Anything strong enough to completely penetrate this tangle and kill every mold spore could harm the fibers or the bonds between them as well. The leather's structure and strength would be compromised. Protecting your leather tack from all contact with airborne mold spores is an impossible goal. Clean leather after each use with products that lift dirt without clogging the leather's pores or building up in stitching lines. Condition periodically using emulsified conditioning products.
Emulsions contain millions of microscopic oil droplets suspended permanently in water.
The liquid emulsion carries the oil droplets deeply into the leather fibers without leaving that greasy or waxy residue where mildew loves to breed. Choose leather products containing ingredients that inhibit growth of mold and mildew. Cleaning your tack after every use helps prevent the build up of dirt and sweat that provides an ideal breeding ground for bacteria and molds. Leather cleaners or conditioners that leave a soapy, greasy or waxy residue behind in stitching lines, crevices, and leather pores only contribute to this fungus-friendly environment. You can find books describing old home remedies that advocate mild solutions of water and either household bleach or rubbing alcohol to stifle leather mold.