- How to Manage Pests
- Pests in Gardens and Landscapes
- Sudden Oak Death
- LIFE CYCLE/BIOLOGY OF PATHOGEN
- ONLINE RESOURCES
- PUBLICATION INFORMATION
- Sudden Oak Death
- Sudden Oak Death and Ramorum Blight1
- Causal Agent and Geographic Distribution
- Host Range
- Disease Symptoms
- Spread of the Pathogen and Control Options
- Additional Resources
- Sudden Oak Death/Ramorum blight
- Additional Resource
- Sudden Oak Death
- Sudden Oak Death: Prevention is Key
- Sudden Oak Death: What can be done to save your oaks?
- Phytophthora ramorum (sudden oak death (SOD))
- Sudden Oak Death: Overview
How to Manage Pests
Pests in Gardens and Landscapes
Sudden Oak Death
In this Guideline:
Tanoaks killed by Phytophthora ramorum.
Annulohypoxylon thouarsianum fruiting bodies often are found on trees affected by sudden oak death.
Viscous sap oozing from trunk of coast live oak infested with Phytophthora ramorum.
Reddish, discolored tissue and distinct, black zone lines beneath the bark of a black oak infected with Phytophthora ramorum.
External bleeding symptoms on a tanoak trunk.
Sudden oak death is a disease of oak trees caused by an invasive plant pathogen, Phytophthora ramorum. It currently occurs in coastal California counties from Monterey to Humboldt and in a small portion of southwest Oregon. It is estimated to have killed more than 1 million oak and tanoak trees during the last decade. In addition, more than 100 other plant species are susceptible to the pathogen, but most suffer only minor damage limited to leaf spots or twig dieback.
Although Sudden oak death is a forest disease, it is common in urban-wildland interface areas—places where development meets or intermingles with undeveloped wildland—and can present many challenges for residential landscapes. Diagnosis of infected trees and proper disposal of contaminated wood and other material are essential to limiting the spread of the disease. Management options include treatment with phosphonate compounds and selective plant removal.
Because P. ramorum can be spread by moving infested soil and plant materials, state and federal regulations are in place to control the potential spread of the pathogen to uninfested areas. The California Department of Food and Agriculture (CDFA) and the U.S. Department of Agriculture Animal and Plant Health Inspection Service (USDA–APHIS) regulates movement of any known host species. A quarantine is in place for the infested counties. Before moving regulated plant material out of quarantined areas, you must contact your agricultural commissioner for a permit.
LIFE CYCLE/BIOLOGY OF PATHOGEN
Phytophthora species are funguslike organisms, related to algae, which occur worldwide. They are water loving and produce plentiful spores in moist or humid conditions. Most known Phytophthora species are soil-dwelling root pathogens; however, P. ramorum acts primarily as a leaf pathogen. In California, it thrives in the coastal tanoak/redwood forests and oak woodlands within the fog belt. Nurseries outside of these cool, moist areas often create microclimates that mimic an environment supportive of P. ramorum and allow it to grow and spread far from the coast.
While most nonoak hosts are not killed by the disease, they do play a key role in the spread of P. ramorum, acting as a breeding ground for inoculum that can spread through water, wind-driven rain, plant material, or human activity. Oaks are considered terminal hosts, since the pathogen does not readily spread from intact bark cankers; they become infected only when exposed to spores produced on the leaves and twigs of neighboring plants.
Research in California forests has shown that the greatest predictor of P. ramorum canker on oak is the presence of California bay laurel (Umbellularia californica). Pathologists believe P. ramorum drips or is blown down onto oak trunks from neighboring bay leaves when it rains. Once on the oak trunk, P. ramorum uses natural openings in the bark to colonize the bark tissues, killing cells and clogging water and nutrient transport vessels.
Infections caused by P. ramorum must be confirmed in the laboratory utilizing either culture techniques or DNA analysis for detection of pathogen DNA. They cannot be identified on field symptoms alone. However, infected plants typically are found near other infected plants, so when oaks or other hosts with characteristic symptoms of Sudden oak death are found within forests or woodlands where the disease already has been confirmed, these plants should be suspected to be infected with P. ramorum. Oaks growing farther from infested forests are not as likely to be infected unless infected material or nursery stock has been brought to the area. Check online mapping resources for the most current data on pathogen distribution.
There are two categories of hosts for P. ramorum—trunk hosts and foliar hosts. Trunk hosts, such as tanoaks and oaks, get infections in their bark. These trunk infections often are fatal. Other organisms often attack diseased oak and tanoak trees once P. ramorum has weakened them. These include ambrosia beetles (Monarthrum scutellare and M. dentiger), bark beetles (Pseudopityophthorus pubipennis), and a sapwood decay fungus (Annulohypoxylon thouarsianum) . Though these attacks are secondary to the original P. ramorum infection, they act to further weaken the trunk and can hasten the tree’s death.
On foliar hosts—such as California bay laurel, rhododendron, or camellia—symptoms can range from leaf spots to twig dieback, but these hosts rarely die from the infection. Rather than Sudden oak death, a P. ramorum infection on these hosts is known as Ramorum blight. Symptoms of P. ramorum infection on some key hosts are detailed below.
Oaks (Quercus species)—Coast Live Oak, Canyon Live Oak, California Black Oak, and Shreve’s Oak
The oak genus is divided into three subgenera, or groups: white, red, and golden/intermediate. Oaks from the white group—valley oak, Garry oak, and blue oak along with some scrub oak species—are not thought to be susceptible to Sudden oak death, although other oaks do get the disease.
Susceptible trees in the red oak group (coast live oak, California black oak, and Shreve’s oak) and intermediate group (canyon live oak) develop a bark canker when infected. External symptoms of canker development can include the bleeding of a thick, sticky sap. It typically smells like the inside of a wine barrel and is a deep burgundy but can vary in color from nearly black to an amber-orange (Figure 3). Recent rains can cause the sap to run, often producing large stains on the surrounding bark. Only larger trees—those that are more than 4 inches in diameter at chest height—show symptoms; infections of smaller saplings are extremely rare. Mosses and lichens growing on the tree trunk die if the sap comes in contact with them. Their death might be the only indication a tree is bleeding.
