It looks like powdery mildew, which is often caused by a lack of water and then over-irrigation. If you think of the past few weeks, we had a dry spell followed by downpours. Well, we did in the south east.
Every variety I've ever grown has those patches. They have also tended to succumb to mildew later on in the summer, but at the moment I wouldn't worry.
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I agree with @BobTheGardener … some varieties have those patches. They’re not mildew and you needn’t do anything about them.