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James Susaimuthu inspects plant material in the Fruit Tree Clean Plant Center's greenhouse.

James Susaimuthu inspects plant material in the Fruit Tree Clean Plant Center’s greenhouse.

One of the goals of Dr. James Susaimuthu, new program manager of the Fruit Tree Clean Plant Center, is to use his diagnostic skills to find ways to improve virus testing of plant material.

Susaimuthu, with a background in plant virology and pathology, joined the Fruit Tree Center in January to take over management duties from Dr. Bill Howell, who has been with the program for more than 20 years. Howell is serving as program consultant until he fully retires.

The Fruit Tree Center is a program with roots going back to 1955 when Washington State University’s Irrigated Agriculture and Research Extension Center in Prosser began hosting what was then called the Interregional Project No. 2. After a series of diseases decimated the tree fruit industry in the 1940s, causing thousands of trees to be removed, the national project was created to help keep devastating diseases out of the orchards and provide clean planting material to industry.

Through the years, the name of the program has changed, from IR-Project 2 to the National Research Support Project No. 5, to the newest Fruit Tree Clean Plant Center, which is part of the Fruit Tree Clean Plant Network. From the beginning, the national program sought to centralize the costly activities associated with testing and producing virus-tested plant material and eliminate duplication of efforts from various states.


The Fruit Tree Clean Plant Network is the result of fruit tree and grape industries working together to form the umbrella organization, the National Clean Plant Network. The umbrella network was created by Congress a few years ago to coordinate the efforts of regional facilities that provide services and house working plant collections for certified planting stock. The network has designated 16 clean plant centers across the nation to represent fruit and nut trees, grapes, hops, berries, and citrus. Three are dedicated to fruit trees, including the Clean Plant Center of the Northwest in Prosser.

The Prosser center provides diagnostic and clean plant services for fruit trees, hops, and grapes. Dr. Ken Eastwell is director of the Clean Plant Center and also manages the hops network, Susaimuthu manages the fruit tree network, and Gary Ballard manages the Northwest Grape Foundation Service, the grape clean plant network.

The Prosser fruit tree program is the nation’s main fruit tree clean plant center, and is responsible for the import and export of new fruit tree selections. The other two fruit tree clean plant centers are located at the University of ­California, Davis, and Clemson University in South ­Carolina.

The Prosser program has developed such a reputation for its rigorous virus testing and virus elimination therapy that Israel and New Zealand require that imported fruit tree selections must be cleared first by Prosser, according to Susaimuthu. “In Israel, if you want to import tree fruit varieties from Italy, you have to send them first to us for screening,” he said.

Susaimuthu explained that center’s mission is threefold:

  • to safeguard the fruit tree industry from diseases
  • to provide opportunities for industry and nurseries to import new stone and pome fruit varieties and ­selections in a timely and safe manner
  • to improve virus testing and therapy techniques through research

The center receives a small portion of its funding through fees collected for the importation, screening, and diagnostic services, but federal funding is needed to help subsidize the program. He said that in the past, WSU provided some funding for the center, but under the new national framework, the network is federally funded.

Research needs

Susaimuthu hopes to identify unknown plant viruses, improve virus diagnostic techniques, and enhance disease-elimination therapies through new research. When the Good Fruit Grower interviewed him in late February, he had already requested funding from the U.S. Department of Agriculture for research to develop a cherry rootstock resistant to cherry virus A, an emerging pathogen of ­quarantine significance.

“We have some viruses, like cherry virus A, that are proving difficult to eliminate by heat therapy, and we need to find other techniques to eliminate them,” he said.

Plants use gene-silencing mechanisms naturally to combat virus infections, Susaimuthu explained. By inserting the cherry virus A gene into the rootstock, the gene would produce a silencing signal that would inhibit replication of the virus. Because the silencing signal is mobile within the plant, it could be transported from the rootstock to infected scion wood grafted onto the rootstock and aid the plant in stopping accumulation of cherry virus A.

“In a few weeks, we could end up with virusfree growth of the budwood that could then be grafted to the rootstock of choice for commercial multiplication,” he said, adding that process would remove viruses that are difficult to eliminate by heat therapy without genetically modifying the budwood.

Heat therapy has been an effective technique for eliminating viruses, particularly for pome fruit. It involves propagating a plant and growing it in a special chamber heated to 100°F for 80 days. Just as a fever is the human body’s mechanism to fight infection and viruses, the high temperatures help the plant thwart and slow down the virus, he explained. “It doesn’t totally eliminate virus from the plant, but the plant grows faster than the virus, and we can collect the last inch of the tip of plant tissue before the virus reaches it. The tip, which is clean, is then grafted to healthy rootstock.”