SUDDEN OAK DEATH IN WASHINGTON
by Tom Wessels, Plant Services Program Manager
Washington State Department of Agriculture
It’s been a year and a half Since the USDA issued the ‘Emergency Federal Order Restricting Movement of Nursery Stock From California, Oregon and Washington Nurseries’, and Phytophthora ramorum, the causal agent of sudden oak death, continues to take its toll on Washington nurseries. There are currently 11 WA nurseries undergoing eradication procedures for P. ramorum with only a third of the 2006 survey completed.
The federal order regulates the interstate movement of nursery stock from Washington and the non-quarantine areas of Oregon and California. Nurseries in the regulated areas that produce or sell P.ramorum host plants must be inspected, tested and found free of P.ramorum annually and enter into a compliance agreement with USDA in order to ship any plants out of state. There are approximately 155 nurseries under federal compliance agreement in WA, and an additional 135 approved WA non-host shippers. This includes not only the nurseries that ship out of state, but also nurseries that supply nurseries that ship out of state. Certified stock shipped interstate from the regulated areas can be identified by a federal certificate certifying the stock as free of P. ramorum. The purpose of the federal order is to prevent P. ramorum from being spread to non-infested areas and to allow the continued movement of nursery stock. As an emergency order, the federal order is scheduled to sunset three years after taking affect, but may be extended.
Prior to the federal order there was a patchwork of state quarantines for P. ramorum that varied widely from state to state, aimed at nursery stock and forest products from the West Coast. With support from the nursery industry, USDA implemented the federal order on January 10, 2005, preempting all state quarantines for P. ramorum.
Parallel to the federal order is the national survey for P.ramorum, which takes place in nurseries and forests throughout the country. The survey has detected infected plants in at least a dozen states. The 2006 survey has so far detected the pathogen in nurseries in WA, OR, CA, AL, CT, FL, GA, IN, ME and MS. The 2004 and 2005 surveys in WA detected 25 and 16 infected nurseries respectively.
When an infected plant is detected in a nursery, either during the compliance inspections for out of state shippers or during the national survey, the nursery immediately comes under federal regulation with a mandatory eradication protocol. The block containing the infected plant is destroyed (a block is defined as a contiguous block of host plants until there is a 2 m break of either no plants or non-host plants), all host plants within 10 meters of the infected block are placed on hold for 90 days, and all host plants on the nursery are held until they can be inspected and found free of P. ramorum-like symptoms. In most infected nurseries additional infected plants are detected during the 90-day hold period and the eradication process is lengthened.
Nurseries receiving host plants from Oregon or California will see a federal certificate with each shipment certifying the stock as is in compliance with the federal order. Unfortunately being in compliance with the federal order does not necessarily mean the plants are free of P. ramorum. The sampling protocol for nurseries under the compliance agreement is designed to detect P ramorum at a 0.5% infection rate, if sampling is biased towards symptomatic plants. Since most production nurseries have good disease control programs, there are very few symptomatic plants and a minimum number of samples are taken, decreasing the chance of detecting the pathogen. Like most plant pathogens, P. ramorum likely survives in apparently healthy plant material at low levels but will grow rapidly once conditions are right.
Most of the infected nurseries in Washington are retail garden centers and most of the infected plants detected originated from Oregon or California nurseries under the USDA compliance agreement. In all cases trace back inspections have failed to detect the pathogen at the supplying nursery even though it’s almost certain some of the plants were infected prior to shipping. In other nurseries it appears the pathogen was not eradicated from previous year’s infections. In most infected nurseries however, it’s not possible to tell the origin of the infection. What is certain is that P. ramorum has the ability to multiply and spread rapidly in the retail nursery environment.
The purpose of the federal order is to reduce the risk of introducing and spreading P.ramorum in infected nursery stock. Because compliance inspections continue to detect infected plants in previously uninfected nurseries and the national survey continues to detect infected plants in nurseries around the country, it’s fair to say the federal order has fallen short of its objective.
To further reduce the risk of nursery stock from the regulated area spreading P. ramorum, the National Plant Board has recommended an enhanced certification protocol for high risk genera (Camellia, Rhododendron, Viburnum, Pieris and Kalmia) which make up the vast majority of infected plants. In the nurseries under compliance agreement, these plants would be inspected monthly by trained nursery personnel and tested and inspected quarterly by state regulators. USDA will probably revise the federal order to include the NPB recommendations by the end of the year. It’s hoped that this enhanced certification protocol will reduce the number of infected plants in the industry.
In Washington where plants of the high-risk genera are an important part of the nursery trade, P. ramorum has not decreased since implementation of the federal order. WSDA has detected 63 infected plants this year resulting in the destruction of 5,765 and the survey is only a third complete. In addition to infected plants, our survey crew is also finding P. ramorum in soil samples taken near infected plants and in water samples from a seasonal stream flowing through a nursery. They are also finding nurseries infected for the second and third time. Of the 16 nurseries found infected in 2005, 9 had been infected the previous year. As part of the USDA eradication protocol infected nurseries must be re-inspected for 2 years. As a result after a nursery is found positive and completes the eradication process there’s a good chance it will be found positive again in subsequent years.
Although a retail nursery can never be certain that purchased plants are free of P. ramorum, there are certain measures that can be taken to minimize the impact if the pathogen is found. For example placing host plants in small distinct blocks separated by at least 2 m; not intermingling host plants carried over from previous years with plants brought in recently; buying host plants from a single supplier or keeping host plants from different suppliers separated; not placing host plants on the same site as previously infected plants; and inspecting and isolating incoming plants. It’s also important to notify WSDA of all incoming shipments of woody plants as required by WAC 16-402.
It’s likely that P. ramorum will be part of the nursery landscape for the foreseeable future. The nursery industry and many state regulators are urging USDA to extend the federal order before it sunsets on January 10, 2008. However certification based on inspecting and testing tends to be expensive and is never totally effective. Up to this point USDA has funded both the enforcement of the federal order and the national survey. The proposal to increase the inspections from annually to quarterly could dramatically increase the cost of the federal order at the same time federal money for P. ramorum is decreasing. At some point the nursery industry will be expected to pick up more of the cost. The cost of inspecting and testing a large nursery would be as high as $10,000. A reasonable alternative to continuing the federal order would be implementation of a clean stock program similar to programs already in use for other nursery commodities such as caneberries, grapevines and fruit trees. These are fee for service programs based on propagation from known healthy plants and isolation from all sources of infection. Clean stock programs require minimal testing and are very effective in eliminating pathogens.
All WA nurseries will be affected by the continued regulation of P. ramorum and should be aware of the options facing the industry as funding continues to diminish, while the risk of P. ramorum being introduced into the environment remains unabated. Detection of P. ramorum in the forest may not necessarily be environmentally significant but will certainly result in severe restrictions on the movement of WA nursery stock and forest products.
