Pierce’s disease is becoming a major threat to grape vines and citrus trees.
The bacteria responsible for Pierce’s disease, Xylella fastidiosa, was first seen on grapes in Southern California in the late 1800’s, when it was called Anaheim vine disease. By the 1930’s and 1940’s, it had spread to California’s Central Valley. By the late 1990’s, the disease had spread to several California counties. This increase is believed to be, in part, a result of warmer temperatures allowing more of the bacteria to survive the winter. According to CABI, Pierce’s disease is now found throughout the Americas, and in Italy, Iran, and Taiwan.
Pierce’s disease is carried by sap-feeding insects. Most commonly, this means sharpshooters, such as blue-green and glassy-winged sharpshooters. [Did you know that sharpshooters can consume hundreds, or even thousands, of times their body weight in sap in their short lives?] Spittlebugs have also been found to carry this disease. Whichever insect is chewing on your grape vines, they inject the bacteria into the vine’s vascular bundle as they feed, making them a disease vector. These bacterium then live and reproduce in the xylem, clogging the flow of nutrients and water through the plant.
Pierce’s disease can occur on a large number of weedy and ornamental crops, such as wild grape, California blackberry, periwinkle, stinging nettle, eucalyptus, live oaks, blue elderberry, and mugwort. These plants are not affected by the bacteria that cause disease in grapes. But they do provide a transitionary location for the insects that carry the disease to your garden.
Symptoms of Pierce’s disease
Infected plants exhibit leaf scorching and stunting. These symptoms start out as slightly yellow or red leaf margins (edges) of white or red grape varieties, respectively. Concentric areas of infected leaves may dry up. You may also see ‘matchstick’ petioles, ‘green islands’ on mature brown stems, raisined clusters of fruit, and dieback. These symptoms do not normally appear until spring, after temperatures are above 65°F.
There is no cure for Pierce’s disease. In some cases, the disease will disappear on its own and we don’t yet know how or why. It seems to be a function of temperature, the timing of the initial infection, and the variety of plant being infected. Generally speaking, a late season infection, one that occurs after June 1st, has a 95% chance of recovery. Water stressed plants are more likely to succumb to the infection. If a plant becomes infected early in the season, the bacteria have time to become firmly established. Once that happens, you will ultimately have to remove the vine completely.
Pierce’s disease control and prevention
This disease triangle consists of the host plant, the feeding insect carrier, and the disease-causing bacteria. Break the connection between any one of those three and you can reduce the chances of disease. The easiest ways to prevent Pierce’s disease is to keep host weeds out of the area and treat for the sap-sucking insect pests. Since insect-eating birds, such as bluebirds, along with several predatory insects, love to eat sharpshooters, keep your garden welcoming to these natural helpers.
Monitor your plants for signs of Pierce’s disease so that you can act quickly, reducing the spread of the disease. Most of the vector insects are low fliers, so physical barriers can be used to quarantine potentially infected plants. During the dormant season, remove any vines that have been infected for more than one year. They will not recover and they will spread the disease to other plants as vector insects feed on them and then move to nearby plants for more feeding.
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