Biofumigation: Using a Mustard Cover Crop to Fight Soil-Borne Diseases

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Summer 2025 was one of the wettest I’ve ever seen for the North Georgia mountains. Throughout July, we had torrential thunderstorms every afternoon for the whole darn month, and I’m not exaggerating. Our dog got so conditioned to it he hid under the bed at 4 pm every day for weeks even after the stormy pattern finally ended.

Combine daily downpours with above average summer temperatures, and you’ve got a recipe for disease in the garden. We had serious leaf blight in the tomato patch, bacterial spot on the peppers and squash, and an alarming first for us: a Southern Blight outbreak that wiped out five pepper plants, two tomato plants, and one summer squash plant.

Southern blight is a soil-dwelling fungus that girdles the stem base of affected plants, cutting off their supply of water and nutrients. This disease, along with a few of its soil fungi friends, is a problem that can come back to haunt you in seasons to come. Many plants are vulnerable, and natural fungicide products like copper won’t touch these hardy fungi.   

So, what can you do to suppress harmful soil fungi and reduce the likelihood of more trouble later? One neat strategy is to plant a mustard cover crop, which can naturally “fumigate” your soil and neutralize the bad guys. Mustard is a beginner-friendly cover crop that’s easy to sow, easy to grow, and easy to manage.

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A Word on Soil-Borne Disease

Soil-dwelling fungal diseases like Southern blight and Fusarium strike fear into the hearts of gardeners because of how persistent they are. They can live in the soil for many years, waiting to attack new victims. Google these things, and you’ll be wondering if you should excavate your soil, set the whole garden on fire, or maybe just nuke it from orbit to be sure.

Here’s the thing, though. These fungal attackers are either native or endemic in a lot of places. They’re always hanging around in the soil, and they need favorable conditions to cause problems. That’s why an aggressive approach (such as removing the top layer of soil, which I’ve seen recommended) isn’t necessary or practical.

Back before the internet was a thing, folks didn’t have access to endless information about plant diseases. Whenever they’d have crop losses after too much rain, they just called it root rot and hoped for better weather next season. I guess with great knowledge comes great…anxiety.

Safe to say the old country folk around here probably can’t tell you the Latin names for all the different fungi that can cause root rot. What they know really well, though, is how to cultivate and manage good, fertile soil. And that’s the first line of defense against soil-borne disease.

How Mustard Can Help Fight Harmful Soil Fungi

Cover crops in general are useful for building healthy soil. They increase nutrient levels, prevent compaction, and support beneficial soil organisms. They also contribute organic matter to improve soil structure and help balance soil pH. Soil with good tilth and a healthy population of microbes and friendly fungi is harder for the bad guys to colonize. Meanwhile, disease-causing fungi tend to thrive in compacted, acidic, otherwise lifeless soil.

Mustard is a particularly good cover crop to employ after trouble with soil-borne disease because of its unique biology. A member of the brassica family, mustard produces high amounts of compounds called glucosinolates, which give mustard greens their peppery flavor.

After the mustard crop is cut down and the residue begins to decompose, the glucosinolates break down into something called isothiocyanates. These naturally occurring chemicals have a toxic effect on harmful fungi and water molds, and they’ve also proven to control certain soil pests. In science-y terms, the process is called biofumigation.  

The science on mustard biofumigation is still young, and some claim there’s not enough hard data to say it works. There’s plenty of proof in the field, though. After the EPA’s recent bans on some widely-used soil fumigant chemicals, many commercial farmers have reported successful disease control using mustard cover crops as an alternative. That’s saying something, since soil-borne disease can be a lot more devastating and hard to manage in monoculture fields compared to small gardens.    

What Diseases and Pests Can a Mustard Cover Crop Suppress?

The compounds produced by decaying mustard residue are toxic to many different disease-causing fungi and oomycetes (water molds) that live in the topsoil. Some notable examples are Southern Blight, Fusarium Wilt, Verticillium Wilt, White Rot, Phytophthora, and Pink Root in onions. Mustard can also help suppress Pythium and Rhizoctonia, which cause damping off in young seedlings.

