Most Brassica species release chemical compounds that may be toxic to soil borne pathogens and pests, such as nematodes, fungi and some weeds. The mustards usually have higher concentrations of these chemicals.
All brassicas have been shown to release biotoxic compounds or metabolic byproducts that exhibit broad activity against bacteria, fungi, insects, nematodes, and weeds. Brassica cover crops are often mowed and incorporated to maximize their natural fumigant potential. This is because the fumigant chemicals are produced only when individual plant cells are ruptured.
Pest suppression is believed to be the result of glucosinolate degradation into biologically active sulfur containing compounds call thiocyanates (Gardiner et al., 1999; Petersen et al., 2001). To maximize pest suppression, incorporation should occur during vulnerable life-stages of the pest (Williams and Weil, 2004).
The biotoxic activity of brassica and mustard cover crops is low compared to the activity of commercial fumigants (Smith et al., 2005). It varies depending on species, planting date, growth stage when killed, climate, and tillage system. Be sure to consult local expertise for best results.
The use of brassicas for pest management is in its infancy. Results are inconsistent from year to year and in different geographic regions. Different species and varieties contain different amounts of bioactive chemicals. Be sure to consult local expertise and begin with small test plots on your farm.