Grapevine leafroll disease is caused by a complex of vector-borne virus species in the family closteroviridae. It is present in all grapegrowing regions of the world, primarily affecting vitis vinifera cultivars, hybrids, and rootstocks. The five serologically distinct pathogens associated with grapevine leafroll disease are known as Grapevine leafroll-associated viruses (GLRaV-1, -2, -3, -4, and -7). Of these, GLRAV-3 is the most widespread and damaging.
Diseased vines experience phloem degradation and decreases in net photosynthesis that affects fruit quality and pigmentation, delays maturity and reduces yield. Since there is no cure for the disease, planting virus-free vines and removing sources of virus from the vineyard are critical management strategies. Mealybugs and soft scale move GLRaV-3 from vine to vine; it is also transmissible by grafting. New developments include the use of hyperspectral imaging technology to remotely identify diseased vines.
Common symptoms of this disease include stunting, chlorotic leaf mottling, and deformation. These typical symptoms have been largely absent in Napa vineyards, even though the virus is present; this is because to date only “asymptomatic strains” of the virus have been found in California. Eriophyid mites of the erinuem strain have been implicated as vectors. As with other viral diseases, there is no recognized cure once a vine is infected.
Although recently “discovered”, there is evidence that Grapevine red blotch virus has been infecting California plant material since at least 1940. The virus interferes with grapevine metabolism; diseased vines remain essentially in a “juvenile” state with respect to ripening, and this affects both fruit and wine quality. The three-cornered alfalfa hopper (Spissistilus festinus) has been shown to transmit the virus under laboratory conditions; field studies are ongoing. Other membracids (Torstitilus spp.) that are morphologically similar to the alfalfa hopper are also being investigated as potential vectors. As with other viral diseases, there is no recognized cure once a vine is infected.
Caused a bacterium and moved from vine to vine by spittlebugs and sharpshooters, Pierce’s Disease (PD) is episodic in the north coast region. During the most recent outbreak, incidence rates as high as 50-60% have been recorded in the most susceptible areas. Ongoing epidemiology studies seek to refine management strategies to lessen impacts. Newly released cultivars with PD resistance are being tested in field trials. Riparian revegetation (although costly) remains an effective way to manage the disease.
Although a perennial concern for growers, grapevine powdery mildew (GPM) is generally controlled with preventative fungicide applications. However, continuous use of these compounds has resulted in the appearance of fungicide resistant populations of GPM in most US grape production regions. Growers suspecting GPM resistance as a factor in failed disease control can submit samples to a laboratory for analysis, or contact the UCCE Farm Advisor-Viticulture. Spore detection devices can be used to determine presence of GPM and adjust spray schedules accordingly.
Pathogens associated with the trunk diseases are characterized by their ability to infect woody grapevine tissue; diseases include Esca (measles), Eutypa dieback, Botryosphaeria dieback, and Phomopsis dieback. Symptoms range from spots or streaks on leaves, to distorted leaves and shoot stunting, and eventual death of spurs, arms, or cordons. Double pruning and pruning wound protectants are the most effective preventative practices. Economic studies have demonstrated that preventative practices are most beneficial when implemented before the vines show symptoms (early in the life of the vine).