The usual cause for a moldy pit is a split pit. Split spits are usually caused by rapid fruit growth following a rainy spell at the same time the fruit is forming the pit. Some peach varieties are more prone to split pits. These tend to be varieties with large pits ripening in late July to early August. If the stem end of the pit is exposed to the air (so-called open splits), the opportunity for mold is greater. Splits happen.