After observing  your photos, it appears that you have a couple of diseases on your dogwoods. The powdery material in the first photo is powdery mildew. The spots on the third photo is dogwood anthracnose, caused by the fungus Discula destructiva. Once the disease is present, as with the case of anthracnose, sanitation is you best method of prevention. As the leaves turn brown,remove them and rake any that fall on the ground.The powdery mildew is more of an environmental condition. The attached publication will help in preventative treatments for both conditions.https://extension.umd.edu/learn/ipm-series-dogwood-hg12

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