The bleeding is the external manifestation of an underlying, diseased area of the tree, or canker. Removing the surface bark will reveal discolored, brown tissue, normally separated from healthy bark by a distinct, black zone line, although this line can be somewhat indistinct during periods of active pathogen expansion, typically in the spring. This zone line represents the active front of the infection. Cankers usually develop 3 to 6 feet from the ground, although they can be as high as 12 feet or greater; they can be as low as soil level, but they are not thought to extend below the soil line. Bleeding sap initially appears on intact bark, without any obvious holes or wounds, although in later stages of the disease the bark might split.
P. ramorum infections on oaks originally were called “Sudden oak death” because of the rapid (2- to 4-week) browning of leaves without an apparent, prolonged period of visible decline. The foliage might appear healthy until shortly before it turns brown, or the leaves might turn olive green, pale green, or yellow green for several weeks to several months before browning. Infected coast live oaks also might lose leaves before they die. There are no other symptoms on leaves or small twigs of most Quercus species, although canyon live oaks, Q. chrysolepis, might have lesions on smaller twigs. While this browning of leaves can appear suddenly, it usually occurs after an extended period of disease, perhaps more than 2 years from the onset of a P. ramorum infection of the trunk.
Tanoak (Notholithocarpus densiflorus)
Tanoak is highly susceptible to P. ramorum, and the disease can infect and kill all sizes and ages—seedlings, saplings, and mature trees. P. ramorum infects trunks, branches, twigs, leaves, and leaf petioles (the slender stems that support leaves). Experiments on tanoak trees revealed they could be infected without showing cankers or bleeding symptoms, making diagnosis difficult. When visible, trunk cankers are similar to those of the red oak group. Death can occur with a sudden browning of the leaves, as with the red oaks, or over time with gradual leaf loss. P. ramorum infection in twigs can lead to shoot tip dieback and wilting. Shoot tip wilting, or flagging, can be useful in identifying trees that are infected but not showing bleeding symptoms. Tanoaks, unlike Quercus species, can produce spores from infected twigs, which can then be dispersed both within the tree and to neighboring susceptible plants, potentially causing new infections. Infected trees with brown foliage are effectively dead, although there might be some sprouting from the tree bases. Many of these new shoots are likely to become infected within a growing season.
Just a few foliar hosts, listed below, support pathogen populations large enough to spread P. ramorum to susceptible oaks and tanoaks. For a more complete list and description of nonoak hosts and symptoms, see Sudden oak death and associated diseases caused by Phytophthora ramorum (PDF) (Davidson et al. 2003) and Nursery Guide for Diseases of Phytophthora ramorum on Ornamentals: Diagnosis and Management (PDF) (Tjosvold et al. 2005). Please note that there are many potential causes of leaf spots on each of these hosts, so these symptoms descriptions should be used only as a guideline.
California bay laurel (U. californica)
On California bay laurel, P. ramorum causes leaf spots, usually brown tips surrounded by a halo of yellow. Lesions typically are found where water collects on the leaf. This is generally its tip, although a leaf spot can develop elsewhere where water rests on the surface. Bay laurel are not thought to die from P. ramorum infection, but these trees are a major source of inoculum for the pathogen and appear to play an important role in spreading disease to other plants in California.
Rhododendron (Rhododendron species)
Leaf spots are the main symptom on rhododendrons, although more severe effects have been noted in some cases. Lesions penetrate through the plant tissue so that spots are identical both on the top and bottom of the leaf. They are often triangular and extend along the leaf midvein, but they can appear anywhere water collects on the leaf surface such as along edges, near the petiole, and at the leaf tip. Leaf spots have diffuse margins and might appear water soaked. In severe cases, twigs, stems, or entire plants can die.
Camellia (Camellia species)
Camellia symptoms usually are limited to leaf spots, which can vary in size from 1/4 inch in diameter to covering nearly half the leaf, depending on environmental conditions. Lesions usually are on the leaf tip or leaf edge, and diffuse margins or thick black zone lines can surround them. Plants will drop their infected leaves, and the lower part of the plant can defoliate. Occasionally flowers or buds will be affected. Tip dieback or small branch cankers have not been observed on Camellia species.
Many common maladies other than P. ramorum infections can cause damage that resembles Sudden oak death. Other possible causes include boring insects, oak root rot (Armillaria mellea), root and crown rots (e.g., P. cinnamomi), physical injury, wetwood bacteria, and inappropriate cultural practices such as summer irrigation. (See Pests of Landscape Trees and Shrubs.) Figures 10 (for trees growing in wild areas or landscapes), 11 (for plants that just came from a nursery), and 12 (for trees that are dead) provide assistance in determining whether P. ramorum is a likely cause of the symptoms on your plants.
Once you have determined that P. ramorum is a probable cause of the symptoms you have observed, the final step in getting a confirmed diagnosis is to submit the symptomatic plant material to a laboratory for testing. Even if you are concerned about the health of an oak, because the pathogen is more readily isolated from leaves than trunks, it is better to focus sampling efforts on foliar hosts such as bay trees that surround your oak. Following are two methods for collecting and submitting a sample for a more thorough P. ramorum diagnosis.
Foliar sampling involves collecting about 10 symptomatic leaves from a foliar host (typically bay laurel) and submitting them for analysis. It is perhaps the easiest and most reliable way to determine if P. ramorum is present at a given location. Because infestation levels vary tremendously throughout the state, it is best to contact your local Cooperative Extension or County Agricultural Commissioner’s office for information on how sampling is being handled in your area.
While bark sampling is the only way to directly confirm disease on an infected oak, this procedure is invasive, it requires special equipment from the laboratory, and improper sampling techniques will greatly influence results. If you do decide to collect a bark sample, you must request sampling materials in advance from the laboratory. It is recommended you work with a trained sampler.
Phytophthora ramorum affects different species in different ways. It can be lethal to trunk hosts and madrone (Arbutus menziesii) saplings, while it might cause only a minor leaf or needle disease for the numerous foliar hosts. Depending on a number of factors, some trees might never become infected, some might become infected and survive for various lengths of time, and others might become infected and die quickly. In a few, rare cases, trees have recovered on their own.
Because Sudden oak death is a relatively new disease in California, it will take time to determine how likely different outcomes are for different tree species. Initial observations tell us that once infected, tanoak has a high probability of P. ramorum killing it, but some trees will survive. Coast live oaks appear to have a lower mortality rate than tanoaks, although the disease has killed many of them. There is little mortality information on California black oak at this time, so it is difficult to predict how this species will fare.
