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Pierce’s Disease requires two pronged approach for prevention, treatment

From the Napa Valley Wine Insider Digest: Oct. 9, 2021 series
  • Updated
Pierces Disease in a Vineyard

Pierce's Disease symptoms showing themselves in an infected vineyard. 

Driving through Napa County, you may recognize billboards warning of bugs called glassy-winged sharpshooters. And if you know anything about vineyard management in California, you’ve also probably heard of Pierce’s Disease, the grapevine bacteria spread through these leafhopper insects.

But tackling the issue that is Pierce's Disease in Napa Valley’s wine country is more difficult than squashing rogue sharpshooters and blasting insecticides across vineyards — it is an issue requiring the minds of California’s brightest when it comes to plant disease prevention.

“Xylella comes in different types within the same species or subspecies, and those different subspecies attack different types of plants,” said Neil McRoberts, professor of plant pathology at UC-Davis. “They’re what is called a fastidious bacteria, so [that means] you can’t grow them in culture on a plate in the lab; they have to be inside of their host plant.”

“That actually makes them really difficult to research and investigate,” McRoberts explained. “You can’t really work with them in isolation.”

Luckily, McRoberts and other experts have grouped up to create the Pierce’s Disease Advisory Task Force as part of the CDFA’s initiative to prevent the disease in California’s winegrowing regions, and will hopefully be able to come up with a management and treatment plan suitable for local vintners. Representatives from the Agricultural Commissioner’s office, USDA and more have all been recruited to mitigate the risks associated with this particular invasive pest, as the nature of its spread complicates treatment.

“Those insects feed on what is called the xylem, the system in the plant that is like the plumbing system inside the plant that brings the water up from the roots,” McRoberts explained. “They’re feeding on the liquid that is coming up from the soil to the leaves and shoots, and when those insects stick their mouthparts inside the plant to get access to the water, they release bacteria into the pipework.”

“Basically, the bacteria multiply and gum up the works and that’s what causes the symptoms that we see,” he said.

These symptoms include yellowing, dried and falling leaves, as well as shriveled fruit and irregular growth like green islands, where patches of young, green tissue are surrounded by brown bark.

“If you take out the vines that are infected and you manage the vector population — the insect population — effectively, you can do a decent job of staying on top of Pierce’s Disease,” said McRoberts. “Particularly in the North Coast area where the glassy-winged sharpshooter hasn't been established yet.”

But the lack of these bugs doesn’t mean Pierce’s Disease hasn’t impacted the region. There are other, indigenous species that also can carry the disease from vine to vine, they just aren’t as destructive as the former.

“The indigenous leafhoppers and sharpshooters don’t move around very much, and because it moves so slowly it's a disease that people can usually stay on top of,” said McRoberts. “It is really when the glassy-winged sharpshooter gets into the picture that people really have trouble with the disease because it spreads much more quickly and has a different behavior.”

Once infected, the vines will show symptoms fairly rapidly, with McRoberts saying you will typically know by the end of the season if there is an infected area of the vineyard.

“That’s good because that means that those infected vines are not in the vineyard for very long-acting as sources of infection, but it is obviously dramatic and bad for the growers because they see the vines going down relatively quickly,” he said.

Historically, folks would remove the affected vines and spray insecticides, but there wasn’t much else that could be done to treat or prevent PD. Biological control has been used to manage vector pest populations, but often lacks the strength and precision needed to save a vineyard from the disease.

Some recent findings have been made, though, and the potential for preventing and treating PD and glassy-winged sharpshooter invasions is promising. On the front end of things, McRoberts points to an interesting project done in San Joaquin Valley.

“The USDA is looking at discouraging the insects from landing on the vines by using little vibrating devices that are attached to the trellising,” said McRoberts. “These are insects that communicate with each other by vibration, and so[they] are trying to work out if they can mimic alarm signals that the insects would send out to each other.”

By emulating the “don’t come down here” pleas of a sharpshooter, the vines are thus left (hypothetically) untouched and uninfected.

As for the treatment side of things, Texas A&M researchers have worked alongside pharmaceutical company A&P Inphatec to identify a way to minimize the symptoms of PD in already-infected vines, and recently announced their newest product. A form of bacteriophage therapy, this treatment is called XylPhi-PD and injects an “otherwise harmless” virus into the plant, which then eats the bacteria that causes PD.

In Sonoma County, five vineyards took part in a XylPhi-PD field study assessing year-to-year symptoms and the resulting impact on yield, which according to reported data, ultimately proved to have about 60 percent efficacy. The treatment has been approved at the federal and state level here in California, and ultimately has the potential to cut costs for vintners that may otherwise need to replant infected vines, but isn’t at fix-all levels yet.

As research continues and those advising legislators — like McRoberts and the rest of the PD Advisory Task Force — continue to advocate for increased funding and prevention solutions, grape growing stakeholders will need to stay on high alert for PD and the presence of glassy-winged sharpshooters, as 2014 numbers estimate its destruction costs California nearly $104 million per year.

Miguel Garcia shows how he uses technology to help sustainably manage the demonstration vineyard owned by the Napa County RCD.

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