One of the reasons growers use Brassica cover crops is to help defend vegetables against abiotic stress, such as flooding, as well as biotic stress, which includes attacks from insects and disease.
Drilling down to the technical aspects, these cover crops produce glucosinolates (sulphur containing substrates), which are secondary metabolites used by plants to guard against biotic and abiotic stress. Glucosinolates are broken down by thioglucosidase (myrosinase) enzymes, which are the enzymes that catalyze the chemical reaction.
These cover crops, which include oilseed radish, brown mustard, oriental mustard, yellow mustard, turnip, and rapeseed, release enzymes when tissue is damaged or there is an attack from insects or pathogens, triggering the breakdown reaction that is sometimes referred to as the “mustard bomb.” Because of the ability of Brassica species to produce these volatile toxic compounds, Brassica cover crops are referred to as biofumigants.
When managed properly, biofumigants can help reduce the populations of nematodes, weeds, and diseases. Some tips presented here will help maximize the benefits of biofumigants. Also included are some pointers on practices to avoid during biofumigation.
Tips For Efficient Biofumigation
1. Species and cultivar selection. Use a species or cultivar with high glucosinolate content, and be aware that mustard cover crops vary in their glucosinolate content.
2. Biomass production. Maximize biomass production by using appropriate seeding rate, method, and timing. Note that high seeding rates may actually result in low biomass production due to plants competing for the same resources.
3. Tissue break- down. Break down plant tissue to trigger the chemical reaction. A flail mower will do an excellent job.
4. Soil moisture. Adequate soil moisture is critical during cover crop incorporation. Efficacy of biofumigation is reduced significantly when the cover crops are incorporated into dry soil. If necessary use overhead irrigation about 12 hours before integrating the cover crop.
5. Residue incorporation. Be sure to add the residue immediately as most of the breakdown products are volatile.
Depending on soil conditions, a roto-tiller or multiple passes of a disk can be used to incorporate residue.
6. Soil surface sealing. Seal the soil surface (with irrigation or a packer if possible). In plasticulture systems, lay the plastic immediately after cover crop has been added. The combined effects of biofumigation and anaerobic soil disinfestation may be achieved with the use of plastic mulch.
7. Plant-back period. Brassica cover crop residue is toxic. Avoid planting susceptible crops shortly after Brassica cover crop incorporation. This is especially important for small seeded crops that are direct seeded. It is important to note that severe injury has been reported on transplants. Observe a cash crop plant-back period of at least two to three weeks (depending on the crop).
What To Avoid During Biofumigation
1. Avoid rotation with other Brassica species. There are several reasons to avoid this practice:
Brassica species do not form mycchorizae. Therefore, monoculture practices could reduce mycchorizae in the soil.
An increase in cabbage maggot populations has been observed in some growing conditions after Brassica cover crops. This could negatively affect Brassica cash crops.
Flea beetles are attracted by Brassica cover crops and may increase the risk of crop injury if Brassica cash crops are a component of the cropping system. It is noted that yellow mustards with hair-like structures, Ida Gold being one, are less attractive to flea beetles than Oriental mustards or oilseed radish.
2. Avoid volunteer cover crops. Viable seed formation by the cover crops could result in serious weed problems with volunteers. Drilling the cover crops (as opposed to broadcast and incorporation) will also reduce the risk of volunteers.
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