This plant disease attacks anything in the cabbage family.  This plant disease is caused by the soil-born fungus Plasmodiophora brasicae. This fungus enters the plant through the little root hairs.  Once inside the root(s) become deformed and sometimes cracking.  In turn, the part of the root affected can no longer take up water and/or nutrients.

The symptoms the plant(s) will display if they have this plant disease start out with wilting during the hottest part of the day.  This wilting goes beyond the slight wilting that unaffected plants suffer.  Then, as the day cools down, the affect plant(s) will recover.

Outer leaves will begin to turn brown and will eventually die.  The affect plant(s) compared to the unaffected ones will have reduce yield.  If the problem continues, the plant can die.

Since this is a fungal disease, it is spread by spores.  These spores can be carried by wind, water, and even unsterilized garden tools.  There is not a standard environmental condition by which this fungus thrives but if you have a lot of moisture, a low pH, and/or a soil temperature of 64 to 77 degrees Fahrenheit then you have the ideal conditions.

I know you are thinking all is lost when it comes to the club root since soil temperature and moisture, to a point, is not controllable but do not give up.  There are a few simple things you can do to reduce the chances of developing club root in your cabbage-related crops.

1. First, take a simple soil test.  If your soil is more acidic, raise it to a pH of around 7.2.  This is easily done by adding dolomite lime to the soil.  Mix in completely before planting.
2. A good habit to get into regardless if you have problems or not is to clean your garden tools in between use.  This means soaking them in a mixture of one part bleach to four parts water.  Soak and then scrub to remove debris.  Once that is done, rinse and allow to dry in the sun.
3. If you think that club root spores may be present, sterilize the soil through soil sterilization.  This is easily done by covering the soil with black plastic.  To maximize this technique, one will need to secure the plastic to the soil surface as tight as possible.  Allow the plastic to stay there for several weeks up to several months.  While this will kill the spores, which can live in the soil for up to 10 years, it also kills beneficial bacteria and/or fungus.  In doing so, you will need to go back into the soil with some organic material.

When selecting plant material, make sure to pick cultivators that are resistant to club root.