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Glassy-winged sharpshooter a continuing threat to grape industry
Anaheim boasted a thriving wine industry in the late 1800s, before an unnamed affliction killed 40,000 acres of the grapevines and put 50 wineries out of business. The problem was later found to have been Pierce’s disease of grapevines. Would...
A New Tool for Studying Sharpshooter Feeding
When an insect pierces the surface of a plant to feed, much of the action takes place in the plant's interior. A device called the Electrical Penetration Graph (EPG) is a critical tool for peering into the process.
Now a new type of EPG developed by U. S. Department of Agriculture (USDA) entomologists is giving scientists the clearest view yet of the wars waged between piercing-sucking insects and the plants they attack.
The EPG was developed by Elaine Backus at the Agricultural Research Service (ARS) San Joaquin Valley Agricultural Sciences Center, in Parlier, California, and her late partner William Bennett from the University of Missouri.
To use an EPG, researchers connect the insect and plant to an electronic monitor that reads electrical charges produced by changes in voltage that occur as the insect feeds. At least eight different systems have been developed, and researchers who study aphids and other piercing-sucking insects have used them over the years to publish nearly 400 peer-reviewed papers. But the new EPG is much more versatile than any of its predecessors, and is being used by researchers around the country in ways expected to broaden understanding of how plant-feeding insects cause so much damage.
Backus and Bennett described their AC-DC monitor in a 2009 issue of the Journal of Insect Physiology, using it in a series of studies published in the Annals of the Entomological Society of America. These studies focusedon the critical role that saliva plays when the glassy-winged sharpshooter injects the Pierce’s disease bacterium, Xylella fastidiosa, into grapes. Backus believes that the saliva loosens bacteria living in the gut and stylets and carries them into the plant when the mixture is “spit up” during feeding. That inoculation process begins the spread of the disease throughout the plant. Backus could not have gained these insights without the AC-DC monitor.
Traditionally, monitors have been designed to work with either AC or DC current. Because of the physics that govern electricity and the flow of electrical current, researchers have been likely to get best results using AC monitors when studying larger insects and DC monitors when studying smaller insects.
Ideally, a monitor should be capable of studying a variety of insect sizes. As the name implies, the team's AC-DC Monitor incorporates design features from both AC and DC monitors, making it more versatile. Researchers can adjust the settings to the sizes of any insect they are studying. Entomologists will be able to view the feeding process in detail for more insects than ever before. They also will be better able to compare the feeding habits of pathogen-bearing insects with those that are pathogen-free.
PIERCE’S DISEASE IN THE SAN JOAQUIN VALLEY
It has been several years since Pierce’s disease (PD), the debilitating grapevine disease caused by the bacterium Xylella fastidiosa, has caused significant problems for San Joaquin Valley vineyards. PD dropped off the radar of valley grape industries and growers partly due to other invasive grape pests (vine mealybug, EGVM, etc.) inhabiting vineyards and to a low incidence of PD found throughout the valley. However, in recent months glassy-winged sharpshooter (GWSS) has been trapped in west Fresno County commercial vineyards and near the San Joaquin River. The recent catches are concerning since they have been found near a major riparian corridor that has had a historically low level of PD. With these recent GWSS catches, growers should survey their vineyards and test grapevines displaying PD-like symptoms in order to minimize potential “hot” spots from developing should GWSS become established.
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Four PD symptoms to look for in late summer
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Pierce's Disease Summer Symptoms
PD infected vines display several symptoms in late summer. Burned or scalded canopies are the first and easiest symptoms to spot. However, there are other diseases (measles), pests (mites), chemical maladies (sulfur burn) and/or cultural practices (poor irrigation management) that can cause foliage to burn. It is important to correctly determine the cause of any leaf burn symptoms that may be noticed. White varieties suffering from PD will display yellowing of the leaf margins followed by necrosis, while infected red varieties exhibit reddening prior to necrosis (see photo). Dead leaves on red or white grapes will detach from the petioles that dry and remain attached to canes, resembling matchsticks. Irregular wood maturity, a prominent symptom on canes, is marked by lignified wood surrounded by green tissue that never fully matures. Fruit from infected vines will shrivel, raisin and drop from the vine.
PD Insect Vectors
In the valley green and red-headed sharpshooters may vector PD, maintaining it at low levels. Those sharpshooters primarily feed on grasses, but as irrigated pastures, ditch banks and weed patches dry, they move into vineyards and feed on new succulent grapevine growth near shoot tips. Sharpshooters carrying PD will transmit the bacterium at the point of feeding but it rarely becomes a systemic infection; hence the low PD levels found in the San Joaquin Valley. Since its introduction, GWSS has changed the presence and spread of PD in CA. The voracious feeding of GWSS, it’s ability to feed on older, more lignified tissues, and its ability to fly longer distances makes it an exceptional vector of PD. Glassy-winged sharpshooters often feed near the base of shoots or on 2-year old wood increasing the chance for late-season infections. It also has a large host range that includes citrus, on which it breeds and may develop into very high populations. Many of the hosts that GWSS can survive and reproduce on have not been of concern with our grass feeding sharpshooters.
More information
Pierce’s Disease ANR Publication-21600 can be purchased at your local UC Cooperative Extension office or online at: http://anrcatalog.ucdavis.edu/
Pierce's Disease
