For wine grape growers facing the threat of grapevine red botch disease, which can cost them tens of thousands of dollars in ruined vines and fruit, precious little is known about the disease, the virus that causes it or the insect that can spread the virus.
That promises to change, if efforts by Frank Zalom, University of California professor and Cooperative Extension entomologist, and USDA-ARS virologist Mysore (Sudhi) Sudarshana at UC Davis, succeed in filling in the critical gaps in knowledge grape growers need to limit the damage from the disease.
Caused by the Grapevine Red Blotch-associated Virus (GRBaV), the disease can be transmitted by a three-fourth-inch long, wedge-shaped green insect – the three-cornered alfalfa hopper. This insect is primarily known as a pest of legume crops, such as peanuts, soybeans, and alfalfa in the southern United States. It can be found in California grape vineyards, but feeding damage, girdling of canes and leaf petioles from its feeding is not considered economically important. Consequently, its biology has not been studied on grapes.
As Zalom notes, a number of basic questions need to be answered about the virus and the three-cornered alfalfa hopper including:
- How widespread is the disease?
- How does it damage grape vines?
- Is there a way to reduce the impact of the disease?
- Are there other vectors of the disease?
- How much time is needed for the vector to acquire the virus?
- How long does the virus persist inside the vector?
- Is there a faster, less costly test for identifying presence of the disease in vineyards?
In red grape varieties, symptoms of the disease include red blotches on leaves later in the season and, frequently, reddening of leaf veins beginning in mid-summer. Patches of pale green to pale yellow appear on the leaves of infected white grape varieties.
The disease prevents grapes from ripening fully, reducing Brix levels by as much as five units on red varieties and adversely affecting pH, anthocyanin and tannin levels and other phenolic factors that affect the quality and market value of wine made from the infested fruit.
“Some premium California wine producers estimate that a 100-percent red blotch infection can reduce the value of a grape crop by as much as $68,000 per acre,” Zalom says. “The juice from those other-wise high-value grapes can‘t be sold.”
Replacing the infected vines with virus-free stock – currently the only way to control red blotch – and the value of several years of lost production until the new vines reach bearing age is very expensive and a difficult choice for growers, he notes.
Grapevine red blotch first attracted attention in 2008 when UCCE researchers began examining some diseased vines in a Napa County vineyard. Some of the symptoms resembled those of grapevine leafroll disease. In fact, that similarity, has probably led some growers and crop advisors over the years to mistake red blotch for leafroll.
However, upon looking more closely, the UCCE researchers suspected that the symptoms they were seeing indicated an entirely different and unidentified disorder. It took four years of testing before Sudarshana could confirm the disease was caused by Grapevine Red Blotch-associated Virus (GRBaV).
The virus does not appear to be new to California. Testing of dried grape leaves in a herbarium specimen by UC Davis plant pathologist Deborah Golino and colleagues at Foundation Plant Services revealed the diseases-causing virus was present in California at least as early as 1940. The extent of the virus in the state then is not known.
Since that 2012 confirmation, red blotch has been found elsewhere in the North Coast area, as well as the northern Sierra Foothills, in the Central Coast from Paso Robles to Santa Barbara County and in the Temecula area of southern California, but a systematic survey has never been conducted. Red blotch has also been reported in the San Joaquin Valley.
Meanwhile, the virus has also been confirmed in Oregon, Washington and eight other states in the country’s major wine producing areas.
So far, other than vineyards, the only other known source of GRBaV are wild grapes growing wild in riparian areas, Zalom says. “We don’t know if it occurs in other plant hosts.
Initially, red blotch was believed to be spread only by planting infested nursery stock. However, some UCCE farm advisors and grape growers began seeing the disease spreading in ways unrelated to planting virus-infected vines.
Based on distribution patterns of the disease, Zalom and Sudarshana speculated that the disease-causing virus was being spread by a vector, probably one that flew. Over an 18-month period Zalom’s team of researchers tested more than 100 insects that could be capable of transmitting the virus. A year ago they announced that they had found one – the three-cornered alfalfa hopper – that transmitted the Grapevine Red Blotch Associated Virus (GRBaV) to grapevines in their greenhouse tests.
Considered a minor pest of vineyards in the North Coast, the three-cornered alfalfa hopper also feeds on alfalfa and other legumes, as well as grasses, shrubs and other herbaceous hosts. The insect spreads the virus by feeding on the vines.
“The three-cornered alfalfa hopper has mouth parts that pierce the plant’s phloem tissue and injects salivary fluid into the plant,” Zalom notes. “This fluid pre-digests the plant material before the insect sucks it up. In the process, the disease-causing virus is injected into the grapevine.”
Currently, Zalom and Sudarshana are testing other closely-related insect species to identify any that may be able to transmit GRBaV.
Vectors, like the three-cornered alfalfa hopper, or grafting with virus-infected scions or root stock, are the only ways that red blotch is known to be spread. “We’ve found no evidence that it can be transmitted by pruning shears or other tools or any mechanical means,” Zalom says. “The virus can live only inside a host plant or a vector. Without a source of the virus, a vector is not a disease threat to a vineyard. If your vines are virus-free, it doesn’t matter if the three-cornered alfalfa hopper is in your vineyard. In fact, if it is, it’s probably been present for years.”
Zalom offers this advice to limit the spread of red blotch:
- Remove virus-infected grapevines and plant only nursery material that is free of GRBaV.
- “Once the virus is inside the plant, there is no cure,” Zalom says. “Currently, the only way you can control it is to remove infected vines.”