Kern County Grape Growers Not Immune to New Problems

Don Luvisi

UCCE Farm Advisor, Kern County

Viticulture - Grapes

November 24, 1999

KERN COUNTY GRAPE GROWERS NOT IMMUNE TO NEW PROBLEMS
(Glassy-winged Sharpshooter Leafhopper)

Kern County grape growers should not become complacent about Pierces Disease (PD) "Xylella fastidiosa" which has presented grape growers in Temecula and the Napa Valley with costly vine losses. All grapes (raisin, table and wine) are susceptible to PD. The current situation is very critical since the introduction of a highly efficient vector, the "Glassy Winged Sharpshooter Leafhopper" (GWSS). Currently, the "Blue Green Sharpshooter Leafhopper" although a vector of PD, feeds close to the edges of a vineyard and at the tips of the shoots. Most of the new infections are pruned out each year, since 90% or more of the one year old wood is removed.

The GWSS is much larger (½ inch), can fly 1/4 mile and feeds on green and dormant canes closer to the base of the vine. Feeding close to the base of the vine makes it difficult, if not impossible, to prune out the infections. Its ability to move to the center of a vineyard increases vine to vine spread throughout a vineyard thus rapidly increasing the number of infected vines.

Kern County is ripe for an increase of PD. The vector GWSS and PD are both present in various areas of Kern County. There has been isolated infections of PD in the Shafter area and in vineyards east of Earlimart. Infections have not been of epidemic proportions but the PD is present in Kern County. In the 1960s, PD was usually found near alfalfa fields. Alfalfa dwarf is caused by the PD organism Xylella fastidiosa. At this time we do not know to what extent alfalfa dwarf is present in alfalfa fields. GWSS has been found in an area east of Lamont and in the area of Hwy 166/I-5. It is only a matter of time before the GWSS meets PD, becomes infected and feeds on Grapevines. Vineyards next to citrus groves will probably be infected first since the GWSS moves from the vines in the winter to citrus, then moves back to the vines in the spring.

What can be done besides wait. Return to weed free vineyards. Blackberries (California and Himalayan), elderberry, wild grape, Periwinkle ground cover, and Umbrella sedge are systemic and propagative hosts to PD. Systemic hosts have the PD bacterium systemic throughout the plant while propagative host means that the bacterium is in the host only through feeding in a localized area and the PD can be spread when a blue-green or GWSS leafhopper feeds on the infected area.
This is just a sketch of what is becoming a major problem to the grape industry. Further information can be obtained from an excellent web site run by Dr. Purcell at www.cnr.berkeley.edu/xylella.