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Left: A Merlot grapevine shows redleaf symptoms on mature leaves in the lower portions of the canopy. Symptoms are easily confused with grapevine leafroll disease. Right: Cabernet Franc clusters from a single vine show infection on the left, nonsymptomatic cluster on the right.

Left: A Merlot grapevine shows redleaf symptoms on mature leaves in the lower portions of the canopy. Symptoms are easily confused with grapevine leafroll disease. Right: Cabernet Franc clusters from a single vine show infection on the left, nonsymptomatic cluster on the right.

A new disease that threatens the health of grapevines nationwide highlights the need for stronger clean plant campaigns. The disease, first reported in a Napa Valley vineyard in 2008, has been identified in infected vines in the top three U.S. grape growing regions of California, Washington, and New York, as well as a few other East Coast and southern states, and Canada.

Misidentified in vineyards?

The symptoms of redleaf disease in red-fruited cultivars may look similar to those produced by grapevine leafroll disease, but the new disease is distinct in several respects. Like leafroll, redleaf symptoms show up just after veraison and look very similar to leafroll, which could be why growers and university researchers never paid much attention to it in the vineyard. Many just thought it was leafroll virus. But the epidemiological aspects are quite distinct.

It’s like a person having the flu or West Nile virus. Symptoms can be similar, but the nature of the virus and mode of its spread are very different, and the same treatment doesn’t work for both. With leafroll disease, growers have focused on controlling grape mealybug and scale infestations to slow the spread of the disease. But with redleaf, a new vector is suspected, and a different set of management tactics is required.

How does it spread?

WSU bench graft tests showed that redleaf disease is graft-transmissible. Symptoms on the scion (virus-free Cabernet Franc) were similar to those observed in the source vines used as the rootstock. Based on the wide host and geographic distribution, and the fact that it is transmitted through grafting, the disease is thought to spread through propagative material. Redleaf disease has been detected in young and old vineyards.

Research conducted at WSU has implicated the Virginia creeper leafhopper (Erythroneura ziczac Walsh) as a carrier or possible vector based on tests under greenhouse conditions. Both nymphal and adult leafhoppers have piercing and sap sucking mouth parts.

Economic impact

The economic prosperity of Washington’s wine industry is at risk of irreparable damage due to the new disease and its negative impact on fruit and wine quality. Initial reports from University of California and USDA scientists at Davis have found the new disease can result in significant reduction of sugar accumulation in fruit—up to 5° Brix.

Dr. Naidu Rayapati’s research team, among the first to conduct extensive yield and fruit quality evaluations of diseased vines, found reduced yields of 22 percent and 37 percent in own-rooted Merlot and Cabernet Franc, respectively, when compared to nonsymptomatic vines. Pruning weights and shoot lengths of infected vines of Merlot and Cab Franc were also reduced—pruning weights by 25 percent and shoot lengths by up to 20 ­percent.

An analysis of fruit quality attributes showed that berries from redleaf disease-affected grapevines of both cultivars had significantly lower soluble solids (12 percent in Merlot, 14 percent in Cab Franc), higher titratable acidity (9 percent in Merlot, 16 percent in Cab Franc), and lower extractable anthocyanins (Merlot 4 percent, Cab Franc 9 percent) when compared to berries from ­nonsymptomatic vines.

In contrast, there was no difference in the pH of juice extracted from berries of ­symptomatic and nonsymptomatic grapevines of both cultivars.

Taken together, these results clearly showed significant negative impacts of grapevine redleaf disease on vine vigor, fruit yield, and berry quality attributes in own-rooted wine grape cultivars of Merlot and Cabernet Franc, under commercial growing conditions.

Preliminary surveys conducted last year by Rayapati’s team indicated the new disease is prevalent in other red-berried varieties, in addition to Merlot and Cab Franc. It’s unknown if the disease causes symptoms in white grape varieties.

Additionally, WSU does not know if the new disease was originally introduced into Washington State through cuttings imported from outside sources.

In California, the disease has been observed primarily in vineyards planted with red varieties, with some detected in Chardonnay, according to the U.S. Department of Agriculture and University of California, Davis. Red varieties infected, thus far, include Cabernet Franc, Cabernet Sauvignon, Merlot, Malbec, Mourvèdre, Petite Sirah, Petit Verdot, Pinot Noir, and Zinfandel.