Trees growing in a mixed woodland or forest environment seem more likely to become infected by P. ramorum, as it spreads naturally in forest settings along coastal California. Oaks in residential landscapes seem less likely to be infected with Sudden oak death, because they usually are not growing as close to foliar hosts such as California bay laurel. An exception would be when horticultural hosts such as rhododendrons and camellias are growing close to oak trunks.
Once Sudden oak death infects oak trees, there is no known way to cure them. Therefore, most of the management practices discussed below are directed at preventing the spread of the disease to new plants or areas and protecting susceptible trees before they are infected.
Inspecting Nursery Plants Before Making a Purchase
Many common horticultural plants are hosts for P. ramorum, and nurseries in California, other states, and other countries have found the pathogen on their plants. Plants are shipped all across the country, but they are strictly regulated. All P. ramorum host plants in California’s regulated counties must be inspected and approved prior to shipment out of the regulated area, although sales within the regulated zone of 14 counties are not. In either case, carefully inspect the leaves of host plants for symptoms before making a purchase. Nurseries often use general fungicides that can mask P. ramorum symptoms, and some plants might have asymptomatic or latent infections that might not be visible at the time of purchase. Even if you do not see signs of infections when you make a purchase, consider quarantining the new plant in moist area of your yard for up to 8 weeks to see if symptoms manifest before you transplant it. You might want to refrain from planting any of these horticultural hosts near susceptible oaks in your yard.
Removing Infected Oaks
A tree with Sudden oak death needs to be considered and treated differently than a tree without the disease, but the disease alone is not justification for removal. In some cases, oak trees infected with the disease can remain relatively healthy for some time. Since data indicate nonoak foliar hosts actually spread the pathogen, removing infected oak trees probably will have little or no impact on local disease levels and spread. However, an important consideration with respect to any tree is whether it presents a hazard to life or property. All trees present some hazard, depending on the tree’s structural integrity and its potential to do harm should it die or portions of it break off. Preliminary research has shown that trees P. ramorum has infected or killed are prone to rapid decay and unpredictable failure. Green infected trees and trees already dead from P. ramorum and/or secondary pests are at an increased risk of trunk and limb breakage.
The decision to remove a hazardous tree ultimately lies with the property owner. In order to get an objective assessment of hazardous conditions, contact a certified arborist or other qualified professional. While a dead tree has an increased risk of causing damage, consider leaving it standing if there is not a risk to life or property, such as when the tree is in a natural area. Standing dead trees provide important wildlife habitat, and after they fall and decay, they are a source of nutrients to be recycled into the soil.
Always consult regulatory officials regarding local tree ordinances before deciding to remove trees. Experienced tree service technicians should conduct tree felling, as infected trees might have an abundance of structural wood decay. If there is an acute emergency, contact your city arborist or local fire or police department.
Removing Nonoak Host Trees
Large-scale removal of nonoak host plants is not a recommended way to prevent disease spread at a residential level. However, selective removal and/or pruning of these foliar hosts when they are in close proximity to uninfected, susceptible oaks might be helpful in preventing particular oaks from becoming infected, especially if there are few other disease hosts nearby. For more information, see Sudden Oak Death and Residential Oak Care: Protecting Trees in Advance of Local Disease Establishment.
Disposing of Plant Debris
Since P. ramorum has been present in many areas of coastal California for a decade or longer, complete eradication is impossible. However, the disease is not uniformly distributed, and there are still many areas that remain uninfested. If infested plant materials are moved, they inadvertently can transfer the pathogen to uninfested areas. Disposal of infested material is extremely important, because branches, twigs, and leaves from California bay laurel, rhododendron, and other host plants can harbor P. ramorum, even after they are removed from the plant. In infested areas, the best option is to leave infested material on site, chipping the small material for use as ground cover and using larger pieces for firewood. Since inoculum levels already are thought to be high in these infested areas, leaving the additional inoculum from the infested plant material on site will not significantly worsen local disease conditions. Composting also can successfully kill the pathogen, but the compost must reach temperatures that probably are not possible or practical in a home-composting site.
Removing plant debris from the property is recommended only if it is the first infected tree detected in the area or if fire risk is high. If infected wood is removed from your property, make sure it is utilized or disposed of in a way that does not spread the disease. Avoid leaving wood next to roads where it could be picked up and transported off site by unauthorized parties. Regulations prohibit the movement of host plants and plant parts out of the quarantined area. If you have infected trees cut down, make sure the wood and other tree parts are not moved outside of the quarantine area.
Sanitation Measures to Minimize Pathogen Spread
As a precaution against spreading the pathogen, clean and disinfect pruning tools after use on confirmed or suspected infested trees or in known infested areas. Sanitize pruning tools before pruning healthy trees or working in a pathogen-free area. Clean vehicles and shoes of mud, dirt, leaves, and woody debris before leaving a P. ramorum-infested site and before entering a site with susceptible hosts.
Replanting After Removing an Infected Tree
If you want to replant, it is important to choose a tree that will suit your needs and adapt well to the site. There are many resources available that can guide you in making the right choice. Check to see if there are any local ordinances or guidelines that govern tree replacement or planting.
Resistance to P. ramorum in oak trees is just beginning to be explored. Resistant planting stock is not available at this time nor is it known if it ever will be available. Coast live oaks do not seem to be infected by P. ramorum until they reach about 4 inches in diameter, so small, new trees should be immune for a number of years, and high value trees can be preventatively treated once they reach a susceptible size. (See Preventative Phosphonate Treatments below.) Species in the white oak group (e.g., valley oak, Garry oak, and blue oak) are not susceptible to P. ramorum. If you have space for replanting many trees, consider replanting the lost species in combination with other trees that do not get the disease. Then, if some trees succumb to P. ramorum there still will be others that survive.
Preventative Phosphonate Treatments
One phosphonate fungicide, Agri-Fos, is registered as a preventative treatment for P. ramorum for use on individual, high-value tanoak and oak trees. Treatment is not recommended in areas where infested plants are not already present. This treatment is not a cure, but it can help protect trees from infection and suppress disease progression in very early infections. The phosphonate compound can be injected or mixed with a surfactant and sprayed on the trunk for absorption through the bark. Booster treatments need to be made every 1 to 2 years.