In addition to disease, mustard cover cropping reduces populations of root-knot nematodes and other plant pest nematodes. There’s even some evidence it can help chase away annoying, root-eating soil pests like wireworms and grubs. Finally, mustard has an allelopathic effect on weeds, preventing weed seed germination and growth.

Is it Okay to Plant a Mustard Cover Crop Before Other Brassica Crops?

Since mustard is related to cabbage, kale, broccoli, cauliflower, and other brassica crops, it may host certain pests and diseases that affect those crops. For that reason, some folks don’t recommend planting mustard as a cover crop in areas where you plan to plant brassica-family vegetables the next season.

Disease carryover isn’t always a problem. Still, it’s worth some caution if you’ve had disease strike your brassica crops recently. In that case, it’s probably safest to follow good crop rotation practices and avoid growing mustard where you’ll plant them next.   

Pests are typically a bigger worry than disease. Your mustard cover crop may attract cabbage worms, especially if it’s grown out during warmer weather. Cabbage worms pupate over winter in the ground or in crop debris, and you don’t want them emerging right where you’ve planted your spring cabbage and broccoli. If you use row cover to protect your brassicas from pests like we do, you’ll end up with cabbage butterflies under your netting!  

In our area, we don’t see much (if any) cabbage worm activity on fall-planted mustard cover crops. In some climates, though, the butterflies stay active for longer. The good news is that you’ll know it, if your mustard crop has cabbage worms. Keep an eye out for skeletonized leaves and little green caterpillars.

Choosing the Right Mustard Variety

Not all mustard varieties are equal when it comes to disease and pest control, since some produce more glucosinolates than others. White mustard and yellow mustard varieties are lower in these compounds, meaning they won’t work quite as well for soil biofumigation.

Indian mustard varieties (Brassica juncea) and black mustard varieties (Brassica nigra) produce the highest amounts of glucosinolates. The Brassica juncea varieties are the ones most commonly sold for cover crops. Some good Brassica juncea mustard varieties you’ll find when shopping for cover crop seed are Pacific Gold, Kodiak, and Caliente Rojo.

How to Sow and Tend a Mustard Cover Crop

Mustard is frost-tolerant and germinates in soil temperatures anywhere from 40 F to 85 F, so you can plant it as a cover crop in early spring, late summer, or fall. For a spring cover crop, sow once the soil is workable and the threat of a hard freeze (<26 F) is over. In the fall, sow at least four weeks before the first frost so it can establish some before cold weather arrives.

Sowing

Mustard seeds need good soil contact in order to sprout, so the first step is to loosen the surface soil in the area where you’ll plant. I like to use a hard rake to loosen up the top inch or so.

To get good, even coverage, you’ll want to sow about ½ pound to one pound mustard seed per 1000 square feet (calculate square footage by multiplying length X width). If you’re sowing a large row garden, a spreader might be helpful. I just broadcast the seed by hand, since I have smaller individual garden beds.    

Once your seeds are broadcasted, use your hard rake or other cultivating tool to gently work them in. You don’t want to disturb them too much, here; just lightly scratch the soil around for better soil to seed contact. Then, soak the bed down thoroughly with the garden hose.

Sometimes, birds like to steal my cover crop seed, and I have to cover planted beds with netting to prevent that. However, our birds don’t mess with the mustard seeds. They’re more partial to bigger, meatier seeds like buckwheat. Still, it’s a good idea to keep an eye out for any mischievous bird business after sowing your cover crop!

Watering

Mustard seeds need plenty of moisture in order to sprout, which takes around 5 to 10 days. It’ll save you some hassle to sow before a rainy spell, when possible. If rain’s not in the cards, you’ll need to keep the seed bed nice and damp with the garden hose. Check at least once a day and water well if it looks dry. You can’t really water it too much; the biggest pitfall that can hinder germination is letting the soil dry out.

Once they’ve sprouted, tiny mustard seedlings need continued watering until they’re established. If it’s warm out and you’re not getting much rain, hose them down every few days until they’ve greened up and have true leaves coming in. After germination, they’d probably survive without attention, but a little extra TLC will help you get a good, healthy stand.  

