Disease
Club rootCabbages wilting, with swollen roots

Plants wilt in the day and stop growing. Their roots are swollen and distorted when lifted.
When it shows up: Summer
What to do
Lift affected plants, roots and all, and bin them, not the compost.
It stays in the soil for years, so grow brassicas somewhere else.
Grow new plants on in big pots before planting out, and lime the soil: both help.
Not quite it?
These look similar. Compare the photos.