Since the treatment must be made to healthy trees and the pathogen’s distribution and activity is patchy and somewhat unpredictable, it is difficult to determine which trees need treatment. Generally, you should consider treating healthy, high-value oak or tanoak trees within 150 feet of other infested plants. You also might want to treat healthy, high-value oaks or tanoaks if they are surrounded by healthy California bay laurel and there are known infections within 150 to 1,000 feet. For more information, see Sudden Oak Death and Residential Oak Care: Protecting Trees in Advance of Local Disease Establishment.
Using insecticides to treat or prevent P. ramorum infections provides no control and is not justified. However, treating individual, high-value landscape trees displaying early bleeding symptoms of Sudden oak death might be justified to control damage from secondary bark beetle attacks. If using an insecticide, apply it only if the disease is not at an advanced stage and with the realization it might prolong the life of the tree only for a relatively short time. For more information, see Pest Notes: Bark Beetles.
WARNING ON THE USE OF PESTICIDES
California Oak Mortality Task Force
UC Berkeley Forest Pathology and Mycology Laboratory
Garbelotto, M. and D. J. Schmidt. 2009. Phosphonate controls sudden oak death pathogen for up to two years. California Agriculture 63(1):10–17.
———. 2002b. Phytophthora ramorum and sudden oak death in California: I. host relationships. (PDF) In R. Standiford and D. McCreary, eds. Fifth Symposium on California Oak Woodlands. Albany: Pacific Southwest Research Station, USDA Forest Service. Publ. PSW-GTR-184. pp. 733–740.
Seybold, S. J., T. D. Paine, and S. H. Dreistadt. Nov. 2008. Pest Notes: Bark Beetles. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 7421.
Pest Notes: Sudden Oak Death
UC ANR Publication 74151
Authors: J. M. Alexander, UC Cooperative Extension, Marin Co.; and S. V. Swain, UC Cooperative Extension, Marin Co.
Produced by UC Statewide IPM Program, University of California, Davis, CA 95616
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Sudden Oak Death and Ramorum Blight1
Philip F. Harmon and Carrie L. Harmon2
Sudden oak death and ramorum blight are emerging diseases capable of causing a range of symptoms, from leaf spots to plant death, on many woody hosts. Because these diseases are emerging, much about the pathogen, host range, and disease epidemiology is currently being researched. The pathogen was introduced into the state through ornamental plant commerce, but eradication efforts have reduced the pathogen to below detectable levels. Only the foliar dieback disease ramorum blight has occurred in Florida in the past; no oak trees have died in Florida from sudden oak death. Currently, neither sudden oak death nor ramorum blight is established in Florida, but state agencies and university educators continue to work together to monitor for these diseases. If you suspect ramorum blight or sudden oak death in Florida, please contact your local UF/IFAS Extension agent or submit a sample through a Florida Extension Plant Diagnostic Center (http://edis.ifas.ufl.edu/sr007).
Causal Agent and Geographic Distribution
Sudden oak death is caused by Phytophthora ramorum. The historical origin of the pathogen is unknown, but it was first described in Europe on ornamental Rhododendron spp. and Viburnum spp. in 2001. In 2002, the pathogen was reported in California and Oregon and has since been found in western Canada. In 2004, the pathogen was inadvertently shipped on infected ornamental plant material throughout the United States.
Various symptoms have occurred on more than 100 species of native and cultivated ornamental plants infected with the pathogen (). The various symptoms are not exclusive because many plant species have not yet been tested. It is likely that species closely related to susceptible hosts also could be infected by P. ramorum.
Symptoms of these diseases vary from host to host; however, roots of plants infected with P. ramorum typically appear healthy. Symptoms may progress rapidly after infection or may not be visible for significant periods of time. Symptom progression is favored by temperatures near 20°C. Ramorum blight is a foliar blight and dieback disease, and sudden oak death is a trunk canker-causing disease of certain species of oak and related trees.
depending on the orientation on the plant), along the midvein, or around the margin. Lesions may first appear water soaked, and a water-soaked margin may be visible on rapidly expanding lesions (Fig. 1). Blighted leaf tissue typically turns tan to brown and may have a reddish tinge (Fig. 2). Lesions commonly expand from the midvein in an angular fashion (Fig.3).
Leaf lesions on Kalmia latifolia (moutain laurel). Note the symptom occurs at the leaf tip, and the lesion has a water-soaked border.
Credit: Plant Management Network
Tan to reddish brown leaf lesions typical of sudden oak death on Camellia spp.
Credit: Plant Management Network
Angular lesions caused by P. ramorum on leaves of Camellia spp.
Credit: Plant Management Network
Cankers are sunken or swollen lesions on the branches and trunks of woody plants. Cankers may occur beneath the bark and can be difficult to distinguish. Cankers caused by P. ramorum often ooze red, sticky sap (Fig. 4). Some bacterial pathogens can cause sap to bleed from cankers, but sap from these cankers has a foul odor. Vascular discoloration is revealed when the bark and outer cambial layer are removed (Fig. 5). Vascular discoloration on mature Lithocarpus densiflorus (tan oak) is characteristically bright red (Fig. 6).
Bleeding canker symptom on coast live oak caused by P. ramorum.
Credit: Plant Management Network
Vascular discoloration of Rhododendron spp. caused by P. ramorum.
Credit: Plant Management Network
Canker and vascular discoloration on a mature tan oak.
Credit: Plant Management Network
Shoot Tip and Stem Blight
Stem blight often begins at a shoot tip and progresses toward the base of the plant, but infection may occur at any point on a stem and move up or down the plant. Blighted stems appear brown to black and may be killed, resulting in death of attached leaves (Fig. 7). Early symptoms of shoot tip blight may include the formation of a “shepherd’s crook” (Fig. 8).
Stem dieback of Rhododendron spp. caused by P. ramorum.
Credit: Plant Management Network
Shepherds crook symptom on tan oak sapling.
Credit: Plant Management Network
Viburnum spp. are among the most susceptible hosts and typically exhibit wilting symptoms that can mimic drought stress. As symptoms progress, individual branches and eventually the entire plant may collapse and die.
