Mustards as fumigants
I was at the methyl bromide alternatives conference, and if my memory is
correct, the problem with brassicas like mustard as a soil fumigant is that
they are phytotoxic to strawberries.
But for many other crops, mustards make good soil fumigants, particularly
when combined with solarization. The plastic tarp used in solarization
holds in the toxic gases that arise from the decomposition of the mustards
[and other cole crop residues, including cabbage], providing quicker and
better results than without tarping.
The key with strawberries is some R&D to find out the cause of toxicity,
and then find a plant variety that can be used as an alternative fumigant
for the crop. But this line of research is, to the best of my knowledge,
not being pursued.
By the way, there is a 10-page article titled "Brassica Alternatives to
Herbicides and Soil Fumigants," with a page of references on the subject,
in the July 1993 IPM Practitioner [BIRC, Berkeley, CA, 510/524-2567; last I
checked they were selling back issues for $5, if you're really interested
in the subject]. I thought then, and still think now, that the concept of
grouping fumigant green manure/cover crops is a good one. The major
limitation is that when dealing with home-grown botanical products, there
will be variations in levels of the toxicants in the plants and efficacy
due to climate, soil, growing conditions etc. Also, these plant chemicals
are not always benign -- remember the famous mustard gas of World War I was
a product of these same brassicas.
disclaimer: I wrote the brassica article for the IPMP referred to above.