a. Risk of mold
Cured, finished tobacco can support mold growth in a humidity range somewhere between that of leather and that of cheese.
If the relative humidity (shown as percent in the chart) is kept below 76%, the risk of mold growth is quite low. As the humidity increases above that (in typical home temperatures), the risk increases dramatically. Tobacco that is damp will usually not mold over a span of a few days, but after that may suddenly bloom with common molds. Since mold spores are ubiquitous, their control is generally limited to manipulating the humidity, in order to inhibit them (in the absence of chemical anti-fungal agents).
Mold typically appears on the thicker parts of the stem first, since this region of the whole leaf is most likely to hold the most moisture and release it slower than the leaf lamina. In stemming a leaf for use, if there is slight mold only on the stem, then it can be ignored once the stem is discarded.
b. Mold concerns
Mold on the lamina creates two issues. The most obvious is its impact on the smell of tobacco. If it smells moldy, it will be unpleasant to smoke or use in a smokeless preparation.
A more important issue with mold is its potential to create toxins within the tobacco on which it grows. Of greatest concern is aflatoxin (commonly produced by species of Aspergillus, which may be white, black, gray, or yellow). Aflatoxins can permanently damage human tissues, including the liver—and can cause liver failure.
When aflatoxin is burned, during the combustion of tobacco, no toxin is detectable in the smoke. By contrast, aflatoxins that are in non-burned tobacco (e.g. snus, snuff, chew, and the wrapper of a cigar in the mouth) are absorbed into the tissues of the mouth and nasopharynx.
So if there is any concern about using tobacco that might be moldy, its use in cigar filler or a cigarette or in pipe tobacco is probably safe. Such tobacco should not be used in smokeless products or as a cigar wrapper.
c. Control of mold
Very slight mold can be inhibited by misting the tobacco lightly with diluted (50:50) hydrogen peroxide. But by far the wisest policy is to prevent mold in the first place, by controlling the ambient humidity.
With regard to controlling humidity, one factor that is often ignored is the storage of a closed container of tobacco in ambient temperatures that swing back and forth. If, for example, a sealed bag of tobacco is exposed to a 20°F increase in temperature, the internal relative humidity drops by half. If the ambient temp decreases by 20°F, then internal humidity will likewise rise.
That in itself is not a major issue under most circumstances. But a container of tobacco exposed to swinging temperatures does not alter its internal temperature uniformly—some parts of the contents become warmer than other parts of the contents. This causes a heat pump effect, by driving moisture from the warmer areas, and then condensing that excess moisture within the cooler areas. The result of this heat pump effect caused by swinging ambient temperatures is to create favorable conditions for mold growth in portions of the contained tobacco. The easiest way to avoid this is to store tobacco containers (bags, tubs, etc.) in living areas of a home that are the most stable with respect to ambient temperature. In large tobacco containers (5 to 10 pounds), periodically inspect the tobacco within the most interior regions.