Sorry WT, I meant powdery mildew...............
Powdery mildew (Erysiphe cruciferarum)
Powdery mildew is a fungus that produces a white powdery mould on the top surface of leaves. The disease can occur through to crop maturity and its development is favoured by warm dry weather. The powdery mildew fungus, which is spread by the wind, survives on living tissue and Brussels sprouts, swedes and cabbages are mainly affected.
See if you can get a close up pic of both sides of the leaves next time you are out there.I looked on line a little and I think you are right about the Powdery Mildew.
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