Answer: I don't think you've harmed your zinnia's with baking soda and water, but it sounds like you've figured out that blotches are not powdery mildew, but another type of disease. Powdery mildew shows up as a whitish coating on the leaves. Brown blotches sound more like Botrytis and a good garden fungicide should fix the problem. Be sure to rinse away any baking soda residue before spraying your plants with a fungicide. To avoid problems in the future, give your zinnia's full sunshine and plenty of elbow room to encourage good air circulation all around the plants.
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