Plum Fruit split is a condition, not a disease, where the fruit is damaged by splits in the skin.

Although they tend to be on the surface, by breaking the skin they allow diseases and pests to attack otherwise healthy plums and gages.

Why are my plums splitting on the tree?

Erratic watering causes plum fruit to split.
The splits are most common when a long hot dry spell is broken by plenty of rain.
The rush of moisture makes the fruit swell too quickly for the delicate skins, which split.

How do I stop my plums from splitting?

The cure is to ensure a consistent supply of moisture, which may require irrigation in some areas, but it’s better to achieve it by mulching deeply every Spring.
Mulch all the way around the tree, at least 30cm past its “drip line”, meaning the edge of its canopy. Wood chips make excellent, long-lasting mulch.