Mustards--a brassica cover crop for Michigan

Michigan farmers rely on a wide range of cover crops as vital management tools. A living cover is the best protection available against erosion and nutrient loss. Cover crops are grown during breaks between cash crops or on fallow land, often as a means to break pest cycles and to provide the extra organic inputs necessary to improve soil organic matter, tilth, porosity and fertility. Recently, interest has grown in the use of brassica cover crops as specific biological interventions to improve crop health. Generally, cover crops reduce pest outbreaks by providing enhanced biocontrol by providing roots, leaves and flowers (nectaries) that promote the survival and growth of beneficial insects and microbes, both aboveground and below-ground. Beyond these general benefits that support soil health and a more balanced ecosystem approach to farming, recent reports indicate cover crops from the brassica family can be used as biofumigants (Ngouajio and Mutch, 2004). Fig. 1. Field of mustard ready to be incorporated in a Michigan potato rotation. Brassicas such as Oriental mustard have been associated in some reports with improved root health in a subsequent cash crop, such as potatoes, grown after a green-incorporated brassica cover crop. Brassica cover crops can be managed as a green manure — when green residues are incorporated — and this may reduce or suppress some pathogens, including Verticillium in potato; Pythium, Fusarium and Rhizoctonia root rots in beans; Pythium in lettuce; pink root in onion; Aphanomyces, Pythium, Rhizoctonia and Fusarium root rot in peas; and cavity spot and Fusarium in carrot (summarized in Sanders, 2005). The disease suppression property of brassica residues is associated with the amount of glucosinolates in the tissue (these are the compounds responsible for the hot taste of mustard and the sulfur-like smell associated with cooking plants in the brassica family). Field experiments during the early 1990s demonstrated that potatoes generally grew more vigorously following use of brassica cover crops such as rapeseed and Oriental mustard. Conflicting reports have occurred regarding the benefits of incorporating brassica cover crops into vegetable and field crop rotations. Effects from mustard and other cover crops are species-, variety-and location-specific; sorting out their value will take a concerted effort. Farmer interest in this alternative cover crop has grown and was the incentive for initiating research in Michigan on the properties and management of a mustard cover crop. Early findings indicate that soil-borne fungal pests may be suppressed when brassica plant material is incorporated in the field and volatile chemicals are released from the tissue (Date, 2004). See, for example, the limited growth of Rhizoctonia on a petri dish exposed to volatile chemicals from cut up Oriental mustard residues (Fig. 2).