Red Leaf Viruses Update
Winegrape growers in California, as well as other scattered locations across the country, are seeing red in their normally green foliage, and they want to know why. Many assume their problem is the latest disease to be identified, red blotch.
But they shouldn’t jump to conclusions, warns Andy Walker of the University of California-Davis Department of Viticulture and Enology. Walker addressed an audience of growers, Extension advisors, and other industry players at a mid-May seminar on the UC-Davis campus titled: “An In-Depth Look At Red Leaf Viruses.”
Walker said there’s an awful lot unknown about what causes leaves to turn red, followed by plunging yields and Brix levels.
“We don’t even know if they are viruses or not,” he said.
Diagnosing such maladies is incredibly complicated, noted just about every speaker. Maher Al Rwahnih, a scientist at Foundation Plant Services at UC-Davis, noted that so far, 63 viruses have been found in grapevines.
“Fortunately not all of them cause problems,” he added.
The director of Foundation Plant Services, Deborah Golino, gave the audience an overview of the history of leaf diseases. She, too, noted that there is a lot that’s unknown about the diseases, and a lot that’s supposedly known, but has since been debunked. For example, it wasn’t until 1987 that it was discovered that mealybug transmitted leafroll.
“I arrived that year at UC-Davis, and like everyone else, I thought it was transmitted in the field,” she said.
Speaking of leafroll, she said that with all the concerns about red blotch, many in the industry are forgetting about all the other viruses.
“A red leaf is a hint, it’s not a diagnosis,” she said. “It’s a hint you might have a viral disease.”
Golino said she has heard many people say that red blotch is worse than leafroll.
“I just don’t think that’s true,” she said, adding, “we don’t have enough data yet on red blotch and yield.”
Complicating matters further, Golino said that diagnosticians usually don’t find just a single virus in a vine with red leaves, but a “cocktail” of viruses.
“Multiple viruses is the common denominator of vines we test at Foundation Plant Services,” she said. “Whether from Europe, Australia, wherever, multiple viruses is much more common than single virus infections.”
Golino was asked what other countries are doing about red blotch.
“They are studiously not looking for it,” she said, drawing a laugh from the audience. She added that Australia is using red blotch as a trade barrier, and her counterparts in Europe are “remarkably mute on the topic.”
The Foundation for Plant Services tests a lot of vines — “I can’t tell you many boxes of latex gloves we go through each month” — including vines from all around the world. Golino was asked if any of those vines have tested positive for red blotch, and she responded “All the time.”
Red blotch has been found all over the U.S., and Golino thinks it will eventually be found all over the world. Just because the disease was only diagnosed a few years ago doesn’t mean it’s a new problem.
“I think it co-evolved with vitis vinifera,” she said. “It would be great fun to go test some isolated 100-year-old vineyards.”
Growers who want to avoid the virus should start with clean vines and stay clean. The good news is that Russell Ranch, from which virtually every nursery the state sources vines, is 100% free of red blotch, she said. They plan to keep it that way through constant testing and by not bringing in any equipment from other vineyards, using separate tractors and tools. They have an advantage in that Russell Ranch is isolated from other vineyards. They convinced the few neighbors they do have with any vines to pull them out.
Golino was asked by one grower if it’s possible to take matters in his own hands in getting rid of red blotch. Golino said she’s seen it done.
“I’ve worked with growers who’ve rogued out viruses, and from then on used only their own scion wood, sourcing rootstock from a source they have complete confidence in,” she said.
Because the problem is so potentially devastating, a lot more money has rolled in to fund research. Golino said there are five groups of entomologists, plant pathologists — not just individuals — working on finding an answer. Among their research goals is to find out why red blotch spreads in once case, and doesn’t in others. Insects have often been found to be vectors in the path, as mealybugs are for leafroll.
“We are frantically screening leafhoppers for a vector of the disease,” she said.
One reason researchers are looking at insects as the potential vector is because of the way it spreads. In the old UC-Davis vineyard, the Classic Foundation Vineyard, only nine vines had red blotch out of the thousands tested in the blocks. What’s odd is that in clones side by side, one vine would have it and one would not. There’s also the fact that no grapevine virus has been found to be mechanically transported.
“Two vines are side by side for 25 years, and one has a virus and does not? If it were mechanically transported, how do you explain that?”
There was an impressive roster of speakers at UC-Davis’ recent “An In-Depth Look At Red Leaf Viruses.” Here’s a brief sampling of what they had to say.
Mysore Sudarshana, USDA-ARS:
- It’s important to remember that this is all quite new about red blotch. Though it was discovered in 2008 because of reduced sugar and delayed harvest, it tested negative for known grapevine viruses. Its symptoms resembled leafroll, but it was identified as red blotch later.
- On some cultivars, such as Merlot, the symptoms are more dramatic.
- Until last year, we didn’t know if it existed in whites — the symptoms are far less obvious.
- It’s found in all the Pacific states, along the East Coast and in Texas.
- Red blotch incidence has become more common than leafroll.
- Grower responses have been all over the map: Some tolerate it, some add phosphorous and reduce load, some make sparkling wine, some remove and replant.
- How to manage red blotch? Source clean material, keep track of its appearance, tag symptomatic vines. Ideally we would carefully manage the vector, but we don’t have clear information on what the vector is.
Kai Blaisdell, UC-Berkeley
- There’s a lot of variation in viruses. For example, Leafroll 3 — commonly referred to as just leafroll — has six variants.
- Five different mealybugs spread leafroll in California.
- Grapevines can pick up more than one virus at one time.
- Red blotch alone is rare — it usually comes with leafroll.
- Different varieties of grapes exhibit symptoms of a given virus in different ways.
Kari Arnold and AJ Campbell, UC-Davis
- Scouting is so important, you have to be out looking for the virus. And since you’re scouting anyway, you should be mapping.
- Infection comes from adjacent blocks, so my neighbor’s leafroll is my leafroll — and my leafroll is my neighbor’s leafroll.
- Red Blotch incidence follows generally that of leafroll.
- Sampling must be random to present as true a picture as possible. Take two samples per vine, one from each cordon — minimum.
- Talk to your diagnostics lab about sampling.
Rhonda Smith, UC-Cooperative Extension, Sonoma County
- Learn to live with red blotch because a lot of young vines have it.
- You’ve got to develop an accurate, efficient strategy to map diseased vines in commercial blocks, though it’s easier said than done.
- Growers have told her they think there are different strains of red blotch disease, but we don’t know for sure.
- Blocks have tested positive for red blotch virus, yet the fruit and wine quality was excellent.
- When a block ripens a little slower than it “should,” will that block become a liability down the road?