Hello Ed,
It is quite common to find this exudate coming from plums. (A bit like
sap coming from cuts on pine trees) and is not disease related.
It is natural but can be caused by a number of items.
First is insect damage, casing leakage of sap from the fruit.
Water availability is another issue. when there is a dry spell,
followed by rapid availability then the surge in pressure can cause the
leakage to occur.
The skin of the plum could be damaged by insects as mentioned, abrasion
or other physical damage (by wasps or birds especially blue tits).
It is often evident when the plums start to ripen. Plums do not ripen
simutaneously, so it is necessary to pick over the tree several times.
Do you know what cultivar you have?
Clifford,
Bawtry, Doncaster, South Yorkshire