Margaret Appleby, IPM Specialist
Biofumigation is defined as suppression of soil-borne pests and pathogens by the use of plants that contain inhibitory chemicals. The plants can be harvested as rotation crops or ploughed back into the soil as green manure. The fumigate properties of these crops have been known for a long time, recently has had a closer look due to the phase-out of the methyl bromide and other fumigants under the international Montreal Protocol. Advances in biopesticides with fumigant properties and application technology make this an economical possibility for commercial agriculture.
Plants in the mustard family, such as mustards, oil seed radish and rapeseed and Sorghum species such as sudangrass and related species, Pearl Millett have shown the potential to serve as biofumigants.
How do they work?
Plants from the mustard family produce chemicals called glucosinates in the plant tissue. The glucosinates contained in the roots and foliage are released when it is cut or chopped, then are further broken down by the enzyme myrosinase to form isothiocyanates that behave like fumigants. These are the same chemicals that are released from metam-sodium (Vapam) commonly used as a chemical fumigant. Sorghums produce a cyanogenic glucoside compound called Dhurrin that breaks down to release toxic cyanide when the plant tissue is damaged.
Mustard, oil seed radish crops and sorghums have been shown in Ontario to have good activity against soil borne nematodes that damage horticultural crops. Work to assess the effect these crops on disease complexes associated with tomato vine decline and apple replant disease is ongoing.
When using biofumigation, a plan is key to get the best return on your investment. You need to implement the 3 R plan: the right seed, right time and the right way.
The Right Seed
Some mustards and sorghums are better than others for their biofumigant activity. Work is being done globally to screen species/varieties for their biofumigant potential.
In Ontario has found that the following work best:
Mustards- Cutlass Mustard
Sudans/Sorghums – Sordan 79, Trudan 8
Pearl Millet – CFPM 101
Marigold – Crackerjack, Creole
Oilseed radish – Adagio, Colonel
The Right Time
For mustards it is possible to have two crops per season. You can plant in late April to early May and then again in mid-late August using in a firm seedbed using a seed drill and pack after planting. For Pearl millet it is critical to get a good stand with no weeds.
The Right Way
Since most mustard and oilseed radish varieties are alternate hosts for some nematodes, the key to using them is mowing to release the fumigant-like compounds. Be prepared to mow before the plants go to seed. Chopping and mangling is really what is needed to release the fumigant-like chemicals held within the plant tissues. Good chopping is critical for incorporation.
Incorporate the chopped plant material as soon as possible after mowing, i.e. within an hour or two at most. Two tractors in the field work best.
After cover crop chopping and incorporation, the soil may need rolling or a light packing to seal the surface and help to hold the biofumigation action. If soil conditions are dry, you may need to irrigate to seal in the fumigant. Basically, if you can smell rotting cabbage, you are losing some of the effective materials.
For pearl millet be prepared to mow 2 to 3 times. Mow millet when it reaches 1 to 1.5 metres, leaving at least 15 cm of millet stubble to ensure regrowth. Mowing will help to encourage deeper rooting and will also keep the millet actively growing. Pearl millet will grow to over 2 metres if allowed. The residue can become quite resistant to breakdown if allowed to grow to that height. Frost in the fall will kill the millet if you have not already worked the cover crop under.
Don’t forget to soil sample for nematodes and soil pathogens before using biofumigation and repeat several weeks after incorporation–you need to measure to manage!