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Disease Management

Because Phytophthora ramorum is a quarantined pathogen that affects plants in both forests and nurseries, disease management is complex. It requires cooperation among federal, state, and county agencies as well as private industry to protect natural resources while not unduly limiting nursery commerce. The difficulty of disease management is compounded by our incomplete knowledge of the pathogen’s biology. As more is discovered about the pathogen, more effective control strategies can be developed.

Diagnosis and Monitoring
It is important to quickly and accurately identify symptoms caused by P. ramorum in the nursery, forest, or landscape. Since symptoms vary by host and are not diagnostic, best results have been achieved by combining polymerase chain reaction (PCR) based assays with the more traditional method of isolating the pathogen from symptomatic tissue on the selective laboratory medium PARP. Enzyme-linked immunosorbent assay (ELISA) kits designed for field detection of Phytophthora species can be a useful screening tool.

Nursery growers must systematically inspect plants and propagative material to prevent introduction of the pathogen to existing stock. In the field, monitoring must continue in both infested and non-infested areas. The use of aerial and ground surveys, stream baiting in high-risk watersheds, and monitoring by trained members of the community must all be used in concert. Effective education and communication are essential for preventing the spread of P. ramorum. Programs and training sessions have been established for nursery employees, Master Gardeners, Extension personnel, arborists, landscapers, utility workers, recreationalists, and homeowners. Web-based resources include photos of symptoms on hosts, questionnaires to help determine the need for submitting a sample, and instructions for collecting a sample.

APHIS and the U.S. Forest Service have conducted several national surveys to estimate the geographic distribution of P. ramorum in nurseries and forests. At the end of 2004, the pathogen had been detected on nursery stock in 21 states and British Columbia. In 2005, 99 nursery sites in 7 states were found to be positive for the pathogen. Despite these findings, national surveys of forest lands and nursery perimeters have not detected the pathogen outside of California and southwest Oregon.

Exclusion
The most effective way to prevent P. ramorum from becoming established is to keep it out of non-infested areas. Preventing human-facilitated spread of the disease is critical. Evidence from Europe suggests that infected ornamental plants can spread Phytophthora species to the forest. Therefore, P. ramorum is a quarantined pathogen, and international and interstate movement of hosts and associated hosts in California, Oregon, and Washington is federally regulated. Much of this entails nursery inspections and testing in quarantined areas. Nursery certification programs have been established to ensure that nurseries are in compliance with preventive protocol. Specific “best management practices” (BMPs) are recommended to prevent establishment and further spread of the disease. BMPs include monitoring all known hosts for symptom development, inspecting all incoming plant stock for symptoms of disease, and confirming that all incoming stock originates from a shipping nursery that has a USDA SOD compliance agreement or has been certified pathogen-free. For a list of BMPs, refer to the California Oak Mortality Task Force Website. Movement of infected logs (including firewood), infested soil, and litter outside infested counties are all strictly prohibited. Even within quarantined counties, one should not move infested materials. To prevent further spread through California forests, signage and devices for cleaning shoes and bicycles are being installed along trailheads in infested areas.

Avoidance
As part of BMP strategies to avoid introduction of P. ramorum, growers are encouraged to segregate the two highest-risk genera, rhododendron and camellia, from the rest of their nursery stock. They are also asked to avoid conditions that are conducive to the development of Phytophthora diseases by improving drainage and irrigating in such a way as to minimize the period of leaf wetness. Nurseries in infested areas should also remove potential native hosts, especially bay laurel trees, from their property.

Eradication
Eradication may be a viable option if the disease is caught relatively early after introduction. For example, the state of Oregon established a proactive eradication program in 2001 in an attempt to control P. ramorum in Curry County in the southwest corner of the state. All hosts within 100 feet (approximately 30.5 m) of symptomatic hosts are cut, piled, and burned. Cut stumps are also treated with herbicides to reduce sprouting. Sites are then monitored for two years to confirm that the eradication process has been successful. Although the pathogen has still spread to new sites in Curry County, the infection rate appears to be less than in similar forests in northern California where eradication programs were not implemented. Approximately 579 ha (1,430 acres) in Curry County have been treated to date (Figure 25). This type of eradication program is not feasible if the disease is already widespread, or if infected vegetation is intermingled with residences.

Figure 25

Figure 26

In all nurseries, infected plants must be destroyed as part of the eradication program. If infected plants are found in a particular nursery, corresponding nurseries receiving or sending these plant materials are also inspected as part of the APHIS protocol. Infected material at these sites, as well as plants within a buffer zone of 2 m, are then destroyed (Figure 26). Healthy-appearing plants within 10 m of infected plants are held for observation for a period of time to see if symptoms develop.

On a smaller scale, eradication of inoculum from soil or contaminated containers can be achieved through treatment with aerated steam (65°C for 30 min) or by treatment with soil fumigants. Two facilities in California are also certified to “hot compost” green waste that is infested with P. ramorum.

Protection
Protection consists of treating a healthy plant before it comes in contact with the pathogen. An important aspect of fungicide application in nurseries is to ensure that treated plants are not already infected with the pathogen. Most fungicides do not kill Phytophthora; rather they temporarily suppress symptom development, delaying detection until after the plants are shipped. Therefore, if fungicides are used, they should only be used to prevent infection and should not be applied to potentially infected plants. Several chemicals, including mefenoxam, dimethomorph, pyraclostrobin, and fenamidone, may protect nursery hosts such as rhododendron from infection. Permitted uses vary for different hosts and in different states and countries. While fungicide resistance has not yet been reported in the U.S., resistance of P. ramorum to metalaxyl has been reported in Europe. There is still more work to be done to determine dosages, application timing, and hosts to be treated. Preventive treatment of forest trees with fungicides is not economically feasible, nor is it permitted on public lands. Individual high-value trees on private property may be treated with phosphite fungicides by direct injection or by mixing them with organosilicate surfactant-penetrants applied to the bark. Both methods have been shown to reduce disease development on canker hosts, and these chemicals are registered in California.

Resistance
There is considerable variation in host resistance among genera and species of susceptible plants. For example, significant variation in susceptibility among individual oaks, tanoaks, and bay laurel has been observed. Variation in virulence has also been seen among U.S. isolates of P. ramorum. So far host specificity has not been found for particular isolates. Of the many plants that have been tested for susceptibility to P. ramorum in laboratory studies, very few species appear to be immune.

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