Crop Comments: Cabbage’s cousins counter soilborne pathogens

The Brassica genus boasts in its membership many of our most well-known vegetables: cabbage, broccoli, cauliflower, rutabaga, turnips, kohlrabi, Brussels sprouts, collards, kale, mustard and more. My first contact with this genus as a source of soil fumigants occurred about 20 years ago. At the time, several associates and I were making biodiesel. Our most common raw material for this alternative fuel was waste vegetable oil, donated by local restaurants.

Most of this cooking waste product was from corn and canola oils. Canola is a specially-bred strain of rapeseed (scientific name Brassica rapa). Over 50 years ago, Canadian plant breeders invented the name canola (“Canadian oil with low erucic acid”). Pressing regular rapeseed was okay for making food for diesel engines (as well as meal for livestock). But its erucic acid wasn’t so good for humans. Through selective breeding, scientists took care of that issue, creating a new cousin to B. rapa.

In studying how to make better biodiesel, I encountered research from North Carolina agronomists that said rapeseed grown before wheat benefited those cereal grain yields by about 20% (compared to wheat following wheat). According to Dr. Angela Post, NC State University small grains specialist, large diameter canola roots make it an excellent cover crop, harvestable for oilseed. Additionally, the vertical hole left by the decaying root greatly assists soil percolation. The other asset here is the fumigant benefit provided by this Brassica.

Brassica (and specifically mustard) cover crops are known for rapid autumn growth, great biomass production and nutrient scavenging ability. They also deserve attention because of their pest management traits. Most Brassica species release chemical compounds toxic to soilborne pathogens and pests like nematodes, fungi and some weeds. Mustards have higher concentrations of these chemicals. Members of this genus are increasingly used as winter or rotational cover crops in vegetable and specialty crop production. Some Brassicas boast large tap roots that can break through plow pans better than the fibrous roots of cereal cover crops. Members of the genus that winterkill decompose very quickly, leaving a seed bed that is mellow and easy to plant in.

With several different species to consider in this genus, growers will likely find at least one that fits into their farming operation. Don’t expect Brassicas to eliminate all pest problems. They are a good management tool and an excellent rotation crop, but pest management results are somewhat inconsistent. More research is needed to further clarify the variables affecting the release and toxicity of the chemicals and compounds involved.

Since these cover crops share insect pests and Brassica diseases with other members of that genus (like those mentioned above), growers are advised not to follow one Brassica with another in the rotation.

This genus provides excellent soil coverage and up to 8,000 lbs. of biomass/acre. Because of rapid autumn growth, Brassicas are well-suited to capture soil nitrogen (N) remaining after harvest. Actual N captured is mainly related to biomass accumulation (not by fixing N with root nodules). Because they bind up less N than some cereal cover crops, much of the N metabolized can become available for uptake by the main crop in early to late spring.

Brassica roots penetrate as far down as six feet, scavenging nutrients from below the rooting depth of most crops. To maximize biomass production, as well as autumn nutrient scavenging, members of this genus must be planted earlier than winter annual cereal crops in most regions. Often this makes it more difficult for them to fit into grain production rotations.

All Brassicas can release bio-toxic compounds that exhibit broad activity against bacteria, fungi, insects, nematodes and weeds. Brassica cover crops are best mowed and incorporated to maximize their natural fumigant potential. This is because fumigant chemicals are produced only when individual plant cells are ruptured. The timing and method of incorporation determines just how effective they are in suppressing pests.

UMass studies show that green manures from this genus suppress Rhizontonia scab and Verticillium in potatoes, and Phytopthora blight in cucurbits. Brassicas are shown to reduce nematode and weed populations in various cropping systems. But the use of Brassica cover crops for disease and insect control remains a work in progress.

In New Brunswick, Canada, agronomists address this subject further. They write that biofumigation is the suppression of soilborne pests and diseases by using plants that produce inhibitory chemicals known as secondary metabolites. “In most cases these biofumigant plants are chopped and incorporated into the soil, so they can release inhibitory chemicals. Mustard (Brassica kaber) is a well understood biofumigant. Its biofumigation properties have been studied for a number of years, and scientists have developed a method to fully use these properties. Mustard and most other plants from the Brassica family produce chemicals called ‘glucosinolates.’ When glucosinolates come in contact with water and a family of enzyme myrosinase (contained in plant cells), they are transformed into another group of compounds called ‘isothiocyanates.’ These isothiocyanates give mustard its biofumigation power.”

They also give many Brassica members their bitter/hot/spicy taste. The isothiocyanate produced by mustard is called allyl isothiocyanate. This compound is very similar to one called sodium methyldithiocarbamate, which is the same as the synthetic active ingredient in the commercial fumigant Vapam®.

These agronomists offer the following cultural suggestions: (1) Soil incorporation should be done when the mustard crop reaches full bloom, and should be done in the morning or evening, avoiding hot sunny days. (2) Incorporation should be done when soil has plenty of moisture. Do not incorporate mustard when the soil is dry. (3) Prior to actual incorporation, chop and crush as much plant material as possible to release fumigant from plant cells. (This can be done with a flail mower.) (4) Mustard must be incorporated immediately after mowing, since 80% of the fumigant gas will be released in the first 20 minutes after mowing.

by Paris Reidhead