Answer: Bougainvilleas are usually insect pest and disease free so what you are seeing may be an indication of a magnesium deficiency, especially if it is affected the oldest leaves rather than the new growth. For positive diagnosis you might want to take a few of the affected leaves to your local cooperative extension office or well staffed nursery. It's much easier to identify a problem when you can actually examine it! Best wishes with your bougainvillea.
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