Answer: The brassica family of crops can prevent certain diseases in crops, especially Rhizoctonia and Verticillium. All brassicas contain glucosinolates, which are believed to be effective against fungi and nematodes. Certain brassica plants are more effective than others, however, in the level of protection they offer. Plants with higher levels, such as Oriental and Idagold mustards, are believed to be the most effective.
According to Michigan State University's Department of Crop and Soil Sciences, brassica cover crops may reduce or suppress Verticillium in potato, and a host of other diseases and nematodes in various crops.
There is extensive information located in the E-Organics online article titled, "Brassicas and Mustards for Cover Cropping in Organic Farming." The following excerpt from this article summarizes some of the research on this topic to date. The full article is available at www.extension.org/pages/18643/brassicas-and-mustards-for-cover-cropping-in-organic-farming#.UqXWB9JDtYU.
"In Washington, a SARE-funded study of brassica green manures in potato cropping systems compared winter rape (Brassica napus) and white mustard (Sinapis alba) to no green manure, with and without herbicides and fungicides. The winter rape system had a greater proportion of Rhizoctonia-free tubers (64%) than the white mustard (27%) and no green manure (28%) treatments in the non-fumigated plots. There was less Verticillium wilt incidence with winter rape incorporation (7%) than with white mustard (21%) or no green manure incorporation (22%) in non-fumigated plots (Collins et al., 2006)."
While the brassica cover crop may help to reduce the incidence of Verticillium in the soil, it is advisable to take into account these other considerations when planting crops that are very susceptible to Verticillium. It is advisable to avoid planting your new crops where you had tomatoes or other Solanacea crops planted beforehand that are equally susceptible and often vector the disease many years beyond when they were planted (up to 10 years). Also, plant in an area with little weed pressure, as even Solanacea weeds can harbor Verticillium. Other common-sense preventive measures include planting in well-drained soil and using certified seed stock.
For more info on managing disease, see the ATTRA publication Sustainable Management of Soil-borne Plant Diseases, available at https://attra.ncat.org/attra-pub/summaries/summary.php?pub=283.
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