Every few years, the wine business hears of a devastating new pest or disease and has to mount a full-court press to overcome it.

The most famous was phylloxera, the tiny bug that migrated from tough eastern grape vines to Europe, then California, and almost wiped out their wine grape industries. The solution was the well-known practice of grafting susceptible European vines to resistant roots, most hybrids of American grape vines.

Other pests and diseases have followed in recent years: the glassy winged sharpshooter that threatened to infect all vines with deadly Pierce’s disease, powdery mildew, Eutypa and brown apple moths — the list seems endless.

The local community has been able to overcome each pest so far with fast action, quarantines, introducing natural predators and different vine management — or pesticides.

Pesticides are an undesirable solution for almost all growers, but they work. Growers use them to grow European grape vines in central Texas, the home of Pierce’s disease, and pesticides have also saved the citrus and grape businesses in parts of Southern California.

The latest pest scourge is scary because we don’t have a cure for it, don’t know how it spreads, and don’t even have a complete characterization.

It’s the red blotch-associated virus that appeared a few years ago. It was first noticed by UC’s Jim Wolpert at the Oakville experimental station in 2008 and is now infesting many local vineyards.

At first glance, it seems similar to other common viruses that turn infected grapevine leaves beautiful reds in the fall, but it’s distinctive and not even officially classified yet.

It keeps grapes from ripening fully and accumulating enough sugar, typically limiting it to no more than 20 to 21 percent compared to 24 to 27 percent in healthy vines. “It’s not devastating — it doesn’t kill the vines — but it’s bad,” said Andy Walker, an expert at UC Davis last week at a seminar in Davis.

Other red leaf viruses have been around for at least 60 years, Walker noted, but they were reduced to about 20 percent of vines by planting clean plants.

Even before red blotch surfaced, fanleaf virus was spreading fast, up to 80 percent of vines in some blocks. We now know that certain grape moths spread fanleaf virus, but there are ways to control them at least

Walker suspects that some of rise in infected vines is caused by growers who demand certain chic grapevine clones even if they’re not fully tested — or may even be suitcase imports.

But the red blotch virus is also spreading by a mechanism researchers have not yet identified. Even if you plant clean nursery stock, the plants may get sick from neighboring vines — and quickly.

On top of that, you basically can’t “cure” viruses, just mitigate them or replace the plants, but we don’t even know how the vines are being infected except through defective plant material. “This disease is way ahead of the research,” Walker warned.

Although some wags suggest that red blotch may help reduce Napa Valley’s notoriously high sugar and alcohol levels, it’s no laughing matter. Walker says it will take a community-wide effort to remove and replant at least — but until we’re sure why it’s spreading, we can’t rest.

A note: The April Wines & Vines and the March Wine Business Monthly have big sections devoted to red blotch. UC Davis Extension also has bulletins available.

For more information on red blotch virus, visit NapaGrowers.org or NationalCleanPlantNetwork.org.

Email Paul Franson at paul@paulfranson.com.

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