Grapevine Pests and Diseases are On The Uptick

In 2022 California wineries and vineyards—along with their allied businesses—delivered a total annual economic contribution of $73 billion to the state’s economy and $170.5 billion to the U.S. economy. They also directly and indirectly generated 422,000 jobs in California and 1.1 million jobs across the nation, according to a new report commissioned by the Wine Institute and the California Association of Winegrape Growers.

Given the economic importance of the wine industry, the sudden rise of vineyard-specific disease and pest threats is alarming. These threats run the gamut from bacterial and viral infections to damage from fungi, molds, mildew, mites, insects and a host of others. And if the diseases weren’t worrisome on their own, the vectors that transmit and transport them can be just as troubling.

In 2021 five glassy-winged sharpshooters (GWSS) were found in a Solano County vineyard. Although there were relatively few, the find sent shudders through the state’s agricultural community, and a rapid-response team was deployed to successfully contain the outbreak.

The small GWSS insect can travel fast and far. It multiplies quickly and feeds on the fluids that course through a grapevine. However, it’s not the loss of fluid that the farmers fear. Instead, it’s a bacterium—Xylella fastidiosa—that these insects transmit while feeding.

Once X. fastidiosa enters the vasculature of a grapevine, it begins to choke off fluids and nutrients, eventually killing the entire vine. The condition, called Pierce’s Disease (PD), is not foreign to California and was in fact first reported nearly 100 years ago. At that time the disease was considered mild. It spread slowly with only one or two vines being affected at a time. However, in 1999 that changed. In the Riverside County city of Temecula an entire 300-acre vineyard was decimated by the disease. Near panic broke out. A frantic search ensued to uncover how a previously benign disease had burst onto the scene like an out-of-control plague.

But the glassy-winged sharpshooter and Pierce’s Disease are not the only such threats.

The parasitic “root louse,” phylloxera, first decimated vineyards in California and Europe in the 1860s and then re-devastated them in the 1980s. And then there’s the Western grape leafhopper (Erythroneura elegantula), which is known to spread the red-blotch-associated virus that causes Red Blotch Disease in grapevines.

Currently there are more than 70 different viruses that can negatively affect vines. Some of these viruses can be transmitted by insects, while others are transmitted by grafting or using infected planting material.

Some of the most common grapevine viruses include the leafroll-associated viruses; fanleaf virus; viruses A, B and E; fleck virus; and rupestris stem-pitting-associated virus. Each of these diseases wreaks its own havoc on the vines. They can include stunting, chlorosis (chlorophyll deficiency), reduced fruit yields, leaf mottling, shortened vine lifespan or death.

And more are on the way. In 2014 the spotted lanternfly (Lycorma delicatula) was found in the eastern United States, and it is quickly spreading westward. Native to China, this invasive planthopper has not yet been found in California, but is known to feed on a wide variety of plants, including grapevines. Like the GWSS, these insects feed on grapevine fluids, which can cause wilting, leaf curling and reduced grape production. The excrement of the spotted lanternfly can also contain other pathogens, including a fungus known to cause black rot—a serious and costly grapevine disease.

To combat vine disease a variety of treatments are applied to the vines or the vineyards. The most common of these is the removal of affected vines (roguling), spraying sulphur dust or other chemicals, or applying chemical agents directly through the irrigation (chemigation). Beyond these solutions, it is also important to redouble efforts to ensure that pathogens, diseased material and contaminants be avoided when transferring material and even personnel between vineyards.

Due to a host of drivers—extreme weather, climate change, novel pest pressures and the risk of rapid monoculture contagions, among others—the threat to vineyard health is expected to increase in the coming years. And because the entire wine industry is dependent on the healthy production of wine grapes, it is imperative that everyone involved rallies around efforts to track, treat and prepare for what are likely to be ever-increasing dangers.