It sounds to me like a fungal disease called anthracnose. The symptoms include small red, brown or purple spots develop on upper leaf surfaces. The center of each spot eventually dries out, turns white and may fall out of the leaf. Leaves eventually turn yellow and fall from the plant. When symptoms appear, begin spraying with a fungicide once every seven days until all signs of the disease are eliminated. Do not water plants from above, since the fungus spreads in splashing water.
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