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Diagnosing Pierce's Disease
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In order to successfully manage Pierce's disease in Texas vineyards, growers need to understand the pathogen and how it behaves in nature and in an infected vineyard.

Although Xylella fastidiosa has been killing susceptible grapevines for hundreds of years, it was first diagnosed in the late 19th century. Pierce's disease was named for Dr. M.B. Pierce who first discovered the disease in plantings of Mission grapevines in southern California. First known as Anaheim disease, the disease was first seen in 1883 in the 10,000 acres of wine, table and raisin grapes near Anaheim California. According to some historical sources, in 1885, two years after the first noted infections, half the acreage in the Anaheim area was dead. On the other side of the country, researchers were working with a similar disease which killed peach trees that is now known as peach phony disease.

In addition to infecting peach and grape, other strains of Xylella fastidiosa are known to cause disease in citrus, alfalfa, oleander, plum and numerous deciduous and evergreen annual and perennial plants. It can generally be thought of as a new world disease of old world crops. Xylella fastidiosa is native to the Gulf Coast of the United States where it resides benignly in many native plants.

The bacterium colonizes the xylem or water conductive tissue of susceptible plants. It disrupts the movement of water by directly occluding the xylem through the congregation of bacterial cells, it causes the production of a substance known as fastidian gum, and by triggering the enlargement of naturally occurring cells known as tyloses. It is also speculated that a toxin may be produced by the bacterium adding to the typical leaf scorch symptoms associated with Pierce's disease.

The bacterium is known to be susceptible to cold and low temperatures limit its distribution to the north. This map produced by Hopkins and Purcell show its suspected range across the United States.

In Texas, we have known for some time that the disease is much more prevalent and likely in areas closer to the Gulf of Mexico. At one time we speculated that constituted the natural range of insect vectors. The pathogen's susceptibility to cold temperatures roughly confined it to areas with 800 hours of winter chilling. We now believe that both are true. Vector populations and survival of Xylella are both much higher in warmer areas of the state.

That said, the range of Pierce's disease appears to be moving. Pierce's disease is confirmed in several locations along the Red River and in Ft. Davis and Alpine in far west Texas where we once believed winter temperatures would keep the disease from becoming established.

There is no cure for Pierce's disease. There are, however things growers can do to manage its impact on commercial grape growing. Selecting a vineyard site with low disease risk and understanding the components of the disease triangle are ways of avoiding Pierce's disease- Being aware of the pathogen and identifying its supplemental hosts, recognizing and managing insect vectors and conducting sound cultural practices in the vineyard are all necessary in managing risk.

Pierce's Disease Vectors
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In California, the introduction of glassy-winged sharpshooter changed the dynamics of Pierce's disease and marked the introduction of a fourth insect vector. This new species increased the probability of infection in permanent parts of the grapevine such as trunks, cordons and spurs and increased the likelihood of vine infection in the interior of vineyards

The Pierce's disease work done in Texas over the past five years has revealed that we may be dealing with over twenty separate sharpshooter species capable of infecting grapevines

As with any plant disease, infection depends on a source of the pathogen, a vector that is responsible for infection and a susceptible host. Understanding this disease triangle, and breaking any one of the links is necessary for successful management of the disease.

When a sharpshooter feeds on a plant infected with Xylella fastidiosa, the bacterium begins to colonize the foregut of the insect. After a sharpshooter acquires the bacterium, it then has the capability of transferring the pathogen with each feeding episode.

In order for grape growers to successfully manage Pierce's disease, they must become familiar with the specific insects that can vector the disease, monitor insects inside and preferably outside the vineyard and be prepared to act when insect populations rise or move into the vineyard. Maintaining good viticultural practices that make the vineyard inhospitable for sharpshooters is a vital component of the management strategies in addition to managing vectors with insecticide inputs.

Its important to know that not all cicadellids are xylem feeders. Leafhoppers such as Erothronura species are phloem feeders and while they can cause a reduction in photosynthesis through their feeding, they are not believed to be involved in the transmission of Pierce's disease.

There are five different sharpshooter sub-families that have been found in Texas. Perhaps most problematic are members of the Proconnii such as glassy-winged sharpshooter. These insects are distant flyers and can appear in large numbers. Cicadellinni species can also appear in great numbers but tend to have higher numbers on the edges of vineyards.

Proconnii are thought to be primarily responsible for infections in the center of the vineyard and are probably responsible for vine to vine spread of the disease down the vineyard row. Cicidellini movement can be in response to cultural practices such as mowing adjacent fields. When they lose their feeding hosts, they by necessity move to what is closely available. While they are not distant flyers, these insects are still important vectors and should be carefully managed.

Where we have a problem with Pierce's disease in Texas, Glassy-winged sharpshooters are present. It probably represents our single greatest species responsible for disease transmission.

This insect is native to Texas and recent genetic studies have shown that the origin of the insects introduced into California was from Texas. While problematic for Texas growers, it also makes Texas the natural laboratory for studying the insect and its natural enemies.

Unlike California where GWSS appear in the vineyards very early in the growing season, for some reason these insects stay in the riparian habitat in Texas until the end of May. At that time, they can move into vineyards in very high number. It makes good sense for growers to use monitoring tools like yellow sticky cards on the vineyard edge and interior to monitor all vector presence and movement.

While cultural practices make a huge difference in how attractive a vineyard site is to sharpshooters, at some point insecticide inputs will probably be needed. Our first line of defense is to use systemic insecticides such as imidicloprid injected through the drip system.  This class of chemistry is a feeding deterrent, and if sharpshooters do feed, they immediately stop feeding, become disoriented and die. There are also numerous foliar insecticides labeled for use in vineyards an most are effective as knock down agents when sharpshooter populations rise. In addition to treating vines, using imidicloprid on trap crops weed control and habitat modification are all additional methods that may be useful in managing insect vectors of Pierce's disease.