The establishment of the glassy-winged sharpshooter (GWSS), Homalodisca coagulata, in southern California in the 1990s introduced a dramatically more dangerous element into the way that Pierce's disease (PD) spreads in California. Basically, where the GWSS is abundant, the spread of PD appears to have changed from a "simple interest" (linear) rates of disease increase to "compound interest" (exponential) rates of spread.
Compound interest diseases reflect the plant to spread of pathogens (in this case the bacterium Xylella fastidiosa) from plant to plant within the same crop. Our best explanation for why PD does has not reflected the vine-to-vine spread of the bacterium in California is that the vine-to-vine phase of bacterial spread established infections that were usually eliminated by dormant pruning. In other words, in "traditional" PD in California, the bacterial infections that are established during the summer months (when bacteria appear in numbers high enough for vectors to pick up the bacteria from grapes) do not travel far enough downward by fall to reach a part of the vine that will remain after pruning. This may have been because our "traditional vectors" in California have a strong preference for feeding at the tips of grwoing stems. We think that GWSS changes this situation because adult GWSS regularly feed low on canes and even on 2 or 3-year old wood. This allows GWSS to establish infections during the summer that pruning will not eliminate. According to this version (hypothesis) of how PD has spread in California in the past, most vector transmissions of Xylella fastidiosa to grapevines occur during the summer months, but do not result in chronic disease, but in infections that do not usually survive winter pruning.
For traditional PD in the North Coast, control consists of avoiding those infections that result in chronic disease. Such infections are established mainly during the early spring, mostly April-May. Control is aimed at keeping the blue-green sharpshooter (BGSS) out of vineyards during these months. Control of BGSS during summer months or eliminating vines with PD from vineyards has not shown any effect in reducing future losses to PD compared to vineyards where vectors were not controlled or diseased vines removed. GWSS should change this.
Removing vines with PD and other sources of Xylella fastidiosa may be essential to controlling PD where GWSS is present.
Control of GWSS throughout the growing season rather than just the spring months will be important to slowing the spread of PD.
GWSS has different plant preferences than BGSS. Irrigated ornamental landscapes are likely to be major sources of GWSS if this insect becomes established in the North or Central Coasts. Natural vegetation may also be a source of GWSS because GWSS has a very wide range of plants on which it will feed and reproduce. However, we cannot yet predict what the kinds of natural vegetation may support significant reproduction of GWSS.
The control of BGSS in the North Coast focuses on the favored habitats of BGSS near vineyards. Vegetation management to minimize the occurrence of plant species that this insect finds attractive during the period from March through May is one approach [link here to Information Manual: Riparian Vegetation Management for Pierce's Disease in North Coast California Vineyards].
Another tactic is the use of insecticides to reduce movements of BGSS into vineyards from habitats such as ornamental landscapes [link to ornamental guidelines] or natural vegetation [link to Guidelines for the North Coast].
More information on GWSS can be found at
http://www.CNR.Berkeley.EDU/xylella/oss.html
The California Dept. of Food and Agriculture web site for latest news
on GWSS at
http://plant.cdfa.ca.gov/gwss/
A site maintained by UC IPM Specialist Phil Phillips at