I pulled out shoes I haven’t worn all winter and found white fuzzy patches growing on the leather. What is this?

During winter, shoes are often stored away in closets or basements, where humidity levels can easily rise above 60%, the threshold where mold begins to thrive. Leather is a porous material, meaning it can absorb moisture from the air. When the ambient temperature fluctuates, condensation can form on the leather surface, providing the moisture mold needs to grow.

Additionally, organic residues like sweat, skin cells, and dirt left on shoes after the last use can serve as a nutrient-rich substrate for mold. Without regular cleaning and maintenance, these residues can exacerbate mold growth, especially during long periods of storage.