What causes white patches on my cabbages?

Hi Phillippa, without a photo to confirm it could be powdery mildew which is predominant at this time of year in the warmer weather. Are they close to harvest or newly planted?

Check at your local garden centre for a suitable spray, or harvest if close. You can remove the outer leaves, the powdery mildew shouldn’t affect the heart, it just reduces the vigour of the cabbage. Regular feeding and applications of Tui Organic Seaweed Plant Tonic will help keep your veges healthy.