In my experience weak plants succumb more readily to pests and disease, particularly brassicas. If you make sure they have a good nitrogenous food supply, don`t let them dry out and help water retention with a good thick layer of compost they should produce the goods. I have gardened with club root, observing a strict, minimum 3 year rotation policy and it “appears” to regress to nothing significant. Some types of brassica are more susceptible and 2 years ago, one patch of land produced some amazing Swedes only for them to develop the worst case of club root I have ever seen, with massive, evil smelling, tuberous growths. The weather may have aggravated the situation because I think it started with good moist growing conditions followed by a hot dry period also that particular plot was still being developed and I did not apply my normal compost mulch, which I always do for brassicas. Another couple of tips for all brassicas is that they (a)prefer to be very firmly planted and (b)like plenty of space. After decades of growing veg, it finally dawned on me and I make sure that the ground is nice and firm, just cultivating the top inch or two if necessary and not planting them too close together. I could not believe the difference this makes to Swedes and in fact I make a point of walking up and down the row in my size 8 boots before I sow them.
Pete