August 28, 2015
If you know grapes, you’ve heard of Pierce’s disease. This disease, caused by a strain of the bacterium Xylella fastidiosa, was discovered in 1892 by California’s first professional plant pathologist, Newton B. Pierce, but did not become a serious threat to our wine industry until 1996, when a new vector appeared in the Temecula Valley. A vector is an organism, such as a biting or sucking insect in this case, that transmits disease from one plant or animal to another. This pesky vector is a leafhopper from the family Cicadellidae — the glassy-winged sharpshooter.
We in Marin have other smaller sharpshooters that have passed along a few incidents of Pierce’s disease, but these outbreaks have been confined to vineyards that are close to the riparian areas (those tracts of land close to water) that harbor these little pests. But the introduction of the glassy-winged sharpshooter has raised the threat of Pierce’s disease throughout California to a dangerous level. This large leafhopper flies greater distances than other smaller leafhoppers, makes its home in a wider range of host plants, and sucks an incredible amount of moisture out of a plant, causing problems even if it is not carrying bacteria.
The glassy-winged sharpshooter is indigenous to southeast U.S., where, luckily for grape growers there, the native muscadine grapes (Vitis rotundifolia) have some resistance to Pierce’s disease, unlike the common grape vine (Vitus vinifera) that is prevalent here in California. The good news is that work is ongoing with some success at the University of California at Davis to breed the resistance to Pierce’s disease found in Vitis rotundifolia into Vitis vinifera. And, while there’s been some Pierce’s disease in Napa and Sonoma, growers in the region are channeling their resources to battle the insect to slow, if not stop, the disease’s march northward into California’s premier grape growing regions.
Xylella fastidiosa. Sounds like a magical spell straight off the pages of a Harry Potter book, but there is nothing cool about it. This plant pathogen is causing problems around the world, and impacting important industries such as Georgia’s peach orchards, Brazil’s citrus groves, Italy’s olive trees, as well as California’s vineyards. This bacterium lives and grows in the xylem, the plant’s vascular system. This tissue conducts water from the roots to all parts of the plant. Drying and browning begin to appear on leaves when the bacteria block the xylem. Disease is spread when insects like the sharpshooter suck up infected xylem fluid, then travel to feed on another plant. There is no cure and it is unusual for an infected plant to survive.
Different strains of Xylella fastidiosa induce severe diseases in many other agricultural and ornamental plants. A strain of the bacterium is causing oleander leaf scorch in Southern California, which has been devastating to public and private landscapes there (it should be noted, though, that this strain is not the same one that causes Pierce’s disease in grapes). Strains of Xylella fastidiosa have been found to cause different diseases in alfalfa, sweet gum, cherry plum, mulberry and almond, and this is just in California so far.
Science is uncovering more plants that are susceptible to the bacteria, more host plants that harbor it and more plants that are attractive homes to the glassy-winged sharpshooter, such as our ubiquitous eucalyptus, coastal live oaks and citrus. The important thing to remember is wherever there are sharpshooters, there is a risk of spreading any number of strains of Xylella fastidiosa, and the glassy-winged sharpshooter is a particularly effective and aggressive vector.
As home gardeners in Marin, we should keep an eye out for the glassy-winged sharpshooter. Not too long ago, inspectors in Novato found one in a shipment of plants from Ventura County. The insect was sent to Sacramento for study and the plants shipped back.
Watch for Xylella fastidiosa symptoms, too. This bacterium, spread the way it is, could show up anywhere, causing Pierce’s disease in your grapes, or some other troublesome problem in your ornamentals and trees. And a tree infested by the glassy-winged sharpshooter can be a real mess (and an embarrassment) as the insect sucks up so much plant moisture that it excretes a great deal of watery frass that can rain down on your unsuspecting neighbors and guests!
So be aware and call the Marin County Department of Agriculture at 415-473-6700 immediately if you suspect this critter is lurking in your neighborhood.