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Sudden Oak Death Education

2016 summer projects

 2016 Summer Projects – WSU Ornamental Plant Pathology

These projects all involve Phytophthora spp., a group of destructive plant pathogens that move on plants, soil, and water to infect new hosts. We are interested in techniques for removing pathogen inoculum and reducing the damage caused by these organisms as well as learning new host/pathogen combinations.

Steam treatment of soil, pots, and media for removing pests and pathogens

  1. Movement of Phytophthora ramorum inoculum in the soil profile

The recommended depth for treating soil to remove inoculum of P. ramorum is 30 cm, however there is little research to support this. A 2007 study by Dart et al. showed that P. ramorum was detected in soil cores from three WA nurseries at depths of organic layer, 0-5 cm, and 5-10 cm. Soil at 10-15 cm was sampled and no P. ramorum was found. No sampling was done at depths >15 cm. If P. ramorum did not move to the depth of 30 cm, much time and money could be saved on steam treatment, as reaching the target temperature of 50C at 30 cm depth is problematic, and impossible in some soil types such as saturated clay soils.

Soil properties influence the ability to steam treat a soil and the time needed. Moisture, compaction, and presence of organic matter are a few of these properties. Container areas in many ornamental nurseries are located over soils that are unsuitable for in-ground production, which usually means that they are compacted.

Results from steaming soil at a nursery in 2014 and a public garden in 2015 show that Phytophthora spp. can be detected from soil samples taken at 20-30 cm beneath the soil surface. No P. ramorum was detected in the samples from 2014 and identification of the Phytophthora species in the 2015 samples is being done. Phytophthora species found at 20-30 cm depth in 2014 include P. cambivora, P. inflata, P. gonapodyides, and an unidentified species.

Information from this study will determine whether shorter steaming times could be used in P. ramorum infested soil. It will also provide data on the movement of inoculum in different soil conditions that can be found at nurseries.

Some preliminary testing will be done in an APHIS-approved biocontainment lab at WSU Puyallup. All tests will be conducted using colonized rhododendron leaf inoculum with isolate 886, the NA1 P. ramorum isolate used at NORSDUC. Colonized leaf inoculum will be placed on the surface of columns containing the material to be tested. Leaf inoculum will be replaced periodically so that a continuous level of inoculum can be maintained. Irrigation will be applied to the top of the columns following a normal nursery watering schedule.

Small scale columns (10 cm diam x 60 cm high) will be used to assess P. ramorum movement under various soil conditions in the biocontainment lab. Three depths of soil will be tested: 0-5 cm, 0-15 cm, and 0-30 cm. Treatments will consist of soil amended with four levels of organic matter (compost): none (0%), low (1%), med (5%),  and high (10%). Soil at four levels of compaction (none, low, med, highly compacted) will be evaluated. Soil compaction will be done using a modification of the Proctor method (ASTM 2007), which involves dropping a weight from a specific height a determined number of times onto the soil surface. Controls will include columns containing pea gravel to test for movement of inoculum in the absence of soil. A subset of these treatments will be selected for further, large scale testing at NORSDUC. These will be chosen based on analysis of data from the small scale column tests, selecting variables that have the most influence on movement of P. ramorum in the soil profile.

  1. Effectiveness of steam for treating used pots and other materials

Recycling used containers, flats, and trays are critical control points where weeds and diseases may infest nursery crops. Most growers may choose to re-use these containers to reduce the costs of buying new materials, but this is a risky practice unless they are properly sanitized. Disinfestation of these items is often overlooked or compromised in nursery production because of the labor and time involved. A recommended method for disinfesting containers before re-use is treatment with a disinfectant solution. Commonly used chemical disinfectants such as chlorine bleach or quaternary ammonium compounds are inactivated by organic matter, so clinging soil and other material needs to be removed by scrubbing or power washing prior to disinfectant treatment. Heat treatments such as hot water dip or steaming do not require that the pots be cleaned first, thereby eliminating one step in the process. Steaming may prove to be more labor-saving and less costly than chemical disinfectants or hot water dip tanks for disinfecting containers that will be re-used. Composting has the potential to effectively eliminate weed seeds and pathogens in recycled media, but most nurseries do not have the facilities to properly compost media so that all of the media reaches the required temperatures.

In this project we will sample pots, flats, and other materials from a commercial ornamental plant nursery for Phytophthora and weed seeds. Steam vs solarization treatments for pots will be evaluated. Materials will be sampled before and after steam or solarization treatment to determine whether it was effective.

  1. Soil steaming in Washington State

There is great interest in the use of steam to treat soil infested with Phytophthora and other pathogens, pests, and weeds, as an alternative to chemical fumigation. More nurseries with P. ramorum positive soil will take advantage of this treatment method, thus reducing the risk of pathogen movement in contaminated soil. Used containers are another critical control point where diseases may infest nursery crops, and cleaning these containers is often overlooked in nursery production. Steaming may prove to be a labor-saving and less costly means to disinfect containers that will be re-used. Steaming of used media is another application of this technique that growers have expressed interest in. As a result of this project, growers will be able to determine whether steaming infested nursery soil, pots, and media is a viable option in WA State, both economically and in terms of space/time.

We will continue replicated trials of steaming soil under varying temperature and moisture conditions at WSU-Puyallup. Three soil types are being evaluated (sandy loam, silt loam, and nursery gravel bed) under three moisture conditions (saturated (20-50%, vol/vol), field capacity (10-35%), and residual water content (1-10%)). Soil moisture will be determined on all plots at all depths before and after steaming. A tool will be created using environmental data for nurseries to use to determine the most appropriate time for steaming and the cost. The costs of steaming nursery soil under WA climate conditions will be calculated.

Phytophthora culture collection

The WSU Puyallup Phytophthora culture collection has hundreds of isolates of Phytophthora spp. that have been collected and used in various research projects. Maintenance of the culture collection involves periodic transferring of cultures to new media, record keeping, and other tasks. We also want to improve the database by including DNA sequence data and other information.

Koch’s Postulates

In our various projects we have discovered new host/pathogen combinations. In order to publish these Koch’s Postulates need to be completed. This involves the following steps:

  1. The pathogen should be found in hosts suffering from the disease, but should not be found in healthy hosts.
  2. The pathogen is isolated from a diseased host and grown in pure culture.
  3. Observe disease symptoms when the pathogen is introduced into a healthy host.
  4. Reisolate the pathogen from the inoculated, diseased experimental host and identify it as being identical to the original specific causative agent.

Some of the pathogen/host combinations:

A new Phytophthora species causing root disease on Fraser fir. We may test it on other fir spp. also.

Phytophthora ramorum foliar disease on Oregon grape, salal, and Vaccinium sp.

P. hibernalis foliar disease on Oregon grape.

For more information or to volunteer contact

Marianne Elliott (Plant Pathologist ,

SOD, nursery research, soil and water management, stream monitoring

Lucy Rollins (Volunteer Coordinator, )