Diseases of grape (
Pierce's disease
Pierce’s Disease is caused by a bacterium, Xylella fastidiosa. This bacterium is unusual since it is found only in the xylem or water conducting tissues of the plant. It is transmitted by sharpshooters, a type of leafhopper insect. The strain of Xylella fastidiosa that infects grape also infects many other plant species, but many are symptomless and serve as reservoirs from which the sharpshooter can transmit the bacterium. There is little spread of the bacterium within a vineyard, so weeds and other hosts in surrounding areas are probably the major source of the pathogen.
The bacteria block the flow of water through xylem vessels causing yellowing and scorching of the leaves that begins on the outer margins and progresses inward (photo 1). Early symptoms may be easily confused with other problems, especially excess salt. When leaves fall, the petiole may often remain attached (photo 2). Susceptible varieties may die within a few months to a few years after infection. Disease is usually restricted to warm areas but has been found in Arizona in both Pinal County and in the wine grape growing areas of Santa Cruz County near Sonoita and Elgin.
Disease control consists of planting tolerant varieties that are certified to be free of the bacterium and avoiding weed infestations. Thompson Seedless, White Riesling, Sylvaner, Chenin blanc and Ruby Cabernet are tolerant to disease, but others such as Mission, Barbera, Pinot Noir and Pinot Chardonnay are very susceptible. Only grape varieties with good tolerance should be planted in an area in which disease has been reported or is good habitat for sharpshooter breeding such as riparian areas and weedy irrigation ditch banks.
Reduction of disease has been reported in Southern California by reducing populations of sharpshooters, particularly the glassy winged sharpshooter, using insecticides in a regional eradication effort. Also, a foliar protectant, Surround (a kaolin clay based material) that deters sharpshooters and leafhoppers from feeding, has reportedly been effective in reducing transmission of the bacterium.
Other sites for information include:
http://www.ipm.ucdavis.edu/PMG/r302101211.html
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September 20 , 2002