Answer: You didn't specify a plant so I'm not sure how to answer your question. Some flowers, such as camellia, are prone to developing brown spots after a rain. Others develop spots when they begin to decline. So, what you're seeing might be perfectly normal for the plant you are growing. Why not submit another question with more details; name of plant, growing conditions, etc. and maybe we can help with the problem.
Good luck with your garden!
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