Black-leggedness (Fall over) of kohlrabi plants in the seedbed is caused by several fungi and favored by too dense sowing and improper watering.
With this disease, the root neck of young plants turns black, who stays white with the sane. Later they fall down. Blackleg occurs mainly when sowing in the cold frame. Quite thin sowing, sparing watering and seed dressing is the best countermeasure. Sand should be used instead of soil to cover the seeds. Rarely, but water thoroughly and air a lot, hence the surface of the earth (Sand) and quickly dry the plants.