Vineyard weeds found to host Pierce’s disease of grapes

New research just released in the September issue of Plant Disease suggests that weeds commonly found in California’s wine country may enable the spread of Pierce’s disease of grapes, one of the most destructive plant diseases affecting grapes.

Pierce’s disease is caused by Xylella fastidiosa, a bacterium transmitted by sharpshooters and spittlebugs. In response to outbreaks of Pierce’s disease in central California, plant pathologists studied 29 weed species commonly found in California’s San Joaquin Valley to see if the bacterium could survive on the weeds. Perennials and known feeding and breeding hosts of the glassy-winged sharpshooter were tested first, then plants particularly abundant in or near vineyards.

“Our objectives were to determine the fate of Pierce’s disease infections in previously untested plant species associated with southern San Joaquin Valley vineyards, and compare survival of the infections in selected field and greenhouse-grown plants,” said Christina Wistrom, staff research associate in the Department of Environmental Science, Policy and Management, University of California, Berkeley, CA.

The study revealed that environmental conditions have a major impact on bacterial growth in host plants. “Multiplication and systemic movement of X. fastidiosa varied among different plant species and environmental conditions, so weed species in vineyards must be evaluated on an individual basis to determine their potential contribution to Pierce’s disease,” Wistrom said. “Currently, Pierce’s disease is controlled by reducing populations of the insect vector, either through insecticide sprays or habitat modification to remove insect breeding host plants. Our study reinforces the need for weed control in irrigation ditches and roadsides adjacent to vineyards, in regions with chronic Pierce’s disease and established populations of sharpshooters, especially in warm weather,” she said.

The researchers recovered X. fastidiosa from 27 of 29 species in greenhouse tests. Sunflower, cocklebur, annual bur-sage, morning glory, horseweed, sacred datura, poison hemlock, and fava bean were most frequently infected. “Our study confirmed that plant species cannot be simply classified as either ’hosts’ or ’nonhosts’ of X. fastidiosa, but vary considerably among plant species in supporting growth and movement of the bacterium,” Wistrom said. In addition, she noted that the joint lab and field experiments showed that environmental conditions strongly influenced how rapidly the bacteria multiplied within the plants.