What causes a peach seed to develop mold/mildew-like places?
What Causes a Peach Seed to Develop Mold/Mildew
Certified GKH Gardening Expert
This is caused by pit split. The reasons for pit split are not quite understood, but it is generally thought to be connected to environmental factors, like cold, underwatering, overwatering and a lack of nutrients.
As of now, the best advice for pit split is to make sure the tree is getting everything it needs. Also, be aware that some varieties seem to be more prone to pit split than others.