GWSS spread becomes major concern The invasion of the glassy-winged sharpshooter (GWSS) into California agriculture has focused on its impact in the grape industry, with only casual concern about how the marauding disease vector might affect other crops.
However, GWSS' role in spreading almond leaf scorch - heretofore a minor problem in California's almond orchards that has shown up only sporadically in the past - is becoming a major concern. Researchers are now concerned that it may become more widespread as the glassy-winged sharpshooter continues to pop up throughout California.
Almond leaf scorch is a disease caused by the bacterium Xylella fastidiosa, the same organism responsible for Pierce's disease in grapes and alfalfa dwarf disease in alfalfa. At a recent meeting held at an almond orchard near Delano, Kern County Farm Advisor Mario Viveros cautioned PCAs and growers to be on the lookout for the disease in almonds.
"I think there's a potential for a major problem with this disease," he says. "We haven't found very many almond trees that have it so far this year, but we know that glassy-winged sharpshooter finds are becoming more common and that's why we are concerned. This pest is a very efficient vector of Pierce's disease, which causes almond leaf scorch. The glassy-winged sharpshooter has large mouthparts, so it can penetrate the tree's xylem more effectively than some of the other vectors."
Although an infected almond tree may live another three to eight years after the disease is introduced, Viveros says it should be taken out immediately. "It will not be a very good producer anyway, and it will only serve as another source of inoculum for Pierce's disease," he says.
Characteristics The disease is characterized by scorched or burned leaf tips as a result of water stress. Pierce's disease blocks the xylem vessels, interfering with the flow of water through the tree. Scorched leaves first appear in affected trees about mid-June or July. At first, the leaves exhibit chlorotic areas either on the tip or on the sides of the lamina that eventually desiccate and die. The scorched areas gradually enlarge over entire lamina. The scorched leaves remain on the trees until fall defoliation. Often, the symptoms are mistakenly identified as salt burn. However, a close inspection can help a grower or PCA discern between the two.
Salt burn usually - but not always - has an abrupt margin between the necrotic and healthy tissue with little or no intermediate yellowing. Almond leaf scorch necrosis, on the other hand, usually progresses from the leaf tip back to the base of the leaf and is not uniform along the leaf margins. Conversely, salt burn is more evenly distributed along the margins as well as the tip. While salt burn will tend to affect several trees in the same area, almond leaf scorch may only be present on one tree or several trees that are scattered randomly throughout the block.
For positive identification, a sample can be sent to a laboratory at the University of California at Davis for analysis. However, this should not be done until later in the growing season.
"There has to be a certain level of bacterium present in the tissue before the lab can detect it," Viveros says. "That's why we don't send samples in early. And that's also why it's important to keep an eye out for the visual symptoms which may become apparent before the lab can detect it."
So far this year, almond leaf scorch has been found in two places in Kern County, according to Viveros. "We found it earlier this year in McFarland and near Delano," he says. "It was pure chance that I happened to find it in an orchard near Delano. I was just driving by one day and happened to spot a tree that was yellowing and generally less vigorous-looking than the other trees."
At this point, no one is sure how much almond leaf scorch is present in the Valley's almonds. Tom Watson, associate plant pathologist with California's Department of Food and Agriculture says that no one is actively surveying for it yet.
"We know it's been around for years," he says. "We ran some surveys in the Valley in the 70s, but didn't find it here. However, it's quite possible that the levels were so low that we couldn't detect it. During that time, it wiped the almonds out of the Lancaster area."
The most susceptible varieties are Nonpareil, Sonora, Mission (Texas), Neplus Ultra and Peerless. Currently, there are no chemicals registered for control of GWSS in almonds. However, other commonly sprayed chemicals are known to have activity against the pest. It's really a moot point, however, according to most researchers. Even a few escapes can have devastating consequences, as grape growers can unfortunately testify. Researchers believe that the greatest danger of the disease spreading exists in the spring when young shoots emerge on the almond trees. The GWSS typically feeds on young shoots.
Right now the only recommendations for managing GWSS and almond leaf scorch are limited to cultural practices. In addition to removing infected trees, weed hosts and unproductive alfalfa fields should be eliminated. The bacterium Xyella fastidiosa infects several weed species including bermudagrass, blackberry, cocklebur, elderberry, fescue grasses, nettle and rye. Eliminating these weeds can help reduce the reservoir of inoculum available to the GWSS.
Overwinter question Whether or not GWSS will become a major problem in almonds remains to be seen. Researchers are questioning the pest's ability to overwinter successfully in the valley. So far in Kern County, the pest has been primarily confined to elevations of 450-700 feet above sea level - frost-free areas. However, even that area extends up to Porterville and represents a sizable amount of acreage.
"It may be too cold for the glassy-winged sharpshooter to overwinter here in most parts of the valley," Watson says. "If that's the case, then it might not develop into a real serious problem for almond growers. We just don't know yet."
Even if the pest cannot effectively overwinter in the valley, it still represents a major threat, as populations will likely continue to keep migrating into the Valley each year. Monitoring the pest and identifying plants with Pierce's disease is a high priority for the University of California's Pierce's Disease and Emergency Response Task Force which has been formed to address the problem. Growers are being urged to monitor their crops closely and report any finds of GWSS or Pierce's disease.