Answer: There are numerous problems associated with dealing with clubroot. The first is that it is a fungus and lives on in the soil for years and years, and can be moved on tools or even on the wind. This means a long crop rotation is suggested with seven years being the typically named length of time although apparently the fungus can remain dormant for as long as 18 years.
During the rotation period, other hosts such as cabbage must be avoided and weed control for related crucifer weeds such as mustard is also necessary.
The soil treatments are not available to homeowners and would only be done on a commercial basis.
There is work being done on developing clubroot resistant varieties, but apparently to date they are not acceptable in terms of the crop they produce so they would not be worth growing even if they were available.
The fungus thrives in cold acid soil, so raising the pH to 7.2 is one method of trying to fight it. This higher pH however causes problems of its own such as making boron unavailable to the plants and thus requiring that it be applied in very small amounts as a foliar treatment. This is the type of calculation best made by professional growers in concert with their county extension and based on extensive soil testing and very careful soil monitoring and crop management.
If you wish to research it further, I would suggest you contact your county extension and you could also do a targeted search through numerous extension publications by using the Plantfacts search engine out of Ohio state:
To be very honest, in my own opinion, once clubroot is introduced in a home garden it is pretty hopeless to continue trying to continue to grow this group of susceptible plants. I'm really sorry.
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