by Bill and Mary Weaver
Red Blotch Disease, a devastating virus disease that can affect all grape varieties and root stocks, was first identified as a separate disease in Napa, CA in 2008. In exceptionally rapid work, Marc Fuchs, plant pathologist at Cornell, had developed a DNA assay for detecting the virus in 2012. “Much of this information on red blotch came from Marc Fuchs,” stated Dr. Gary Pavlis of Rutgers, at the Mid-Atlantic Fruit and Vegetable Convention in late January. “We’re really relying on him to move this along.”
Red blotch somehow jumped from the west coast to the east coast. It was positively identified in New Jersey, including in a Rutgers experimental plot near Cream Ridge in Syrah vines, in late summer of 2014. “I’m planning to pull those vines,” said Pavlis.
“Nurseries have access to clean material,” continued Pavlis. “If you are considering planting to replace winter-killed vines or to start a new vineyard, do it. But you have to do your homework. You can’t just get on the phone, place your order, say ‘Thank you very much,’ and hang up.
“Ask the nursery for documentation that the material has been tested for red blotch virus and is clean. Also, ask when it was tested. The technology has totally changed. If you’re planting a vineyard, red blotch has to be foremost in your mind.” Microshoot tip culture, useful in eliminating viruses in some plant material, is only 24 percent effective in cleaning up this virus.
While a tremendous amount has been learned about red blotch in the few years since it was recognized as a distinct disease, much is still not known.
Scientists know that red blotch is caused by a virus in the Gemini family that does not usually infect fruits, other than citrus. The Gemini virus quietly does its work by clogging the phloem tubes that transport sugars to the grapes.
There is probably a latency period of + or – 2 years between infection with the virus and visible symptoms on the plant, but again, no one knows for sure. Once symptoms appear, the brix of the grapes will not develop fully. In one New Jersey vineyard unaffected vines had a Brix level of 18, and infected vines were at 14 Brix, according to Pavlis. Thus, ripening will be delayed, and the juice chemistry will be altered, giving the juice higher malic acid levels and higher titratable acidity.
Simply put, vines infected with red blotch do not produce fruit that can be used to make the finest wines. “If less than 25 percent of the vines are infected, rogue those vines,” Pavlis advised. “If more than 25 percent are infected, the standard advice is to rip out the vineyard and replant,” an expensive proposition indeed.
Not everyone with high levels of infection chooses to rip out and replant. “As Marc Fuchs said, ‘If you were making Merlot, you can still make rosé out of those grapes.’” An attendee from a Pennsylvania vineyard and winery commented, “We haven’t taken our vines out. Even though the fruit is not as good, we can still use it in table wine.”
But the tragedy is still there. As Pavlis noted, “South Jersey Winery was making a high end Merlot. Making rosé didn’t fit into its business model at all, which was to produce fine wines. That winery is tearing out a whole block of established vines. These were seven year old vines. Did the virus come in on the wood and slowly emerge, or did it arrive later?”
How is the red blotch virus spread? First, it is not spread by pruning, as far as is known. Marc Fuchs has said that mechanical transmission from pruning has not been shown for any grape virus. There is no evidence that it is necessary or even helpful to disinfect pruners between plants.
The virus IS spread, however, by infected canes and grafts. Scientists believe that the disease probably existed for years before it was identified, and was assumed to be leaf roll, which it can resemble, or possibly potassium deficiency or spider mite damage, which can also cause reddening of grape leaves. Red blotch differs from leaf roll in two noticeable ways, however. First, it does not have the distinctive downward curling of leaf margins found in leaf roll virus, and second, while the leaf veins in leaf roll virus remain green, the secondary and tertiary leaf veins in red blotch turn red.
A lot of infected wood was unknowingly sold during that period when the nursery industry was blissfully ignorant of the threat. However, there is also the question of a vector that could be spreading the virus from vine to vine within a vineyard. If there is a vector, though, no one has figured out what it is. In one case, there appeared to be transmission from vine to vine within a greenhouse by the Virginia creeper leafhopper, but there has been no other direct evidence of vectoring, and this instance raises as many questions as it might appear to answer.
No one is quite certain how the virus has spread so widely so quickly, but likely this has resulted from infected canes and grafts. Red blotch is found today on the west coast, in the mid-Atlantic, the southeast, in Texas and in Idaho — in a total of 11 states — and in British Columbia.
But there is evidence of another mechanism at work. Pavlis cited Chambourcin vines in Arkansas that tested positive for red blotch. “It’s fairly safe to say,” added Pavlis, “that the wood did not come out of California.”
Sadly, red blotch affects grapes of all varieties, colors, and types. Red blotch infects wine grapes, table grapes and raisin grapes, and it also infects all rootstocks. If there are varieties or rootstocks that are more tolerant of the virus than others, that has not yet been established.
The virus can be found in all plant parts in the winter as well as during the growing season. “I once stuck a piece of vine in the freezer and forgot about it for a while. When I brought it out, it tested positive,” commented Pavlis.
The virus infects the leaves, petioles, canes, dormant canopy, clusters, and rootstocks. “It infects everything!” summarized an attendee.
To make the situation even more confusing, Pavlis explained, the same plant may exhibit symptoms at some times of the year and not at others, symptoms can vary according to weather conditions, and white grapes exhibit symptoms differently.
“In summer, fewer symptoms will be visible because in hot weather, the vines can better fight the effects of the virus,” stated Pavlis. “With the arrival of cooler weather in the fall, the symptoms are generally more evident. So the same plant, in different seasons and under different weather conditions, can appear to be infected or not infected.
“The only thing to do,” Pavlis continued, “is to flag suspicious plants, date the flagging, and watch them over a period of time. Don’t guess! The only way to be certain whether you have infected vines is to have them tested. At Rutgers, we send all our suspicious samples to Marc Fuchs at Cornell, although a number of labs do offer the test.”
Research in New Jersey is currently focused on what potential vectors may live in that area, and whether one of those potential vectors may transmit red blotch. “We’re testing plants throughout the state. This is a moving target,” added Pavlis.
Red Blotch Disease: What is known and not known
by Bill and Mary Weaver