Spread of the Pathogen and Control Options
The fungus can be spread by movement of infected host material, infested soil, irrigation water, and wind-blown rain. Unintentional movement of infected but asymptomatic nursery stock is a potential means of pathogen dissemination. Because this is an emerging pathogen, the best option for controlling spread of these diseases is preventing the introduction and establishment of the pathogen in new areas. Quarantines and eradication programs in conjunction with extensive surveys are the most effective way to deal with potential introductions. Infected plant material should be destroyed by burning or deep burial in a landfill. Composting infected plant material is not recommended because it is difficult to reach temperatures that are high enough to destroy the fungus.
Fungicides are recommended in nurseries where these diseases occur. Fungicides can prevent disease caused by this and other Phytophthora spp., but fungicide resistance in the pathogen has been observed. See “Additional Resources” below for more information.
This document is PP197, one of a series of the Plant Pathology Department, UF/IFAS Extension. Original publication date March 2004. Revised August 2014. Visit the EDIS website at http://edis.ifas.ufl.edu.
Philip F. Harmon, associate professor, Plant Pathology Department; and Carrie Harmon, associate director, Southern Plant Diagnostic Network, Plant Pathology Department, UF/IFAS Extension, Gainesville, FL 32611.
The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county’s UF/IFAS Extension office.
U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.
Sudden Oak Death/Ramorum blight
Symptoms on infected rhododendron
To date, Sudden Oak Death has not been found infecting oak trees in Maryland. The name “sudden oak death” is somewhat misleading and the disease is increasingly being referred to as “ramorum blight”, or “ramorum dieback”. Popular landscape plants including rhododendron, azaleas, camellias, viburnum, mountain laurel (Kalmia),and lilacs purchased and planted in Maryland since March 2003 could be providing a safe harbor for the pathogen. The disease was brought into Maryland on infected plants from the west coast. In April of 2004, in the course of conducting routine surveys for P. ramorum, Maryland Department of Agriculture (MDA) discovered rhododendrons testing positive for the pathogen in a retail garden center.
Until 2003, P. ramorum was believed to be confined to native plants in the environment in northern coastal California, Oregon, and British Columbia in North America. In 2002, USDA imposed a quarantine that now includes 13 counties in California and an area in Oregon to prevent its artificial spread through commerce. In 2003, plants in Oregon and Washington nurseries tested positive for the pathogen. Although the introduction was contained and eradicated, concerns about the role of nursery plants in spreading the disease mounted.
In Maryland, if you suspect a plant is infected with ramorum blight contact the University of Maryland Home and Garden Information Center (HGIC) for assistance.
Adapted from August 11, 2004, Invasive Species of Concern in Maryland-Carol Holko
Sudden Oak Death, (SOD, Phytophthora canker disease, Ramorum blight) Phytophthora ramorum is a devastating disease that has killed thousands of trees in California. Sudden oak death is caused by Phytophthora ramorum, which is a fungus-like pathogen that spreads by sending spores from infected leaves, and twigs.
Sudden Oak Death Photo gallery
Symptoms on liliac
Symptoms on viburnum
Symptoms on Kalmia “Minuet”
Symptoms on Douglas fir
Symptoms on Rhododendron
Symptoms on blueberry
USDA APHIS Phytophthora ramorum
Nineteen citizen science-based SOD Blitzes (largest number of blitzes to date) were held this spring, two of which were new this year – one in Trinity County and one on Kashia Band of Pomo Indian land in Mendocino County. The 504 volunteers surveyed nearly 10,000 trees from San Luis Obispo County, north to Mendocino and Trinity counties. Each volunteer was trained to identify Phytophthora ramorum (the plant pathogen known to cause SOD) symptoms on California bay laurel and tanoak leaves. “Blitzers” had up to three days to collect and record locations of symptomatic samples, which were then sent to the Garbelotto lab for DNA analysis to determine the presence or absence of the pathogen.
The SODMap mobile app shows a map of sampled trees. SOD management workshops
Garbelotto is sharing results from the spring blitzes as well as new recommendations for SOD management at workshops being held around the Bay Area. Workshops will be held in Sebastopol on Nov. 3, in Berkeley on Nov. 4, and in San Rafael on Nov. 13. For details, see “Community meetings” at sodblitz.org.
For landowners in infested areas concerned about protecting their oak trees, Garbelotto will reveal his updated three-step SOD management plan. He will show them how to:
- Use the SODMap mobile app to help assess risk of oak infection (see sodmapmobile.org).
- Determine if California bay laurel trees near high-value oaks should be considered for removal (using a new buffer zone new chart – http://nature.berkeley.edu/garbelottowp/?page_id=2345).
- Apply phosphonates to high-value oak and tanoak trees to boost immunity (updated dosages and application frequencies at http://nature.berkeley.edu/garbelottowp/?page_id=2348).
Infection on California bay laurel and tanoak leaves indicates arrival of P. ramorum to an area, but true oak (California black oak, coast live oak, canyon live oak and Shreve’s oak) infection typically requires a couple of years with wet conditions after pathogen arrival. Therefore, preventatively treating oaks to help ward off infection is best done when early indicator species first show symptoms, prior to oak infection and optimal conditions for the pathogen – cool and moist.
These surveys are made possible thanks to funding from the USDA Forest Service and the PG&E Foundation as well as help from the California Native Plant Society.
For more information on the workshops, go to sodblitz.org or contact Katie Harrell at (510) 847-5482 or [email protected] For more information on sudden oak death and P. ramorum, visit the California Oak Mortality Task Force website at suddenoakdeath.org.
Sudden Oak Death
Sudden Oak Death: Prevention is Key
Sudden Oak Death, or SOD, has caused widespread dieback of a variety of oak species throughout California and Oregon. Homeowners who notice trunk cankers, spots on leaves and shoot dieback should seek assistancefrom an ISA certified arborist to help protect their landscapes.
What is Sudden Oak Death?
Caused by a water mold known as Phytophthora ramorum, SOD can have a fatal effect on the following species:
- Coast live oak
- California black oak
- Shreve’s oak
- Canyon live oak
It is also the culprit of twig and foliar diseases in other plants, including Coast redwood and Douglas-fir.
How does it spread?
SOD spores can disseminate via blowing rain or flowing water. They can also be on soil or items such as shoes, tires or pet’s paws. Additionally, SOD can be transported through shipments of ornamental plants.
Identifying symptoms may not be enough to confirm SOD is present. Laboratory confirmation by a local county agriculture department or a university extension office may be required. And while no cure exists, a preventative treatment can be applied to oaks and tanoaks.
