It must be our incredibly humid weather because I often have a problem with mould on my leather stuff. Even belts are not spared. I’m sure it’s a common phenomenon for lots of people. Even the Thirsty Hippo never quite helps.
Not all my leather goods have this problem, which leads me to suspect that some are better treated than others. My most horrific experience was with a Kate Spade black bag, which consistently grew green jungles that could rival the Amazon forest.
Mould (or mold if you’re American) is actually part of the fungus family along with mushrooms and yeast. Mould spores are very small and they travel easily through the air. I guess that accounts for the fact that no matter where I keep them, the mould always manages to find its way to my belongings.
I use mink oil for my leather goods to stop the invasion. It may not work immediately and some stubborn ones may need a few applications. If I recall correctly, the KS bag needed three rounds. I usually wipe the leather item first and then use an old towel to rub the mink oil in. It helps to condition and soften the leather as well, so even if there isn’t any mould, you can use it for general leather care.
Mink oil is as its name suggests is obtained from the fatty hide of a mink (best not to think too much) and I buy mine from Bata. Surprisingly, other than selling school shoes, Bata has a range of leather care products and it was a friendly salesgirl there who recommended it to me.