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

and pests.
Regularly check stored shoes for signs of mold or mildew, especially during prolonged storage. Rotating shoes in storage can also help, as it allows for periodic airing out and reduces the chance of mold growth.
11. When to Call in a Professional Cleaner for Moldy Leather
If mold persists despite home cleaning efforts or if the shoes are particularly valuable, it might be time to consult a professional cleaner. Experts in leather care have access to advanced cleaning techniques and products that can effectively eliminate mold without damaging