Do you think your tree may be affected by Sudden Oak Death? Contact your local certified arborist for a free consultation
Sudden Oak Death: What can be done to save your oaks?
The fungus commonly known as Sudden Oak Death or SOD (Phytophthora ramorum) is fatal to many oak species including coast live oak, sheve oak, interier live oak, holly oak, and black oak, among others. The fungus is spread during cool, moist periods of spring and fall. The fungus acts quickly on the vascular tissue of the tree, blocking the tree’s ability to transport nutrients and water to its canopy and roots. Bleeding cankers are often associated with SOD infections. Large trees can succumb to the fungus rather quickly, causing them to die and require removal. Once a tree has become infected, usually there is not much that can be done to save it.
Pictured above (clockwise from top left): dying tree; SOD-affected trees; bleeding kankers
What can we do?
Prevention is the key to keeping trees from becoming infected. One form of prevention is to ensure that host plants are not touching healthy oaks. This is difficult to do because the host list is about 100 species strong and growing. Applying a preventative systemic treatment to the base of susceptible trees in fall and spring is the best course of action. The treatment acts upon the Sudden Oak Death fungus if it is present and disables its ability to spread and multiply.
Pictured above: California Bay tree leaves with SOD – species is highly likely to carry and spread SOD
Arborwell’s team of ISA Certified Arborists are experts on Sudden Oak Death prevention.
(sudden oak death (SOD))
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Beales PA, Brokenshire T, Barnes AV, Barton VC, Hughes KJD, 2004. First report of ramorum leaf blight and dieback (Phytophthora ramorum) on Camellia spp. in the UK. Plant Pathology 53(4):524
Beales PA, Schlenzig A, Inman AJ, 2004. First report of ramorum bud and leaf blight (Phytophthora ramorum) on Syringa vulgaris in the UK. Plant Pathology 53(4):525
Bilodeau GJ, Lévesque CA, Cock AWAMde, Brière SC, Hamelin RC, 2007. Differentiation of European and North American genotypes of Phytophthora ramorum by real-time polymerase chain reaction primer extension. Canadian Journal of Plant Pathology, 29(4):408-420. http://pubs.nrc-cnrc.gc.ca/tcjpp/plant.html
Brasier C, Webber J, 2010. Plant pathology: Sudden larch death. Nature, 466(7308):824-825
CABI/EPPO, 2006. Phytophthora ramorum. Distribution Maps of Plant Diseases, No. 978. Wallingford, UK: CAB International
Cobb RC, Chan MN, Meentemeyer RK, Rizzo DM, 2011. Common Factors Drive Disease and Coarse Woody Debris Dynamics in Forests Impacted by Sudden Oak Death. Ecosystems, DOI: 10.1007/s10021-011-9506-y
COMTF, 2017a. http://www.suddenoakdeath.org/library/ California Oak Mortality Task Force, 5 pp
Davidson JM, Hansen EM, Garbelotto M, Reeser P, Rizzo DM, 2002. Another canker-causing Phytophthora on oaks and tanoak in forests of California and Oregon. Phytopathology, 92:S18
Davidson JM, Rizzo DM, Garbelotto M, 2002. Phytophthora ramorum and Sudden Oak Death in California: II. Pathogen transmission and survival. In: Standiford R, McCreary D, eds. 5th Symposium on California Oak Woodlands, USDA Forest Service. Gen. Tech. PSW-GTR-184, 741-749
EPPO, 2019. EPPO Global Database. Paris, France: EPPO.https://gd.eppo.int
Florida Department of Agriculture and Consumer Services, 2004. Florida Department of Agriculture and Consumer Services. Florida, USA. http://www.doacs.fl.us
Garbelotto M, 2003. Composting as a control for sudden oak death disease. BioCycle, 44(2):53-56
Garbelotto M, Davidson JM, Ivors K, Maloney PE, Hnberli D, Koike ST, Rizzo DM, 2003. Non-oak native plants are main hosts for sudden oak death pathogen in California. California Agriculture, 57(1):18-23; 15 ref
Garbelotto M, Rizzo DM, Hayden K, Davidson JM, Tjosvold S, 2002. Phytophthora ramorum and Sudden Oak Death in California: III. Pathogen genetics. In: Standiford R, McCreary D, eds. 5th Symposium on California Oak Woodlands, USDA Forest Service. Gen. Tech. PSW-GTR-184, 765-774
Garbelotto M, Rizzo DM, Marais L, 2002. Phytophthora ramorum and Sudden Oak Death in California: IV. Chemical control. In: Standiford R, McCreary D, eds. 5th Symposium on California Oak Woodlands, USDA Forest Service. Gen. Tech. PSW-GTR-184, 811-818
Garbelotto M, Svihra P, Rizzo DM, 2001. Sudden oak death syndrome fells three oak species. California Agriculture, 55:9-19
Gillman, D., 2011. Fact Sheet: Phytophthora ramorum. https://ag.umass.edu/landscape/fact-sheets/phytophthora-ramorum
Goheen EM, Hansen EM, Kanaskie A, McWilliams MG, Osterbauer N, Sutton W, 2002b. Eradication of sudden oak death in Oregon. Phytopathology, 92:S30 (abstract)
Gomes Mde J, Amaro PT, 2008. Occurence of Phytophthora ramorum in Portugal on Viburnum spp. (Ocorência de Phytophthora ramorum em Portugal sobre Viburnum spp.) Revista de Ciências Agrárias (Portugal), 31(2):105-111
Grünwald NJ, Garbelotto M, Goss EM, Huengens K, Prospero S, 2012. Emergence of the sudden oak death pathogen Phytophthora ramorum. Trends of Microbiology, 20(3):131-138
Hall KM, Albers HJ, 2009. Economic Analysis for the Impact of Phytophthora ramorum on Oregon Forest Industries. Unpublished report. Department of Agricultural and Resource Economics, Oregon State University, 97331-3601. Corvallis, Oregon, USA: Department of Agricultural and Resource Economics, Oregon State University, 14.
Hansen EM, Sutton W, 2002. Log inoculations to assess tree susceptibility to sudden oak death. Phytopathology, 92:S33 (abstract)
Hayden KJ, Nettel A, Dodd RS, Garbelotto M, 2011. Will all the trees fall? Variable resistance to an introduced forest disease in a highly susceptible host. Forest Ecology and Management, 261(11):1781-1791.
