Entomologists can now comfortably recommend how to limit damage of Pierce's disease in Texas vineyards.
A research effort lead by Dr. Forrest Mitchell, Texas Agricultural Experiment Station entomologist based in Stephenville, has identified with a “great amount of certainty” the insect vector that spreads the disease in vineyards. Just as important, he said, a statewide insect survey utilizing data from 21,000 traps, now means Mitchell's team can say when the insects arrive at vineyards.
“We suspected for some time that the glassy-winged leafhopper was responsible for the spread in Texas,” said Mitchell, who has a joint appointment with the Experiment Station and Texas Cooperative Extension. “It's what's devastated the California wine grape industry.”
Bacterial in nature, Pierce's disease causes leaves to become discolored, grape clusters to shrivel and new canes to grow stunted and misshapen. New shoots may be stunted even on vines that did not show symptoms the previous year.
The disease has been a threat to Hill Country viticulture since the industry began to expand in the 1980s. The Texas wine industry now has a total economic impact of more than $200 million a year with more than 110 licensed and bonded wineries and is currently the nation's fifth-largest wine-producing state, according to the Texas Wine and Grape Growers Association.
But Pierce's disease threatens to slow or halt the growth of the industry.
In tackling Pierce's disease, Mitchell faced several problems. One was to identify the glassy-winged leafhopper as the primary vector.
The glassy-winged leafhopper feeds on the grape vine's xylem, the plant's vascular system that transports nutrients and moisture from the root to stems and leaves. It was believed that the insect transmitted the Pierce's disease bacterium plant to plant as it fed, directly contaminating the xylem. Incidences of Pierce's disease increased as the number of glassy-winged leafhoppers increased.
To complicate issues were other potential insect vectors, including a blue-winged leafhopper and a spittlebug. Researchers needed to verify that one or the other insects were transmitting the bacterium.
Mitchell enlisted the help of Jeff Brady, assistant research scientist and genetic specialist. Using a technique called “polymerase chain reaction,” Brady identified the strain of bacterium in the glassy-winged leafhopper as the one found in vineyards where it was feeding.
Polymerase chain reaction is used to generate virtually unlimited copies of a small fragment of DNA. The technique can multiply selectively a very short unique DNA sequence and not the millions of others found in a plant or insect. In this case, the unique DNA sequence Brady identified all but ruled out any insect but the glassy-winged leafhopper.
“That left us with the glassy-winged leafhopper as our main culprit, which we suspected but could not prove,” Mitchell said.
In a second line of research, funded by the U.S. Department of Agriculture, Mitchell used insect traps to find when the glassy-winged leafhopper arrived in Texas. The traps, colored yellow to attract the insects, were sampled twice a month on sites from El Paso eastward to Galveston, and from Spring Lake on the High Plains south to Del Rio. The traps revealed two population peaks in Texas, one in July and a second in August, Mitchell said.
These two pieces of data — that the glassy-winged leafhopper is the vector, and when the insect's population peaks — makes precision control possible, Mitchell said. Vineyard managers can use an insecticide that targets insects that feed on a plant's zylum, and they can time spraying to when the insects commonly arrive.
“Methods in California involve wholesale spraying. We're trying to avoid that,” Mitchell said. “All we have to do is to kill the (insect) vectors.”