Clubroot is getting worse

Clubroot has got worse and worse in my vegetable beds. What can I do to prevent it? I’ve tried resistant varieties to no avail. Everything is grown from seed, sown in drills on a different part every year.

Once it is resident, clubroot is virtually impossible to eradicate. The cause is a slime mould – a kind of fungus – which can persist for many years. Acid soils are the worst affected, but it’s absent from alkaline land – chalk or limestone-based – and less troublesome on neutral soils.
Liming is an option. It will reduce the symptoms and growing resistant varieties will also help, particularly on limed land. Among resistant brassicas, Unwins recommend cabbage ‘Kilazol’ and Brussels sprout ‘Crispus’.
The remainder of the cure lies in your growing methods. Instead of direct sowing, start your seedlings off in cell trays and pot them up to 13cm (5in) pots before planting them out. You don’t need a greenhouse for this, but you must use sterile potting compost.
Plant young brassicas in open ground when their root systems have had time to develop in the pots.