by Deborah Jeanne Sergeant
Controlling disease challenges many organic growers. The University of Vermont Extension presented “High Glucosinolate Mustard as an Organic Biofumigant in Vegetable Crops” as a webinar recently in partnership with eOrganics. Dr. Heather Darby, professor and University of Vermont Extension representative, Abha Gupta, crops and soils coordinator with the University of Vermont Extension, and Katie Campbell-Nelson, a University of Massachusetts Cooperative Extension agent, presented the webinar.
Darby said most farmers are familiar with using cover crops to prevent soil erosion, increase soil aggregation, smother weeds, promote soil microorganism biodiversity, scavenge nutrients and increase soil organic matter to increase yield. Biofumigants used as cover crops such as high glucosinolate mustard may also help suppress disease.
“These plants contain high levels of glucosinolates throughout the whole plant,” Darby said.
Though many use biofumigant crops to help control pests, the goal of her research is to explore their use in the Northeast.
She has found high glucosinolate mustard as a biofumigant useful in controlling insects such as black vine weevils, nematodes, wireworms and symphilids. It can reduce instances of diseases such as Rhizoctonia solani, Pythium species, Cladosporium cucumerinum, root rot of peas and beans, Fusarium sambucinum, Gaeumannomyces graminis , Sclerotium rolfsii, Aphanomyces, Verticillium spp. and Phytophthora.
“This is not an exhaustive list,” Darby noted.
She tested high glucosinolate mustard in a few forms, such as meal after its seeds have been pressed for its oil. The meal can be used to amend soil with its 5 to 6 percent total nitrogen and as a biofumigant.
“Other research has shown the meal and seeds can have a high level of glucosinolates,” Darby said. “We’ve used the meal after the oil is pressed as a soil amendment. The organic nitrogen breaks down quite rapidly.
Weed control with mustard seed meal proved about twice as effective as sunflower meal and canola meal.
Gupta shared research from University of Vermont’s High Glucosinolate Mustard Research project.
“The earlier planting dates yielded significantly more biomass, which is advantageous for generating a higher concentration of glucosinolates,” Gupta said.
She recommended planting in July, when the soil is very dry.
“As long as it’s at 45 degrees or more and it’s 70 days before frost, you can plant,” she said. “Anytime you have a window that meets that need, you can plant.”
She added the main results showed that overall, the July 31 planting date mustard “yielded significantly more than the Aug. 17 planting date.”
Earlier planted beans yielded 700 pounds more per acre.
Gupta said because 2015 was very dry, irrigation may be needed to achieve the highest efficacy of biofugmigant release.
Campbell-Nelson said four trials she conducted from 2014 through 2016 studied precautions and benefits of mustard and how to grow mustard as a biofumigant. Since phytophothora blight can last for up to a decade in a field, she chose the ethical method of testing plants: in pots in a greenhouse bioassay test to avoid contaminating fields.
“We added the fumigated soil and inoculated pots with phytophothora spores and rated incidences of the disease over time,” Campbell-Nelson said.
Mustard fumigated soil took longer to show phytophothora, which may be long enough for a pepper crop to mature.
Campbell-Nelson said, “the mustard fumigated soil had the highest vigor” compared with oat fumigated soil or sterile soil.
From 2015 to 2016, Campbell-Nelson conducted root knot nematode trails. These parasites are “very difficult to control,” she said.
Also using a lettuce bioassay study, Campbell-Nelson studied how soon it took for the plants to begin developing galls indicating root knot nematodes.
Within 4 days to 8 weeks, they started forming galls on the roots. Campbell-Nelson said root gall severity (RGS) thresholds can help establish a threshold tolerable to different types of plants. For carrots, it’s two. Onions can tolerate three and potatoes and beans, four.
She compared bare land that’s repeatedly tilled as a management practice to a mustard-treated area and an undisturbed area. The mustard treated area still had significantly greater nemotodes than the land that was undisturbed, but over time, it greatly reduced.
The mustard and bare fallow management methods were enough to reduce the populations to grow onions and potatoes, but not carrots, Campbell-Nelson noted. “Ideally this spring, I’m going back out to the same fields to see if the mustard fumigation had the positive effect,” she said.
Among her precautions about mustard, she said it attracts many of the same pests as brassica crops such as imported cabbage worm, flea beetle and cabbage root maggot, and diseases like black rot and alternaria leaf spot. But its benefits include reduce weed pressure, reduce populations of parasitic nematodes and reduce soil-borne pathogens such as pythium, rhizoctonia, sclerotinia, verticillium and phytophthora. It also serves to amend soil and attract pollinators.
To grow mustard as a biofumigant, producers must prepare and fertilize the field, seed with mustard, chop and incorporate the mustard, and then plant the main season crop. It costs about $5 per pound, or $50 an acre. It should not be a field used for other Brassicas.
Soil should receive 50 to 80 lbs. of nitrogen and 20 to 30 lbs. of sulfate. Ideally, its pH should be between six and seven. Campbell-Nelson further recommended using a no-till grain drill set to 9 to 12 lbs. per acre, Brillion Sure Stand set to 12 to 15 lbs. per acre or broadcasting 15 to 20 lbs. per acre.
To chop, incorporate and seal, “use a flail or rotary mower to chop the plant tissue while the mustard is in bloom,” Campbell-Nelson said. “Immediately incorporate the residue with a chisel plow, roto-tiller or heavy disc.”
Afterwards, sealing the soil surface with a heavy board, roller or culti-packer lets the mustard do its work.