No, we're not talking about raiding the condiments cupboard for some Grey Poupon! But research done at Cornell University has demonstrated that mustard plants, as well as other related brassicas such as canola, when grown as a cover crop, act as a biofumigant that helps to suppress certain soil-borne pests and diseases.
While all members of the Brassica clan, which includes cabbage, broccoli, kale, mustard and turnips, among others, produce compounds called glucosinolates that are toxic to microorganisms, the ones that are used as cover crops have been developed to have very high levels of these anti-microbial compounds.
When the plant cells of these cover crops are damaged by chopping, the glucosinolates are released and come into contact with an enzyme that, in the presence of water, causes the formation of the natural gas isothiocyanate that suppresses the plant pathogens. After the harmful soil organisms are killed, beneficial microorganisms quickly repopulate the soil. Research has shown that Phytophthora, Verticilllium, Rhizoctonia and Fusarium are among the pathogens that are checked with this biofumigation.
The cover crop residues need to be incorporated into the soil immediately after chopping in order to keep the gas produced from being lost and the soil must be irrigated for maximum effectiveness. In addition to the disease-suppressing effect of a mustard cover crop, the soil benefits from the addition of the large quantity of organic material when the cover crop is turned under.
For more information on research using mustard crops for biofumigation, go to: Cornell Cooperative Extension of Suffolk County.