Hicks, C. B., 2018. 26 pp. Unpublished Report. USDA-APHIS, Raleigh, USA
Hüberli D, Sant-Glass Wvan, Tse JG, Garbelotto M, 2003. First report of foliar infection of starflower by Phytophthora ramorum. Plant Disease, 87(5):599; 2 ref
IPPC, 2006. IPP Report No. NL-5/1. Rome, Italy: FAO
Kelly NM, Meentemeyer R, 2002. Landscape dynamics of the spread of Sudden Oak Death. Photogrammetric Engineering and Remote Sensing, 68:1001-1009
Lane CR, Beales PA, Hughes KJD, Tomilson JA, Inman AJ, Warwick K, 2004. First report of ramorum dieback (Phytophthora ramorum) on container-grown English yew (Taxus baccata) in England. Plant Pathology 53(4):522
Lee C, Valachovic Y, Garbelotto M, 2010. Protecting trees from sudden oak death before infection. University of California, Agricultural and Natural Resources Publication 8426, 8426:14
Linderman RG, Parke JL, Hansen EM, 2002. Relative virulence of Phytophthora species, including the sudden oak death pathogen P. ramorum, on several ornamental species. Phytopathology, 92:S47 (abstract)
Maloney PE, Rizzo DM, Koike ST, Harnik TY, Garbelotto M, 2002. First report of Phytophthora ramorum on coast redwood in California. Plant Disease, 86(11):1274; 2 ref
McPherson BA, Wood DL, Storer AJ, Kelly NM, Standiford RB, 2002. Sudden oak death: disease trends in Marin County plots after one year. In: Standiford R, McCreary D, eds. 5th Symposium on California Oak Woodlands. USDA Forest Service. Gen. Tech. PSW-GTR-184, 751-764
Meentemeyer, RK, Rank NE, Shoemaker D, Oneal C, Wickland AC, Frangioso KM, Rizzo DM, 2008. Impacts of sudden oak death on tree mortality in the Big Sur ecoregion of California. Biological Invasions, 10:1243-1255
Moralejo E, Werres S, 2002. First report of Phytophthora ramorum on Rhododendron sp. in Spain. Plant Disease, 86(9):1052; 4 ref
NAPPO, 2016. http://www.pestalert.org/oprDetail.cfm?oprID=646&keyword=PHYTOPHTHORA%20RAMORUM
Orlikowski LB, Szkuta G, 2002. First record of Phytophthora ramorum in Poland. Phytopathologia Polonica, No.25:69-79; 10 ref
Parke JL, Linderman RG, Hansen EM, 2002. Susceptibility of Vaccinium to Phytophthora ramorum, cause of the sudden oak death pathogen. Phytopathology, 92:S63 (abstract)
Rizzo DM, Garbelotto M, Davidson JM, Slaughter GW, Koike ST, 2002a. Phytophthora ramorum and Sudden Oak Death in California: I. Host Relationships. In: Standiford R, McCreary D, eds. 5th Symposium on California Oak Woodlands, USDA Forest Service. Gen. Tech. PSW-GTR-184, 733-740
Rizzo DM, Garbelotto M, Davidson JM, Slaughter GW, Koike ST, 2002b. Phytophthora ramorum as the cause of extensive mortality of Quercus spp. and Lithocarpus densiflorus in California. Plant Disease, 86(3):205-214; 34 ref
Rizzo DM, Garbelotto M, Hansen EM, 2005. Phytophthora ramorum: integrative research and management of an emerging pathogen in California and Oregon forests. Annual Review of Phytopathology, 43:309-335. http://www.annualreviews.org
Rytkönen A, Lilja A, Vercauteren A, Sirkiä S, Parikka P, Soukainen M, Hantula J, 2012. Identity and potential pathogenicity of Phytophthora species found on symptomatic Rhododendron plants in a Finnish nursery. Canadian Journal of Plant Pathology, 34(2):255-267. http://www.tandfonline.com/loi/tcjp20
Sansford CE, Inman AJ, Baker R, Brasier C, Frankel S, Gruyter Jde, Husson C, Kehlenbeck H, Kessel G, Moralejo E, Steeghs M, Webber J, Werres S, 2009. Report on the risk of entry, establishment, spread and socio-economic loss and environmental impact and the appropriate level of management for Phytophthora ramorum for the EU. Deliverable Report 28. Forest Research, Central Science Laboratory, York, UK. EU Sixth Framework Project, RAPRA. York, UK: Forest Research, Central Science Laboratory, 310.
Shrestha, S. K., Zhou, Y., Lamour, K., 2013. Oomycetes baited from streams in Tennessee 2010–2012. Mycologia, 105(6), 1516-1523.
South Carolina Forestry Commission/USDA Forest Service, 2010. United States Department of Agriculture Forest Service, Southern Region, State & Private Forestry, Asheville, NC. 2 pp
Swiecki TJ, Bernhardt E, 2002. Evaluation of stem water potential and other tree and stand variables as risk factors for Phytophthora ramorum canker development in coast live oak. In: Standiford R, McCreary D, eds. 5th Symposium on California Oak Woodlands, USDA Forest Service. Gen. Tech. PSW-GTR-184, 787-798
Tooley PW, Englander L, 2002. Infectivity of Phytophthora ramorum on selected Ericaceous host species. Phytopathology, 92:S81 (abstract)
US Forest Service/Animal Plant Health Inspection Service/National Association of State Foresters/National Plant Board, 2011. National Framework for Managing Sudden Oak Death caused by Phytophthora ramorum in Forests and Wildlands.1-14. Last accessed December 13, 2017,
USDA-APHIS-PPQ, 2016. https://www.aphis.usda.gov/plant_health/plant_pest_info/pram/downloads/pdf_files/quarantine_map.pdf
Valachovic YS, Lee CA, Scanlon H, Varner JM, Glebocki R, Graham BD, Rizzo DM, 2011. Sudden oak death-caused changes to surface fuel loading and potential fire behavior in Douglas-fir-tanoak forests. Forest Ecology and Management, 261(11):1973-1986
Vettraino AM, Sukno S, Vannini A, Garbelotto M, 2010. Diagnostic sensitivity and specificity of different methods used by two laboratories for the detection of Phytophthora ramorum on multiple natural hosts. Plant Pathology, 59(2):289-300. http://www.blackwell-synergy.com/loi/ppa
WDNR/USFS, 2017. Forest Health Highlights in Washington—2016. Washington Department of Natural Resources (WDNR) Department of Agriculture, Forest Service (USFS). 37 pp.
