White patches on my caulis
There could be several explanations to this problem. If there is a white powder on the leaves, this could be powdery mildew and you may have to spray with a fungicide or remove the infected leaves. If the white patches are actually aphid skins, then aphids could be feeding on the plants, They often inject plant viruses or just damage the leaves by extracting sap.
If it is an insect pest, I suggest you try either Savona Fatty acid or Insect Killer spray containing pyrethrum. Both are available from Harrod Horticultural and both are safe to use on edible crops. Leave the crop 24 hours before harvesting after spraying. I hope this helps.