New Sharpshooter Takes Aim at Crops
Fast-moving insect increases threat to almonds
Almond leaf scorch, also known as golden death, isn't new to California, but a new carrier of the disease is. The glassy-winged sharpshooter, a highly mobile vector for the bacterium that causes almond leaf scorch and Pierce's disease in grapes, recently appeared in large numbers in Kern County. This discovery dramatically increases the potential threat to almonds, grapes, citrus, alfalfa and other crops susceptible to the Xylella fastidiosa bacterium. Almond leaf scorch first appeared in California in the 1940s in Mendocino County, some fifty years after Pierce's disease had been identified in the state's vineyards. Soon after, it appeared in the Lancaster area of Los Angeles County and the Brentwood-Antioch area of Contra Costa County, according to the University of California. Eventually, scattered infestations appeared in almond orchards in the San Joaquin and Sacramento valleys. While the disease is lethal to almond trees, its discovery did not raise widespread concern because the primary carriers of the bacterium are spittlebugs and several species of leafhoppers called sharpshooters, which do not travel far or fast. However, the arrival of the fast-moving glassy-winged sharpshooter increases the threat of serious infestations of a variety of important crops. X. fastidiosa has caused millions of dollars of damage to California vineyards in recent years, even before the glassy-winged sharpshooter arrived. The seriousness of the infestations in grapes triggered the formation of a UC task force last year to address the issue and a state appropriation of $750 thousand to fund research into Pierce's disease. The results of that work could be useful to almond growers as well.
According to the university, sharpshooters and spittlebugs pick up X. fastidiosa from any of a wide variety of infected plants when they insert their mouthparts to feed in the xylem (water carrying tissue). The insect transmits the disease when it travels to an almond tree and feeds on it. After once acquiring the bacterium from an infected plant, the sharpshooter can transmit it to healthy plants throughout its life. According to the UC IPM for Almonds manual, the disease infects the xylem, choking off the flow of moisture and nutrients to the plant. Leaves on infected trees develop symptoms in early to mid-July. Tips and margins of leaves turn yellow, then brown, and a golden yellow band forms between the green and brown leaf areas. Affected leaves do not wilt, but eventually the entire leaf turns brown and dies. Some leaves may drop prematurely, but many remain on the tree until the end of the growing season. Newly infected trees typically have one terminal branch that is affected. But on closer examination, several developing infections on the same or nearby scaffolds will be apparent. As the disease progresses from year to year, trees have reduced terminal growth, large numbers of dead spurs and small branches, and a dieback of terminal shoots. Within three to eight years, the disease spreads over the entire tree, yields are greatly reduced on affected branches, and eventually the tree dies.
Not much is known about controlling the sharpshooter, except to minimize the amount of host material within flying distance of the orchard, advises the University of California. The main breeding areas for grass-feeding sharpshooters, UC notes, are irrigated pastures, weedy alfalfa fields, ditch banks, and the edges of ponds. Typical cover crops do not appear to be important hosts, but some newer types of cover have not been evaluated for their potential to support sharpshooter populations. However, permanent cover that includes grasses such as Bermuda are known to support high populations of sharpshooters. UC advises frequent monitoring of cover crops for sharpshooters. This requires direct sampling with a net and is best done by an experienced person. Sticky traps are not effective for sharpshooters. And insecticides generally are not satisfactory for controlling the grass-feeding sharpshooters because their sharpshooter eggs that have been deposited within plants survive, and the nymphs emerge some weeks later. Cultivation of host plants is the best known preventive. For more information on Pierce's disease and the sharpshooter vector, visit this UC website: http://nature.berkeley.edu/xylella/. Blue Diamond® will continue to monitor developments with the UC task force and provide updates as appropriate.