sprouts club root

Club root is a nasty disease - If you're sure one has it, then you'd be pretty certain to bet on the others having it too. It can last a long time in the soil too, so simple rotation each year won't help prevent it. Either grow them away from the area, grow a club root resistant cultivar (not sure if there are sprouts that are resistant though - I've only seen cabages etc)..

Quite often it'll cause your plants to fail, and it's easy to spread - either by walking over the infected soil, then onto "clean" soil, or via the plants themselves.

That said, as far as I'm aware clubroot doesn't like alkaline soil - so your soil may be on the acidic side. Adding lime can help, but I'd suggest you research this (and becareful with it, as lime can burn skin/eyes!) to find out the exact methods of dealing with it.

I can't see any roots in the picture you've posted, but they look healthy to me. Can you upload a picture of the root for more knolwedgable folk here to confirm this with you?

If you do find it's clubroot, and lift your brassicas, don't just chuck them on the compost heap. Burn them (or bin them if you don't want to burn them.. but burning will kill the cysts/spores)

Some commercial growers use liquid nutrimate in their brassica crops where there is a danger of infection from club root. The nutrimate helps the plant to continue to absorb moisture from the soil when it would otherwise just have succumbed and collapsed.