Vineyard Weeds Found To Host Pierce's Disease Of Grapes

Pierce's disease is caused by Xylella fastidiosa, a bacteriumtransmitted by sharpshooters and spittlebugs. In response to outbreaksof Pierce's disease in central California, plant pathologists studied29 weed species commonly found in California's San Joaquin Valley tosee if the bacterium could survive on the weeds. Perennials and knownfeeding and breeding hosts of the glassy-winged sharpshooter weretested first, then plants particularly abundant in or near vineyards.

"Our objectives were to determine the fate of Pierce's diseaseinfections in previously untested plant species associated withsouthern San Joaquin Valley vineyards, and compare survival of theinfections in selected field and greenhouse-grown plants," saidChristina Wistrom, staff research associate in the Department ofEnvironmental Science, Policy and Management, University of California,Berkeley, CA.

The study revealed that environmental conditions have a majorimpact on bacterial growth in host plants. "Multiplication and systemicmovement of X. fastidiosa varied among different plant species andenvironmental conditions, so weed species in vineyards must beevaluated on an individual basis to determine their potentialcontribution to Pierce's disease," Wistrom said. "Currently, Pierce'sdisease is controlled by reducing populations of the insect vector,either through insecticide sprays or habitat modification to removeinsect breeding host plants. Our study reinforces the need for weedcontrol in irrigation ditches and roadsides adjacent to vineyards, inregions with chronic Pierce's disease and established populations ofsharpshooters, especially in warm weather," she said.

The researchers recovered X. fastidiosa from 27 of 29 speciesin greenhouse tests. Sunflower, cocklebur, annual bur-sage, morningglory, horseweed, sacred datura, poison hemlock, and fava bean weremost frequently infected. "Our study confirmed that plant speciescannot be simply classified as either 'hosts' or 'nonhosts' of X.fastidiosa, but vary considerably among plant species in supportinggrowth and movement of the bacterium," Wistrom said. In addition, shenoted that the joint lab and field experiments showed thatenvironmental conditions strongly influenced how rapidly the bacteriamultiplied within the plants.

A full article is available in the September 2005 issue of PlantDisease. Published by The American Phytopathological Society (APS),Plant Disease is a leading international journal of applied plantpathology. APS is a non-profit, professional scientific organization.The research of the organization's 5,000 worldwide members advances theunderstanding of the science of plant pathology and its application toplant health.