when I had a allotment I would grow the cabbages etc in large pots then plant out it made all the difference and would always get a crop, always used the old tomato compost to save £££ , plus I was on a loser as the whole allotment had it

An old gardener told me to dip the roots in lime before planting, and make a little circle of lime around the young plant. The calcium cyanamide is a lime compound (lime plus nitrogen) but using ordinary lime is cheaper.

Also sure you know not to walk on the infected plot and then walk somewhere else in the same boots - but hard to do if the whole plot's infected. Wonder if it would be worth making a raised bed for brassicas and taking care to avoid any cross contamination from the rest of the plot?