Werres S, Marwitz R, 1997. Triebsterben an Rhododendron: Unbekannte Phytophthora. Deutscher Gartenbau, 21:1166-1168
WSDA, 2016. Annual Report – Sudden Oak Death (SOD) – Phytophthora ramorum. State of Washington, Department of Agriculture. 6 pp
Zerjav M, Munda A, Lane CR, Barnes AV, Hughes KJD, 2004. First report of Phytophthora ramorum on container-grown plants of rhododendron and virburnum in Slovenia. Plant Pathology 53(4):523
Sudden Oak Death: Overview
Sudden Oak Death, Phytophthora ramorum, Phytophthora ramorum Blight
Tree Solutions arborists have been involved in the recognition and treatment of Sudden Oak Death (SOD) disease since the early period of discovery, 1995-1998. Early on, we offered our assistance to the research effort in order to obtain a thorough understanding of this disease. We have treated thousands of trees and continue our efforts to help preserve one of our precious resources.
Sudden Oak Death mainly affects trees and shrubs along the Pacific coast beginning in central California from the Big Sur coast extending northward into Curry County in Oregon. Although the pathogen mainly exists in coastal areas, it also extends inland. The disease is present in other parts of the United States and Europe, our focus is the California coastal areas from Monterey to Santa Clara counties. A comprehensive list of affected trees is available from the California Oak Mortality Task Force website.
The tree disease known as Sudden Oak Death was first reported by arborists, in Marin County in 1995. At that time it was primarily observed in tanbark oak trees. A Phytophthora infection was suspected because of the characteristic pattern of discoloration in the bark and sapwood of infected trees, but laboratory tests obtained by the UC Cooperative Extension and other sources consistently came back negative for any known disease organism. The primary sign of the disease recognized at that time was a rapid wilting and browning of the foliage and death of the tree, hence the name Sudden Oak Death or SOD.
The phenomenon became known as “SOD Syndrome” because the affected trees showed signs of multiple associated disease and insect pest invasions. One of the principle factors in the rapid decline of affected trees was heavy infestation of oak bark beetles and/or ambrosia beetles; tiny borers that are known invaders of stressed oak trees. Because of the lack of definitive identification of a causal disease organism (pathogen), and because of the increasing association of beetle activity with SOD, emphasis on control during the late 1990s was focused on beetle control.
By 1998 and 1999, SOD in Coast Live Oak trees became prevalent and was reported in Sonoma, Santa Cruz and other counties. The loss of live oaks became a much more serious event than the loss of the less-valued tanbark oaks.
SOD task forces were formed and serious research began. Finally in 2000, a UC forest pathologist, Dr. Matteo Garbelotto, implemented an advanced laboratory process that resulted in the positive identification of the apparent causal pathogen for SOD as Phytophthora ramorum.
Many concerned arborists from several counties offered their assistance to the research teams. Tree Solutions president Jim Neve and arborist associate Don Cox were involved in the early process of discovery and field work supporting research on SOD.
Don Cox reported one of the first cases of SOD in Marin County in 1995 and participated in collaboration among scientists and tree care professionals with the first “oak task force”. He alerted scientists to new areas of infection and organized work parties for the dissection of infected trees in the field and collection of samples for lab analysis.
Jim Neve volunteered his arboricultural and pest control expertise for field work and assistance to the research teams, collecting samples and utilizing new injection technique with systemic fungicides, including potassium phosphonate, the treatment that would become the current standard for control of SOD.
Species of Phytophtora (Sudden Oak Death)
There are over 100 known species and mating types of Phytophthora to date. Phytophthora ramorum is the terrestrial form of the water mold pathogen. It is in the class of Oomycetes from the Kingdom Stramenopila. Literally translated from Greek origin; Phyto,-phyte (plant) and phthora (destruction) or “plant destroyer”. It thrives in cool moist environments.
Most of us were first introduced to this pathogen through history recollections of the Irish Potato Famine. The pathogen Phytophthora infestans is responsible for the death and migration of thousands of Irish citizens when it destroyed a large percentage of their staple food source, the potato.
The pathogen destroys the plant growing tissue called cambium as well as the vascular system phloem and xylem cells which are responsible for tree life. The presence of the pathogen in this tissue causes girdling, which eventually leads to blight.
Reddish brown sap oozing from the tree trunk is a wound response to the pathogen. It usually takes the form of droplets. It is one of the visual diagnostic indicators of disease presence. A thorough site and tree examination and assessment as well as an experienced understanding of the disease complex is very important for diagnosis, several other pathogens produce similar bleeding symptoms.
Removing the bark over an infection area reveals dark zonal lines and diseased tissue within the zonal borders.
An oak may be infected up to one year or more before symptoms appear. In our experience, once a bleeding lesion appears, tree mortality of coast live oaks can follow approximately two years later. We have also experienced trees infected by Phytophthora ramorum and ambrosia beetles to the point of girdling of over 50% of a tree circumference that stay green and healthy appearing after four years of treatment. This in field observation proves that there are no hard and fast rules that apply to each and every tree.
Beetles invade diseased or stressed trees. It is their job in the forest to seek and destroy these trees. It is said that weakened trees emit a chemical, signaling stress, thus attracting the beetles. Beetles are attracted to the canker area. Just like the pathogen, these beetles girdle infected trees. However, they girdle at a much faster rate than the pathogen. If one is trying to preserve and protect a tree, it is important to protect from beetle attack as well as the pathogen itself.
Sawdust material (frass) is produced by one of three possible beetle species. Red frass is from the western oak bark beetle (Pseudopityophthorus pubipennis), and white frass is produced by ambrosia beetles (Monarthrum dentiger and M. scutellare).
Beetles can attack before Sudden Oak Death signs are present, so it is important to look for these pests during regular scheduled inspections.
The combination of Potassium Phosphonate and a surfactant which is sprayed onto the trunk as a drench, or P. Phosphonate alone injected into the tree vascular system was approved for treatment in October of 2003. It is the only approved treatment to date. It is not a cure and it is best to treat trees before infection occurs. The material has both an indirect and direct effect on the pathogen. It primarily helps a tree to boost its defense system which helps fight off infection.