A year ago, a team of researchers at University of California, Davis who were digging into the mysteries of sudden vine collapse acknowledged: “We don’t know the full extent or probable cause at this point.” A year later, although some progress is being made, it’s still an unexplained mystery as to why patches of grapevines are collapsing without apparent reason.
Since the initial appearance of this malady in 2010 in some Lodi vineyards, growers have seen as much as a 30% collapse of their vines over a year’s time. Loss of vines is to be expected --- from injury, stress, rot, disease --- and these stunted, dying, or dead vines are removed without undue concern. But a third or more of an orchard dying, and replants also collapsing, that’s flat-out alarming.
Initial analysis a decade ago turned up specific pathogens in a vine, but they were not consistent across the vineyard site and didn’t match the pattern of spread. About the only commonality was mature Freedom rootstock showing leafroll virus which can decrease a yield and contribute to mature vine demise, but probably not cause its ultimate downfall.
In that intervening time, researchers have postulated many theories “from phylloxera to phytophthora,” according to Lodi Wine Growers. As of a couple of years ago, the working hypothesis revolved around a combination of leafroll virus, certain rootstocks, and some kind of stressor as the cause.
Around that time, plant pathologist Akif Eskalen at UC Davis became the latest detective to try and solve the mystery by building on previous research and working across scientific disciplines.
“We’ve had grape grower reports of sudden vine collapse throughout the San Joaquin Delta, Coastal Counties, Modesto, Fresno, Tulare, and Kern Counties. In some cases, the patches are so large they can be seen via Google Earth satellite image," he says.
Not a single pathogen
At this point in his research, Eskalen notes: “Hypothesis is that SVC is not caused by a single pathogen, but a disease complex where vines grafted on virus-sensitive rootstock are predisposed to root stress due to co-infection by a leafroll virus (commonly GLRaV-3), vitiviruses (like GVA and GVF), and possibly others.
“They appear to be making the initial infection which results in girdling at the graft union that allows other fungal pathogens to thrive and cause the collapse.”
It all happens quickly. Stunted shoot growth, less than half the size of healthy shoots, is the early season omen before entire vines start dying quickly within the vineyard, in some cases, death coming so rapidly that the leaves and berries remain dry on the plant. The entire vine can go from having green shoots to being completely dried up --- without cankers --- in a matter of just a few weeks. The patch of collapse itself tends to spread in the direction of the wind.
Once an infected vine dies and dries up, hungry virus-carrying vine mealybugs and scales in search of a food source move on to a live vine, potentially compounding the spread of virus and contributing to a collapse.
While further research specifics are being awaited, initial management options include testing to confirm co-infection of the viruses (all the leafroll viruses and vitiviruses common in California), rogueing and destroying the infected grapevines, vector control, and in the case of replanting, a possible change to less-sensitive rootstock.
As Dr. Stephanie Bolton at the Lodi Wine Growers notes ---“There is no cure for grapevine viruses now and in the foreseeable future. There is financial assistance through the USDA Tree Assistance Program for eligible growers who need to remove leafroll virus-infected vineyard.
“This is one of the more important diseases we have to figure out because its graphic reach is changing,” says Eskalen. “Our next step is an extensive survey throughout the grape-growing areas of the state to determine just how wide-spread the problem is. We’re also planning pathogenicity testing to confirm our hypothesis of this disease complex. We haven’t published a scientific paper yet since we haven’t completed our research, but it’s our job to find out how to fix the problem and develop a sustainable solution for the future.”