As my Victoria plums are ripening, they are splitting. They are small splits and then they fall off the tree.
Plum fruit split is normally the result of erratic watering. The splits in the skin tend to happen when a hot dry spell rain is broken by plenty of rain. The moisture causes the fruit to swell too quickly and the skins split. The "cure" is simply to make sure there is a consistent supply of moisture to your plum trees.