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Vacuum Steam Heat: A New Method to Kill Oak Wilt Fungus in Logs

Researcher inserting temperature probes in oak wilt fungus-colonized oak logs prior to vacuum steam treatment. Photo by Jennifer Juzwik, USDA Forest Service

International export of oak logs from the United States requires fumigation of the logs with methyl bromide. However, methyl bromide causes severe damage to the Earth's ozone layer. Scientists recently discovered that using vacuum steam heat to treat oak logs kills the oak wilt fungus and is environmentally friendly.

The oak wilt fungus (Bretziella fagacearum), which is only known to exist in the eastern United States, is a significant cause of oak death in the region. To contain the fungus, officials established quarantines to regulate movement of oak logs to other regions and countries. Over 30 years ago, fumigation of oak logs with methyl bromide became the standard practice to kill the fungus and prevent its spread. When methyl bromide was found to be a serious threat to the Earth's ozone layer, scientists began looking for alternative treatments that would kill the fungus while not causing damage to the environment. In recent studies, a Northern Research Station scientist working in collaboration with USDA Animal and Plant Health Inspection Service, Plant Protection and Quarantine, and university colleagues tested the feasibility of using vacuum steam to treat oak logs. Prior to treatment, the fungus was consistently isolated from an average of 18 percent of log samples taken from naturally-infected red oaks and 38 percent of those taken from artificially-inoculated trees. Treatments involved raising the temperature of the wood to a 2-inch depth to 133 or 140 degrees Fahrenheit for 30 or 60 minutes, respectively. In no case was the fungus isolated from the study logs after treatment. Future work on this promising alternative to methyl bromide will focus on questions related to use for commercial-size log loads.



Research Partners

External Partners

  • University of Minnesota, Department of Forest Resources
  • Virginia Polytechnic Institute and State University
  • USDA Animal and Plant Health Inspection Service, Plant Protection and Quarantine