Plum Fruit split is a condition (as opposed to a disease) where the fruit is damaged by one or more splits in the skin. These are generally forked or branched, and although they tend only to be surface deep, by breaking the skin they do allow diseases and pests to attack otherwise healthy plums and gages.
The condition of plum fruit split is a consequence 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. This can obviously be achieved by regular watering, but in the long run it is better (and easier) to improve the soil by incorporating plenty of well rotted organic matter when planting and by mulching well every spring.