Challenges of Red Leaf Viruses in Wine Grapes
UC Davis gives updates about research and management of leafroll and red blotch in grapevines
The University of California, Davis, Department of Viticulture & Enology presented “An In-Depth Look at Red Leaf Viruses” as part of its Current Issues seminar series. The meeting highlighted the fact that many variables influence the impact viruses have on grape production and vine health, including: the type of virus (or combination of viruses), vineyard site, soils, rootstock, variety and clone, rootstock/scion combination, presence of virus vectors and viticultural practices. Vines can test positive for viruses but display no symptoms or negative effects. Symptoms can vary by season and climatic conditions. White varieties do not display red leaf symptoms, but they may show yellowing or leaf curl symptoms. Asymptomatic vines can remain productive, but they also harbor viruses and act as potential reservoirs for virus spread to susceptible vines.
Explaining one reason for the meeting, UC Davis Foundation Plant Services (FPS) director Dr. Deborah Golino said, “I’ve heard people say that red blotch is worse and more harmful than leafroll diseases, and I don’t see evidence to support that.” Golino cited research data from wine grape-producing countries worldwide that showed leafroll-related seasonal reductions in yields of up to 50%, and sugar level reductions of up to 20% compared with healthy vines at the same sites. Red blotch has shown similar (but no greater) levels of reduced sugar levels in grapes of up to 5° Brix. Researchers’ field studies to date indicate that both leafroll and red blotch have similar aggregation patterns in vineyards and similar rates of spread.
Golino provided a history of grapevine red leaf diseases and viruses, beginning with grapevine fanleaf virus (GFLV), first documented in 1841. Other red leaf diseases include (by common names) corkybark, leafroll, vitiviruses, stempitting and fleck viruses. Almost all viruses cause red leaf symptoms in red grape varieties. Golino said red leaf is a hint the vine may have a virus, but she cautioned, “Don’t use vine symptoms to diagnose any of these diseases. Don’t guess, get a test.”
Up to 11 leafroll viruses—more specifically grapevine leafroll-associated virus (GLRaV) species—have been taxonomically identified in the past, but Golino said a new taxonomic proposal suggests five GLRaVs, some of which would have multiple strains or variants. Of most concern is GLRaV-3, the major causative agent of grapevine leafroll disease, and the most widespread species worldwide.
Golino observed, “We rarely see just one virus alone, we commonly see a cocktail of viruses, and this can have a worse effect on vine health.” She added: “Leafroll—and leafroll in combination with other viruses—can be very damaging. I don’t want the current concern about red blotch to take away from the concern we should have about leafroll viruses.”
Red leaf disease research challenges
FPS project scientist Dr. Adib Rowhani pointed out that red leaf symptoms in grapevines could be from a number of causes (including physical vine damage) and from biotic agents (pathogens such as viruses). Chronically affected vines produce weak shoots, small and poorly developed fruit clusters, and experience premature vine death.
Rowhani highlighted challenges in studying red leaf diseases in grafted grapevines associated with graft union disorders: Vines can test positive for viruses but not have symptoms; and certain rootstocks and rootstock/scion combinations (varieties and clones) may have more tolerance, or more sensitivity, to viruses. Rowhani reviewed several red leaf disorders where specific viruses were associated with specific rootstocks and cultivars. One example is 110R necrotic union, with symptoms of red leaf and graft union necrosis. It has a virus associated with 110R rootstock grafted to specific clones of Pinot Noir and Chardonnay.
Rowhani also discussed Syrah decline, with red leaf symptoms and wood necrosis at the graft union. Viruses were found in diseased plants, but no correlation could be found between isolated viruses and the disease. Viruses could be present in symptomatic and asymptomatic vines. Sensitive rootstocks were 110R and 99R, and six Syrah clones were more sensitive than others. Summarizing issues with red leaf symptoms in grapevines with graft union issues, Rowhani said, “Scion/rootstock combinations may elevate expression of symptoms, and plants will eventually die on sensitive rootstocks. But in general, in the majority of cases, the causal agent is unknown.”
