Sudden Oak Death - Control - Nursery Management

Nursery Management

Research and development in managing P. ramorum in nursery settings extends from P. ramorum in the individual plant, to P. ramorum in the nursery environment, to the pathogen’s movement across state and national borders in infected plants.

An array of studies have tested the curative and protective effects of various chemical compounds against P. ramorum in plants valued as ornamentals or Christmas trees. Many studies have focused on the four main ornamental hosts of P. ramorum (Rhododendron, Camellia, Viburnum, and Pieris). Several effective compounds have been found; some of the most effective include mefenoxam, metalaxyl, dimethomorph, and fenamidone. Many of these studies have converged upon the following conclusions: chemical compounds are, in general, more effective as preventatives than in curatives; when used preventatively, chemical compounds must be reapplied at various intervals; and chemical compounds can mask the symptoms of P. ramorum infection in the host plant, potentially interfering with inspections for quarantine efforts. In general, these compounds suppress but do not eradicate the pathogen, and some researchers are concerned that with repeated use the pathogen may become resistant to them. These studies and conclusions are summarized by Kliejunas.

Another area of research and evolving practice deals with eliminating P. ramorum from nursery environments in which it is established to prevent human-mediated pathogen movement within the ornamental plant trade. One way of approaching this is through a robust quarantine and inspection program, which the various federal and state regulatory agencies have implemented. Under the federal P. ramorum quarantine program implemented by USDA APHIS, nurseries in California, Oregon, and Washington are regulated and must participate in an annual inspection regime; nurseries in the fourteen infested counties in coastal California, plus the limited infested area in Curry County, Oregon, must participate in a more stringent inspection schedule when shipping out of this area.

Much of the research into disinfesting nurseries has focused on the voluntary Best Management Practices (BMPs) that nurseries can implement to prevent P. ramorum’s introduction into the nursery and movement from plant to plant. In 2008, a group of nursery industry organizations issued a list of BMPs that includes subsections on Pest Prevention/Management, Training, Internal/External Monitoring/Audits, Records/Traceability, and Documentation. The document includes such specific recommendations as “Avoid overhead irrigation of high-risk plants”; “After every crop rotation, disinfect propagation mist beds, sorting area, cutting benches, machines and tools to minimize the spread or introduction of pathogens”; and “Nursery personnel should attend one or more P. ramorum trainings conducted by qualified personnel or document self-training”.

Research on control of P. ramorum in nurseries has also focused on disinfesting irrigation water containing P. ramorum inoculum. Irrigation water can become infested from bay trees in the forest (if the irrigation source is a stream), from bay trees overhanging irrigation ponds, from runoff from infested forests, or from recirculated irrigation water. Experiments in Germany with three types of filters—slow sand filters, lava filters, and constructed wetlands—showed that the first two removed P. ramorum from the irrigation water completely, while 37% of the post-treatment water samples from the constructed wetland still contained P. ramorum.

Since P. ramorum can persist for an undetermined period of time within the soil profile, management programs in nurseries should also deal with delineating the pathogen’s distribution in nursery soil and eliminating it from infested areas. A variety of chemical options have been tested for soil disinfestation, including such chemicals as chloropicrin, metham sodium, iodomethane and dazomet. Lab tests indicated that all of these chemicals were effective when applied to infested soil in glass jars. Additionally, tests on volunteer nurseries with infested soil demonstrated that dazomet (trade name Basamid) fumigation followed by a 14-day tarping period successfully removed P. ramorum from the soil profile. Other soil disinfestation practices under investigation, or in which interest has been expressed, include steam sterilization, solarization, and paving of infested areas.