As reported by Pierce, the first records of PD come from Southern California in the late 1800s, during an apparently severe epidemic in vineyards in and around the Los Angeles area. Grape production in the region never fully recovered. More recently, starting in the late 1990s, a severe epidemic occurred again in Southern California – this time in Temecula Valley in Riverside County, associated with the invasion of the glassy-winged sharpshooter (GWSS; Homalodisca vitripennis) several years prior. Although other Xf vectors are present in the area, including the native smoke tree sharpshooter (Homalodisca lacerta), GWSS is the most dominant vector. Vineyards most at risk to Pierce’s disease are those located adjacent to citrus groves, with perhaps some vector pressure also coming from ornamental plants on nearby residential properties. Over the last 15 years PD management in the region has relied on a combination of area-wide vector control, vegetation management, and within-vineyard chemical control.
Area-wide GWSS control
Following the onset of the epidemic in the late 1990s an area-wide GWSS control program was initiated by state and federal regulators. In Southern California this encompassed the Temecula Valley and Coachella Valley growing regions, and involved a combination of regular monitoring of GWSS populations in citrus throughout the region, chemical control of citrus, and biological control using parasitoid wasps.
Chemical control of GWSS was a central part of GWSS management efforts in Southern California for nearly 15 years. This program relied largely on soil-applied systemic insecticides (i.e. imidacloprid), with applications made in the spring to allow sufficient time for uptake into citrus trees to be effective against GWSS prior to the summer population peak. Applications were put on hold in 2012 due to sustained low GWSS populations in the region, but monitoring efforts are ongoing.
As part of the area-wide control program the USDA and CDFA initiated a mass rearing and release program for several species of parasitoids (Gonatocerus spp.) that attack GWSS eggs. Although active releases of these parasitoids are no longer occurring in Southern California, observations suggest that the parasitoids are widespread throughout the region and are active in suppressing sharpshooter populations, especially in the summer and fall.
Vine roguing: Due to the mild winters in Southern California fewer infected vines will recover over the winter compared to colder regions to the north. Therefore, diseased vines should be removed to minimize the potential for them to be pathogen sources for vectors that are not already carrying Xf. Vines should be inspected in the fall when PD symptoms are most apparent. Suspected vines, particularly those with severe PD symptoms, should be flagged for later removal. Areas within vineyards that have persistent PD problems should be replanted with less susceptible varieties.
Weed control: Riparian habitat plays a less important role in PD pressure in interior Southern California growing regions. Instead, like the San Joaquin Valley, vegetation management efforts should focus on weed control in and immediately around vineyards. This includes tilling, mowing, or herbicide applications to control weedy grasses that may support vectors and other plants that may be reservoirs for Xf.
Landscaping: GWSS’s wide host range means that many common ornamentals may contribute to vector pressure in vineyards. Vineyards and wineries should avoid landscaping with certain plants, including olive, citrus, roses, photinia, and other key GWSS reproductive hosts.
Within-vineyard chemical control
In addition to area-wide treatments, growers in Southern California have employed successfully chemical control within vineyards to reduce further vector pressure. For conventional growers, soil applications of systemic neonicotinoids (i.e. imidacloprid) are recommended as the typically sandy soils in the region allow for rapid uptake into vines. Applications made in the late spring can yield GWSS-effective concentrations for much of the rest of the growing season. For organic growers, options include kaolin clay (i.e. Surround) and pyrethrins (i.e. Pyganic). Given the short residual efficacy, especially for the latter, successive applications at 1-2 week intervals are recommended starting in the late spring or early summer depending on the results of vector monitoring.