The long-term impact of fumigants on vineyard microbes

A well-tended grapevine in a modern vineyard produces a bountiful harvest for at least two decades. In fact, a winery in Maribor, Slovenia claims the oldest vine in the world, planted along the city wall in the 17th century. The wall no longer skirts the city, but you can still see the mayor of Maribor ceremoniously harvest The old vine’s green grapes, which are fermented and bottled in miniature bottles, according to a website for the adjacent winery known as the The Old Vine House.

Most vineyards can’t boast of harvests from a vine that’s been through both World Wars, but viticulturists still hope to plant seedlings that will stay healthy and produce grapes as long as possible. And a healthy permaculture crop starts with healthy roots.

Modern vineyard managers rely on soil preparation methods like fumigation to make sure grape seedlings—Vitis vinifera L.—get a vigorous start. Methyl bromide (MeBr) used to be a key tool in the viticulturist’s kit. It effectively knocked back parasitic nematodes and pathogenic species of Pythium and Fusarium microbes, which cause root rot and fusarium wilt, respectively. But on 1 Jan. 2005, MeBr—a potent greenhouse gas—was phased out from agricultural use when the U.S. signed the Montreal Protocol.

“We knew, back in 2005, that MeBr was the major compound for fumigating,” Dong Wang says. “It was our aim with the USDA to find compounds that can still do the job but easily dissipate and degrade without causing atmospheric problems.”

This phaseout spurred Wang, a USDA-ARS research leader, and Sadikshya Dangi, an ARS microbiologist, to ask: what can vineyard managers use instead? And how do different fumigants impact the soil microbes in the long term?

In a 10-year project, Dangi and Wang teamed up to test the effectiveness of alternative fumigants compared with MeBr. They evaluated the microbial communities in an active vineyard before and after planting seedlings in soil treated with three different fumigants and various application techniques, publishing their findings in the Soil Science Society of America Journal (SSSAJ; https://doi.org/10.1002/saj2.20186).

Dangi and Wang found that there are good alternatives to MeBr and that the initial impacts of fumigants do, indeed, change the microbial population structure in vineyard soil—impacting the overall health of grapes, an important permaculture crop.

Planting Grapevine Seedlings

California is the number one grape-growing state in the U.S. with grape acreage in 2020 totaling 895,000 acres according to the USDA National Agricultural Statistics Service. Wine-type grapes made up 620,000 of those acres.

Though wine grapes like Chardonnay and Cabernet Sauvignon can return a hefty profit once they turn into value-added products (namely, wine), planting out new acreage is incredibly costly. Seedlings, soil preparation, fumigation, labor, and infrastructure like posts and trellises for the vines all rack up costs in record time. In 2015, the University of California–Davis Cooperative Extension estimated that planting a single acre of vines costs about $20,000, with maintenance expenses over the next four years totaling another $14,300 per acre.

Fumigation is an important first step for quick-growing, healthy grapevines. Fumigation eliminates excess stress to the roots by preventing pathogenic fungi and bacteria from attacking the root systems at their most vulnerable stages right after planting. Poor establishment translates to small, persistent yield losses. Over time—decades, even—those losses can mean a shortfall in revenue for the growers.

Wang and Dangi’s research seeks alternatives to the once-ubiquitous MeBr. One powerful combination pairs the compounds 1,3-Dichloropropene and chloropicrin, sold in different formulations for different application methods. They selected two to test: Telone C35 and InLine, each with about 60% 1,3-Dichloropropene and 34% chloropicrin.

Telone C35 is applied through shank injection, in which a machine with licensed operators drags a hunk of metal (the shank) through the soil about 45 cm below the surface. The liquid form of 1,3-Dichloropropene and chloropicrin is ejected from the shank, and with their low boiling points, turn to gas when they hit the soil, dispersing throughout the root zone. Another formulation, InLine, applies the chemicals in water through subsurface drip.

For the long-term SSSAJ study, Wang compared these two 1,3-Dichloropropene treatments with the traditional MeBr fumigant applied through shank injection. The team also left the soil bare or covered it with virtually impermeable film (VIF). For MeBr, the application area was covered with high-density polyethylene (HDPE) film to minimize greenhouse gas movement into the atmosphere.

It comes down to the pair’s original question: are Telone C35 and Inline reasonable alternatives to MeBr?

“From a pathology perspective, the combination of chloropicrin and 1,3-Dichloropropene is quite effective,” Wang answers.

For vineyards taking on the incredibly expensive process of fumigation, from deep-tilling soil to setting up infrastructure, the long-term results of Wang and Dangi’s work makes a strong argument for using some form of fumigation early in the process, particularly with covered soil.

Next on their list?

“We collected crop response data in these systems, too,” Dangi says. “We want to analyze that data and see, exactly, how fumigation impacted yield in grapevine.”

From their preliminary data, though, Wang and Dangi are hopeful that growers will see the benefits of fumigation during the grapevine’s establishment, providing vineyards with healthy grapes for years to come. And who knows? Maybe a vine in San Joaquin is next on the list to become a centuries-old attraction, giving lucky future wine lovers a taste of the terroir of this gorgeous California valley.