How can you store leather to keep mold off it?

Depending on the region in which you live some mold growth may be inevitable. But you can do a few things to reduce it and make your life a bit simpler. Moisture is probably tack's worst enemy. If at all possible keep your tack in an insulated room, hopefully one that has been sheetrocked. It tends to fight dampness and keeps the interior temperature even. Cover your saddle(s) with a saddle cover and bridles and leather halters in a bridle bag. Check your tack frequently and clean as needed. At this point I recommend a good saddle cleaner and/or leather conditioner. Some of the higher priced conditioners have mold retardent added. It will pay off in the long run with less work for you. It may also help to paint the interior walls of your tack room with mildew-inhibiting paint.