Sudden oak death. USDA National Invasive Species Information Center, U.S. Department of Agriculture.
• APHIS. (2022). Phytophthora ramorum. U.S. Department of Agriculture.
• California Oak Mortality Task Force. (n.d.). Sudden oak death. California Forest Pest Control. 
• California Oak Mortality Task Force. (2022), December. Current California oak mortality task force newsletter. 
• EDDMapS. (2022). Early detection & distribution mapping system. Center for Invasive Species and Ecosystem Health, University of Georgia. 
• Forest Pathology and Mycology Lab. (n.d.). SODmap project. University of California - Berkeley. 
• Funahashi, F., & Parke, J. L. (2020). Soil solarization to eradicate soilborne Phytophthora spp. in container nurseries with surface gravel. Journal of Environmental Horticulture 38, 158-167.
• Grünwald, N. J., LeBoldus, J. M., & Hamelin, R. C. (2019). Ecology and evolution of the sudden oak death pathogen Phytophthora ramorum. Annual Review of Phytopathology 57, 301-321.
• Johnston, S. F., Cohen, M. F., Torok, T., Meentemeyer, R. K., & Rank, N. E. (2016). Host phenology and leaf effects on susceptibility of California bay laurel to Phytophthora ramorum. Ecology and Epidemiology 106, 47-55.
• National Park Service. (2022). Sudden oak death (SOD). Redwood National and State Parks, California National Park Service, U.S. Department of the Interior.
• Rosenthal, L. M., Fajardo, S. N., & Rizzo, D. M. (2021). Sporulation potential of Phytophthora ramorum differs among common California plant species in the Big Sur region. Plant Disease 105, 2209-2216.
• Simler-Williamson, A. B., Metz, M. R., Frangioso, K. M., & Rizzo, D. M. (2020). Wildfire alters the disturbance impacts of an emerging forest disease via changes to host occurrence and demographic structure. Journal of Ecology 109, 676-691.
• Vercauteren, A., Riedel, M., Maes, M.,Werres, S., & Heungens, K. (2013). Survival of Phytophthora ramorum in rhododendron root balls and in rootless substrates. Plant Pathology 62, 166-176.