By Tom Burfield
Growers are finding they can cut costs, preserve the environment and even boost yields by using plant-based biofumigants rather than conventional fumigants like methyl bromide to control soilborne pests in fields.
The most commonly used biofumigants are members of the Brassica family, and mustard species in particular. They contain glucosinolates— compounds that makes some mustards hot—that can be deadly to weeds, soilborne pathogens and nematodes, says Mark Uchanski, assistant professor of vegetable physiology at New Mexico State University in Las Cruces.
When Brassicas are disked, cells are broken and release glucosinolates. They combine with a naturally occurring enzyme and water, producing a fumigant that’s similar to metam sodium.
Not only are biofumigants a more economical, safer way to control pests, but Uchanski says that plowing the plants into the ground leads to increased soil organic matter, which can result in better yields and larger, healthier plants.
Jack Brown, professor of breeding and genetics at the University of Idaho in Moscow, views the plant-based system’s safe environmental profile as its strong suit.
“[Biofumigants] are natural products that don’t pose a risk to the environment, and they don’t pose a health risk to the people who use them,” he says.
But the plant-based system carries its own set of challenges, including additional management and water requirements, timing of incorporation, and mixed results, depending on the soil pest spectrum.
How effective biofumigants are depends on where you farm, says Sieg Snapp, associate professor and soils and cropping system ecologist at Michigan State University’s W. K. Kellogg Biological Station in East Lansing.
Michigan has a short production window, since the weather has to be warm enough to grow the mustards, she says. You don’t want them to interfere with your cash crops.
After winter wheat is a good time to plant mustards, she says. Or in the spring, before planting late-sesaon vegetables, such as snap beans.
“The soil has to warm up enough to grow your cover crop,” she says.
But like others who work with biofumigants, Snapp advises allowing plenty of time between the biofumigant crop and planting your cash crop.
“It’s important to get some incorporation of the biofumigant in time for it to decompose,” she says. “You certainly don’t want to kill your crop.” Small-seeded crops, such as lettuce and carrots, are most vulnerable, so allow about three weeks to be safe, she says. For large-seeded crops, including potatoes, one or two weeks may be sufficient.
The Midwest has cooler temperatures and produces crops that typically are rain fed. But crops in New Mexico and the Southwest are irrigated, and that can present a bit of a conundrum for growers, Uchanski says.
“It is a little bit of a double-edged sword,” he says.
Growing and disking under a cover crop can offer a yield benefit and pest control benefit, but growers have to determine whether the input of additional water is worth it.
“You have to manage an additional crop, incorporate that crop and then hope to see the benefits,” he says. “If you don’t, it’s hard to justify continuing to do that.”
Uchanski and Snapp have had different experiences with some aspects of biofumigants.
Uchanski has used biofumigants in conjunction with chemical fumigants, but Snapp says she has seen growers use primarily one strategy or the other.
In addition, Uchanski says you’ll see better results with mustards bred specifically as biofumigants, whereas Snapp says her experience has show conventionally grown mustards seem to work just fine.
Biofumigants aren’t always a cure-all.
Raymond Viramontes, owner of Viramontes farms in Deming, N.M., says he tried using rapeseed from Germany to combat nematodes and suppress soilborne diseases in chili peppers.
“My results weren’t very good on chili peppers,” he says.
While the biofumigant controlled the nematodes, it did not control verticillium wilt. He lost half of an 11-acre block where he used rapeseed, but he only lost 10 percent of a block where he used a chemical fumigant.
Viramontes says he’ll continue to use rapeseed in a test plot, and he says he hopes that it may become more effective as it is incorporated into the soil over a number of years.
The biofumigant cost only one-third as much as the chemical fumigant, he says.
Ideal for organics
The big winners with biofumigants are organic growers, who have virtually no chemical tools to fight soil-borne pests, Brown says.
“For the first time ever, [organic growers] are going to have tools that are natural, that are environmentally safe, ecologically friendly, and they are going to be able to use them in organic systems,” he says.
Indeed, Diane Green, who, with her husband Thom Sadoskiv, operates Greentree Naturals, a certified-organic farm in Sandpoint, Idaho, found that mustards can serve as a catch crop— or trap crop—to kill pests as well as an effective biomass that helps reinvigorate the soil.
Green first used Pacific Gold developed by Brown between rows of broccoli to help control flea beetles.
“We planted the mustard because flea beetles love mustard,” she says. So instead of going into the broccoli, they went into the mustard plants.
“After using the mustard as a trap crop, we ended up cutting it up and tilling it into the soil to use it as a cover crop,” she says. “We were so happy with the results that we repeated that process the following year.”
Mustard works especially well as a cover crop in northern Idaho because it can grow quickly and can endure the cold and early frost season, she says.
Greentree grows 37 kinds of salad greens and more than 100 kinds of vegetables, berries and tree fruit. Green uses Mighty Mustard products as trap crops and as cover crops, she says.
Mighty mustards help battle nematodes, flea beetles
Jack Brown, professor of plant breeding and genetics at the University of Idaho in Moscow, has developed a handful of commercially available mustard cultivars with specific glucosinolates in the plant tissue to serve as green manure biofumigants.
Davidson Commodities Inc. of Spokane, Wash., markets seed from these new products in 2- and 25-pound bags. The cultivars are called IdaGold (yellow mustard) and Pacific Gold (Indian mustard) and are sold under the Mighty Mustard brand, says Kim Davidson, president of Davidson Commodities.
Brown’s newest cultivar—Kodiak—should be available later this year, though it is not part of the Mighty Mustard line.
IdaGold is a weed suppresser, and Pacific Gold helps combat insects, nematodes and fungal pathogens, such as sclerotinia and Verticillium wilt, Brown says. These cultivars were developed through traditional breeding methods and are not genetically modified organisms, or GMOs. Kodiak is similar to, but more pungent than, Pacific Gold, he says. Growers can plant Mighty Mustard products in the fall and again in the spring or once a year, Brown says.
He advises waiting at least three weeks after disking Mighty Mustard glucosinolates into the soil before planting fruits or vegetables. That will allow time for their biofumigation properties to diminish so they won’t injure the planted crops.
The products can be mixed together before planting, but tests have shown that IdaGold will bolt and flower first, only to be taken over by Pacific Gold. Brown doesn’t recommend mixing Mighty Mustard with other cover crops.
In at least one case, Pacific Gold was planted in a mixture with rye, and most of the rye was killed by the mustard’s natural systemic herbicide— also known as an allelochemical.
Mustards repel most pests because they can’t tolerate the plants’ natural heat, Brown says. But flea beetles and cabbage aphids are attracted to Mighty Mustard plants.