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Controlling PD in texas and missouri: researchers present latest information about pierce's disease in grapevines.

The southeastern region of the United States is a challenging place to grow wine grapes. The Deep South is prone to spring frosts, stays hot and humid in the summer and is home to many types of mildews and other diseases. In places like Texas the climate may be drier, but it's still very hot in the summer. Rick Naber, owner of Flat Creek Estate Winery, noted that wine grapes in Texas experience an early bud break (often by March 1) and have a threat of frost until late March or early April, after which they suffer high temperatures in the summer and are subject to every disease a grapevine can get. Native grapes are adapted to this climate but make a very different style of wine.

One disease that has been killing wine grapes such as vinifera since settlers first brought those grapes to the southeast region is Pierce's disease (PD). Dr. Newton B. Pierce first identified PD in Mission grapes in Southern California in the late 19th century, when the disease damaged thousands of acres around Anaheim, and was then called Anaheim disease.

PD is caused by a bacterium, Xylella fastidiosa, which is native to the Gulf Coast region. Vines native to the Gulf Coast appear to be tolerant of this bacteria, and other indigenous plants serve as hosts to the bacteria while not exhibiting any symptoms of disease themselves.

In order for a grapevine to be infected by PD, it must be a susceptible variety and there must be an insect vector present to transmit the bacteria from a host plant to the susceptible vine when the insect feeds on the vine's tissue. Once introduced into a vine, the bacteria colonize the xylem (the tissue that conducts water throughout the plant). When the bacteria occlude the xylem, and water cannot move through the plant, the vine may exhibit symptoms often seen under heat or drought stress. After being infected, vines with Pierce's disease usually die within approximately two years.

In the 1970s it was thought that PD was confined to regions with mild winter temperatures. However, with the changing climate, the potential range for PD has expanded, and symptoms of PD have been seen in (formerly) colder western areas in Texas, on vines in Virginia and even as far north as Maryland.

Potential PD problem in Missouri?

Southeastern Missouri has been on the borderline of the region defined as susceptible to Pierce's disease, but with warmer winters in the past 10 years, growers began to report symptoms that could be attributed to PD. Texas Pierce's disease program outreach coordinator Jim Kamas, assistant professor and extension fruit specialist in the Department of Horticultural Sciences at Texas A&M, assessed the risk of PD in Arkansas and Missouri in 2006-07. While he found host plants and vectors for PD, he did not find positive symptoms in grapevines at that time.

However, in 2013 a vineyard in central Missouri had significant vine death and sent samples of those Norton vines to Dr. Andrew Walker, professor of viticulture and enology at the University of California, Davis, who does research on PD. Walker found high levels of Xylella fastidiosa, and concern mounted in Missouri that growers were about to face a major problem.

To address those concerns, Dr. Misha Kwasniewski, assistant research professor and enology program leader at the Grape and Wine Institute at the University of Missouri, held a PD workshop titled "Evaluating and Mitigating Risk" on May 6. To provide growers with the latest information about what to look for and how to assess their vineyards, Kwasniewski invited Kamas from Texas and Dr. James Wolpert, viticulture extension specialist emeritus at the University of California, Davis.

Kamas encouraged growers not to panic, noting that some vineyards in Texas have survived even with significant pressure from PD. He underscored the point that even if PD is a part of the landscape, it probably won't jump into a high-pressure situation and probably will be more chronic than acute.

Part of the problem, according to Wolpert, is that the potential source for PD in Missouri is not clear. It may come from infected plant material, or there may be host plants in Missouri's woods. Research has shown that Xf cannot survive for long in sub-freezing temperatures, and during the 2013-14 winter in Missouri, temperatures dropped to -15[degrees]F and remained "stuck there" for several weeks

While there is a strong relationship between cold cycling and the survival of PD-causing bacteria, there may be other factors involved. Wolpert stressed that it is most important for lines of communication to be open between growers and researchers, since growers know their vineyards and can report possible symptoms if they see something. Their vigilance can help extension personnel know what is happening and work on problems in a timely way. Pierce's disease in California

The glassy-winged sharpshooter (GWSS) was identified as a PD vector and evidence of the disease was found in Southern California in the late 1990s. Approximately 840 acres of vineyard were lost due to Pierce's disease during 1998--2000, which was roughly 30% of the total vineyard acreage in Temecula, Calif. PD has been a minor problem in Napa Valley for a long time in riparian areas and of concern to some vineyard managers even before the GWSS introduction to Southern California. As concern about the spread of PD around California increased, the Pierce's Disease Control Program (PDCP) was established in 2000 as a partnership of government, industry and university personnel.