Plant pathologist Dr. Mysore Sudarshana of the USDA Agricultural Research Service based at UC Davis has studied grapevine red blotch-associated virus (GRBaV) in Napa Valley vineyards since 2009. Sudarshana said red blotch can be detected by lab analysis in young, 1-year-old vines, but symptoms will not be observable until at least the second leaf. “An infected vine needs at least one dormant season before it shows symptoms of red blotch,” Sudarshana said.
Red varieties observed with GRBaV in commercial vineyards in California to date are Cabernet Sauvignon, Cabernet Franc, Malbec, Merlot, Mourvedre, Petite Sirah, Petit Verdot, Pinot Noir and Zinfandel. White varieties have more recently been found positive and include Chardonnay, Riesling and Roussanne.
As an indication that GRBaV is not just in young vineyards, and may have been in California for some time, G olino and Sudarshana said a 25-year-old vineyard block in Chiles Valley (Napa County) tested positive for red blotch, but vines in adjacent blocks all tested negative.
GRBaV has been found in the following states: California, Oregon, Washington, Idaho, Arizona, Texas, Georgia, North Carolina, Virginia, Maryland, Pennsylvania and New York.
The distribution of red blotch is officially described as being in the U.S. and Canada, but Golino said it exists in other grapegrowing countries worldwide, although to date many have been reluctant to test for or report its presence. Golino said grapevine material coming into the United States through the importation and quarantine program has commonly tested positive for GRBaV. The good news is that GRBaV, like other viruses, can be removed from vine material through meristem shoot tip culture, a standard practice at FPS before vine material becomes part of the Foundation nursery. All vine material planted at the new FPS Russell Ranch Foundation Block has been tested and is red blotch negative.
Red leaf vectors and transmission
Leafroll viruses and other types of red leaf viruses (including red blotch) can be graft transmissible, so it is important to source both virus-free rootstocks and scion material before grafting and planting.
The vine mealybug was the first insect demonstrated capable of plant-to-plant transmission of GLRaV-3 in 1987. Since then, several species of mealybugs and soft scales have been shown to transmit GLRaV species where these vectors occur in grapegrowing regions worldwide. Mealybug species that are found in California and capable of GLRaV transmission include the vine mealybug, grape mealybug, longtailed mealybug and obscure mealybug. The mealybug’s younger nymph (crawler) life stage appears to be more efficient at transmitting GLRaV-3, although all life stages may be capable of transmission.
Kai Blaisdell of the University of California, Berkeley, Department of Environmental Science, Policy and Management is a researcher working with entomologists Dr. Kent Daane and Dr. Rodrigo Almeida to study possible insect vectors of red blotch, leafroll and other grapevine viruses. The UC Berkeley lab is studying mealybugs, leafhoppers and whiteflies as possible red blotch vectors. Blaisdell said, “We started doing virus-transmission studies for red blotch in 2013, but we have not found any vector to date in our lab studies.”
Sudarshana reported that researchers in Washington state in 2013 showed that the Virginia creeper leafhopper could transmit red blotch from vine to vine under lab study conditions. This insect has been found in California, but it is not known to be in any vineyards where red blotch exists.
Blaisdell said based on greenhouse and field studies, there may be a relationship between GLRaV-3 and the vitivirus Grapevine Virus A (GVA) (associated with Kober stem grooving) in terms of the efficiency of virus transmission. In a Napa Valley field trial, nine of 10 vines were co-infected with both GLRaV-3 and GVA.
Although most California mealybug species occupy aboveground vine tissues, some vine mealybug life stages are found on vine roots. This can cause vine replant problems as root remnants from removed infected vines can remain viable in the soil for years, supporting GLRaVs and mealybugs that can reinfect new replanted vines.
General guidelines recommended for preventing and managing red leaf diseases: source and plant virus-free vine material, monitor vines in fall for red leaf symptoms and tag symptomatic vines, test vines for presence of viruses, remove virus-infected and symptomatic vines during the dormant season, monitor and manage insect vectors, and work with vineyard neighbors to prevent disease spread.