Terminating a Mustard Cover Crop for Max Biofumigation Benefits

Timing when you terminate your mustard crop is important, for getting the most out of its bio-fumigant properties. Mustard’s glucosinolates are at their highest concentration when the little yellow flowers first start to appear, usually around six weeks after sowing. This is the best time to terminate the crop, since levels gradually fall after this point. That said, if you want to let pollinators forage the blooms, you’ll still get plenty of effect as long as you terminate before seeds form.

Mustard releases the most glucosinolates when the plants are chopped at terminating time. The finer you can chop up the crop residue, the more benefits you’ll get. A weed-eater on a low speed setting works well to cut down the mustard (too fast, and you may sling the plant bits out of the garden!). I usually just use a pair of sharp scissors, for my roughly 4 x 12 garden beds.

Once the mustard crop is cut down, the bio-toxic compounds will disappear pretty quickly if left on the soil surface. To seal them in, you’ll need to turn the plant material into the soil with a tiller or cultivator on the same day you terminate. If you strictly use no-till methods, you can cover the bed(s) with a thick layer of compost instead.   

Note: After terminating mustard, wait at least three weeks to plant any veggie crops. Otherwise, those allelopathic compounds that suppress weeds may also stunt your plants’ growth.

A Few More Tips for Dealing with Soil-Borne Disease

Usually, an integrated approach is best when you’re fighting disease and pest problems in the garden. I.e., don’t rely on one strategy alone to tackle the problem. Here are several other things you can do to keep soil-borne diseases in check after an incident:

Remove the victims from the garden.

Disease-causing fungi often stay concentrated on or near the dead host plant, either as spores, mycelia, or some other creepy survival structure. For example, the Southern Blight fungus produces sclerotium, which are hard little fungal structures that can survive deep freezes during winter and germinate again when conditions are right.

To keep the fungi from building up in your soil, remove affected plants, including the root ball and any dirt that comes with it. Don’t compost them unless you’re a hardcore composter with a 140 F smoking hot pile. Instead, burn them or toss them out somewhere far away form your garden or lawn (yep, some of those pesky fungi can cause dead patches in your grass, too).

Test and amend for soil pH.

In our garden, the areas where we had cases of Southern Blight had something in common. They were newly-added beds that hadn’t received several years’ worth of compost and mulch additions, yet. Their soil pH tested at 5.8, which is a bit more acidic than what’s ideal. Our older beds, which have been well-amended and had pH readings from 6.4 to 7.1, had ZERO incidents of Southern Blight.

Generally, a soil pH of 5.8 won’t cause crippling growth problems for most vegetable plants. It is, however, more favorable for disease-causing fungi, which enjoy acidic soil. Per what we’ve seen with our older garden beds, adding organic matter will raise soil pH over several years. Since we had fungal issues over the summer, though, we’ll be adding some ash to the newer beds to raise that pH a bit faster. Soil with a pH closer to neutral is a lot less hospitable for fungal pathogens like Southern Blight.   

Here’s a more in-depth guide to addressing problems with soil pH: Gardener’s Guide to Soil pH: Why it Matters and How to Correct pH Problems.

Boost friendly fungi like mycorrhizae.

A diverse population of beneficial fungi in your soil makes it harder for the bad ones to spread. Fungi called mycorrhizae support your plants’ health, too. These friendly fungi bind with plants’ roots, help them take in more nutrients, and help protect them from invaders. Now, we’ve even got evidence that plants can communicate through mycorrhizal networks. If one plant is attacked by a soil pathogen, nearby plants get the memo and bolster their defenses.

Beneficial fungi are naturally present in soil with a high organic matter content, so add plenty of compost and natural mulch. If your soil is still a work in progress, you can also buy granulated mycorrhizae and sprinkle it in transplant holes.

One big thing to keep in mind when dealing with disease-causing soil fungi is that they don’t tend to knock out whole crops in a given year. They strike at random, often attacking only a few plants in a bed and leaving the others alone. That’s why drastic measures to eradicate them aren’t necessary (or successful, in most cases).

With a practical strategy like good soil maintenance, proper clean-up, and planting a mustard cover crop for good measure, these diseases won’t stop you from having a successful and productive garden.