I recently purchased some seed for Kodiak mustard cover crop without reading much about it before. After getting home and researching it, I found the following description: "Kodiak (Brassica juncea): Suppresses soilborne fungal pathogens and nematodes, produces more biomass than other varieties".
My initial plan was to use this in beds that will be planted in late spring. Will this destroy the soil life I have been working to build? Would it be effective at reducing symphalans? Are garden beds an appropriate place to use it? How about under fruittrees? I am particularly thinking it might be effective cover under a peach tree that has had some off and on curl.
You have purchased canola, or rape seed... just under a different name. Mustard is a good plant to grow in a new garden as it (and most brassicas) do not require a microbial community in the soil to flourish. This is probably the reason that Monsanto chose canola as it's premier crop for development in chemical agriculture using the broadleaf herbicide Roundup (Glysophate), and then heavy fertilizers, since they can kill the soil's fertility and still get crops through 'better chemistry'. Mustard grows as a weed in my garden; quite a tasty one, and I think it hybridizes readily with other mustards, like chinese greens such as gai lan. I've had some very odd looking but very tasty weeds come up in my patch. I doubt that it will do harm to your garden. I'm a big fan of field peas for soil building; this could possibly be done in concert with canola. A spring planting of mustard could be followed shortly be a spring planting of field peas which would climb the mustard. Before either go to flower, cut them to the ground and plant a summer crop in their mulch. Regardless of whether you use the peas, If you do not let the mustard flower, and do not till them in, you are a great deal less likely to have a mass fumigation as the flowers and especially the seeds and pods are the place where the oils are concentrated.
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Our local extension agent recommended it for stopping the spread of a fungal root disease in raspberries. It probably would not help much for airborne spores like peach leaf curl. That gives me an idea though of a possible spray for the leaf buds before they open and get infected. I put my peaches under cover which stopped most of the curl.