Club root of cabbage
How to get rid of the disease
This fungal disease causes nodules and tubers to form on the roots of cruciferous vegetables (cabbage, radish, etc.). This results in poor growth. On warm days, wilted leaves indicate an infestation. Older leaves sometimes turn yellow.
Treatment
The nodules and tubers hinder the plant's water uptake, similar to the root gall (round thickenings, hollow inside). Control is hardly possible. In case of infestation, the only remedy is to replace the soil or to avoid cruciferous plants for at least 10 years.
Leafhopper65, Public domain, via Wikimedia Commons