“Red blotch is not only novel,” said Cornell researcher Mark Fuchs, speaking at the Napa Grape Growers’ Rootstock conference on November 19. “Red blotch does not behave like anything else we are familiar with.
“Forget about making analogies with Pierce’s disease. Forget about making analogies with leaf roll. We have to empty our hard drive and start rewiring everything, the way we see everything, the way we think about red blotch. So, it requires a whole village to start thinking outside the box.”
Over the last 15 years, grapevine red blotch virus (GRBV) has impacted countless winegrape vines, spreading in mysterious ways with no known cure other than vine removal, which has an enormous impact on profitability. Experts recommend replanting a vineyard when GRBV has infected 30% of the vines. Typically, vintners remove individual infected vines, a challenge as the disease can take three years for vines to show symptoms.
A native of Alsace, Fuchs is professor in the School of Integrative Plant Science Plant Pathology and Plant-Microbe Biology Section at Cornell AgriTech.
In 2012, Fuchs began studying GRBV in Rutherford on a Cabernet Franc vineyard when, he said, he naively made the mistake of thinking the virus was moving in from the riparian areas. But subsequent DNA analysis did not fit that hypothesis. He studied the vineyard for 10 consecutive years, seeing the disease spread year after year.
Over time, he realized there were two distinct epidemics going on in the same vineyard. The clue was the disease spread differently in different parts of the vineyard. There was random distribution (what he called “salt and pepper” type of distribution) in a portion of the vineyard, resulting in a 1% annual increase. In another part of the vineyard, the disease spread quickly and increased 6% per year.
“There are two factors that influence the rate of spread: the level of disease, or the prevalence in any particular block, and the population density of the three-cornered alfalfa hopper (TCAH). These two factors are key to explaining fast spread or slower spread, or very little spread or almost no spread,” Fuchs said. “Because how many times have we seen a fully infected block adjacent to a less infected block and there is barely any spread here? And sometimes it spreads almost like wildfire?”
Untangling the way GRBV spreads is even more complicated because weather events affect the rate of spread, he said.
“This is something very unique to red blotch,” he said. “I’m not aware of any equivalent, whether there is a statistically supported correlation between weather events and disease increase three to four years later. Let that sink in for a second. What I’m trying to say is what we see visually today is the result of an event that happened three or four years prior.”
Weather “doesn’t affect the virus, it doesn’t affect the vine. It affects the three-cornered alfalfa hopper.” Fuchs said TCAH eat mostly from sources outside the vineyard. But in dryer years, they find less to eat beyond the vineyard and gravitate to the vineyards, which are green and irrigated, to find food. Hence, in wetter years, the insect spends more time in the vineyard, but the resulting infection is not seen until three years later.
Fuchs and collaborators came to this conclusion after studying the guts of the hoppers and found TCAH eat more than 150 species. “It eats almost everything, but the plants we found, mostly in their gut, are not grown in vineyard settings. They are in the surroundings. They are in natural habitats. They are in ditches, in dirt roads, in riparian areas, in woodlands and the kind of things rarely grown in vineyards, rarely part of cover crop mixes.
“They don’t spend as much time in vineyards when it’s wet. They don’t need to. They have the preferred food outside vineyard settings,” he added.
Managing Red Blotch: Zonal Roguing
The takeaway from all this research? Fuchs recommended tailoring management to each block and keeping the hoppers out when it rains.
When removing infected vines, Fuchs advocated a specific type of vine removal, zonal roguing, which calls for removing adjacent vines that have not yet shown GRBV symptoms.
“Zonal roguing is a new concept whereby you eliminate diseased vines and some of the neighboring vines whether they are infected or not, or whether they are diseased or not,” he said. Growers can choose from two patterns, he said.
In closing remarks, UCCE Viticulture Farm Advisor Monica Cooper in Napa County said her team was learning more and more about TCAH habitat. “We’re more likely to catch TCAH when our study site is surrounded by grasslands or oak woodlands, and less likely when it’s surrounded by vineyards or riparian woodlands.”
She reiterated zonal roguing is the best way forward.
“We cannot get ahead of GRBV by just pulling out the symptomatic vines every year,” she said. “The issue that we’ve been having is our vine-by-vine roguing efforts have not reduced spread in aggregated areas, so we keep removing all those symptomatic vines and instead of the epidemic going downhill, it still continues to climb.”
She said the best practice is zonal roguing, which removes vines adjacent to infected ones. Though the adjacent vines are asymptomatic, they are likely to show signs of infection in future years, she said.
