Bamboo, probably not fully dried yet, gets packed into shipping containers in the tropics with heat and high humidity and and sits there for weeks or months in ideal conditions for mold. Probably it is not coated with anything and the fungus is feeding on the carbohydrates and other stuff in the bamboo itself. Once it has a chance to dry out all the way it will probably die. Bleach should help knock it down too. Penetrating epoxy thinned down with a little acetone and applied as a finish would lock out moisture and mold, but thats probably way overkill for some bamboo stakes
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