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Mold on the stems is fairly common, and usually not a problem for smoking the leaf. Discard any leaf lamina with visible mold. (If it does not smell like mold, and you will be burning it in a cigar filler or a pipe or cigarette, then a small amount of mold on the leaf is not a health risk.) Spread out the leaf and allow it to dry so that it is just barely flexible. Then store it again. Mold spores are everywhere, no matter what you do. Controlling it is by reducing the moisture content of the leaf.