Eliminating the potential host plants for the bacteria was not possible, so the PDCP established programs to control PD by controlling or eliminating the vector transmitting the bacteria. The PDCP has since inspected tens of thousands of loads of plant material from nurseries in southern California for infection by glassy-winged sharpshooters. The PDCP also funded research that developed a treatment program for plants before they are shipped from the nursery. A Rapid Response Program was set up to monitor for glassy-winged sharpshooters in regions not yet infested and to provide a quick and effective response if a new infestation was found.

While several researchers have worked to transform cultivars genetically so that vines will produce a protein that will stop the spread of Xylella fastidiosa once it is introduced, Walker took a different tack at UC Davis and used breeding techniques to gradually develop vinifera cultivars that are resistant to PD. In a talk during the Challenging Environments Symposium at the national meeting of the American Society for Enology & Viticulture in June, Walker gave an overview of the work he has done to develop seedlings that are 97% vinifera but resistant to the bacteria causing PD. (See previous coverage in Wines & Vines including "PD-Resistant Vines Available in 2015?" at

Multiple vine trials have been planted in Davis and Yountville, Calif., and wines have been made from most of the selections. Three PD-resis-tant rootstocks are also being tested. Test plots of different selections have also been established in Temecula, Texas and Alabama.

Control of PD in Texas

In the past 15 years, the wine industry in Texas has increased dramatically--especially in the Texas Hill Country west of Austin, an area long considered to be a transition zone between the hot zone for PD and a lower probability of the disease being present. Today, Texas has 204 wineries; the Hill Country region includes 42 wineries, and many of them are growing vinifera vines. How are they succeeding when PD is a problem that probably will not go away?

First of all, researchers in the state now recognize that PD is cyclical: Cold winters or drought can kill back both the bacteria causing the disease and the vectors that carry the causal agent from a host plant to a susceptible vine, which in turn causes the disease pressure to lessen. The cyclical nature of the problem gives growers hope that they can address the source of the pathogen--the bacteria and the vectors that are responsible for the infection--even though there is no known control for the disease.

Selection of a vineyard site with low disease risk is an important initial step. Native plants such as wild grapevines that may harbor the bacteria can be removed from the vineyard area, and susceptible varieties can be planted away from grapevines that are tolerant to Xylella fastidiosa.

Growers are learning to recognize signs of the disease, confirm the symptoms by laboratory diagnosis and then immediately pull out infected vines to prevent spread of the bacteria from vine to vine within the vineyard.

Timely application of spray materials to control sharpshooters is also beneficial. In California, parasitic wasps have been used as a biological control of the sharpshooter population. Imidacloprid is one pesticide that does kill sharpshooters, but it may also impact parasitic wasps as well as the sharpshooters. It also has been cited as a possible cause of bee decline and colony collapse disorder.

According to Dr. David N. Appel, professor of plant pathology and Texas A&M AgriLife Extension specialist, climatic factors also affect the sharpshooter population in PD-prone vineyards. Texas experienced a severe drought in 2011, which reduced the number of sharpshooters, and the cold temperatures during this past winter may also impact the insect population. Kamas agreed that weather is a big component in controlling the presence of sharpshooters, but he also noted that it was vital to make the vineyard a "sharpshooter inhospitable place."

These scorched Merlot clusters are presenting symptoms of Pierce's disease.

The potential source for Pierce's disease in Missouri may come from infected plant material, or there may be host plants in Missouri's woods.

The cyclical nature of the problem gives growers hope that they can address the source of the pathogen, the bacteria and the vectors that are responsible for the infection.


* This article reviews the threat of Pierce's disease, particularly for southern vineyard regions.

* Scientists in Texas, California and Missouri explain their latest findings about preventing and detecting the insect-borne vine disease.

* New grapevine crosses bred to resist PD may be available for planting in two years.
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Title Annotation:Grapegrowing
Comment:Controlling PD in texas and missouri: researchers present latest information about pierce's disease in grapevines.(Grapegrowing)
Author:Jones, Linda
Publication:Wines & Vines
Geographic Code:1U4MO
Date:Sep 1, 2014
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