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Site preparation is the first step to preventing pests and diseases from becoming a problem in the vineyard, and in the case of crown gall disease, that approach seems to be working.

Dr. Ken Eastwell, plant pathologist at Washington State University’s Irrigated Agriculture Research and Extension Center at Prosser, told growers at the British Columbia Wine Institute’s annual viticulture conference that greater awareness has helped prevent and control crown gall in Washington’s vineyards.

“The first defense is exclusion. You need to keep crown gall out of the site from the very beginning,” Eastwell said.

Crown gall usually follows a traumatic event, such as cold injury or damage by equipment. Rhizobium vitis (formerly known as Agrobacterium vitis) causes root lesions and inhibits the movement of water and nutrients.

The disease can cause grape production to drop by 40 percent per vine, even if girdling affects only 50 percent of the vine.

The cost to the grower can be significant, Eastwell said. Research on a sample of 200 vines at Pennsylvania State University estimated that lost sales would total $40,500. Removing and replacing those infected vines could add $6,000 to the financial impact of the disease.

Although crown gall can affect a wide range of plants, grapevines are particularly susceptible to the disease because they are a common host for most strains of R. vitis, Eastwell said. This facilitates transmission of the disease between vines and makes clean soil a particular focus of prevention and control efforts.

Site preparation before replanting could include a number of measures, Eastwell said. These include:

• removing as much old root material as possible to eliminate R. vitis, which may survive in old root tissue;

• fumigation to eliminate nematodes that may cause lesions on the rootstock and give bacteria a chance to infect the vine;

• letting a site lie fallow for two years.

“These control strategies are not absolutes,” Eastwell said. “It is a matter of percentages, and the lower the concentration of bacteria that survive in the soil, the lower are the chances that new vines will become infected.”