Tulip Poplar, Ash, Maple ... I had problems with all of these mildew/molding to the point of fairly deep stains. (oak in the same stacks did not have any of these problems) In all cases, my lax handling/storage of the timbers was to blame. Boracare did _not_ keep them from mildewing/molding when I had them stored in a damp barn with only 1/2" of air separation. Solution was to keep them inside a bigger barn, higher off the ground, separated by plenty of air, with kiln dried stickers. Bleach and/or lots of planing helped remove stains from the affected timbers. These were floor joists, so the final dimensions were not critical.

Just throwing jello at the wall. Hope some of this helps.