Plum trees are normally relatively sappy trees, so a little sap leaking from plum trees may not be reason for alarm. However, if you notice your plum tree is bleeding sap, your tree may have a problem that should be solved as soon as possible. There are a number of factors that may be to blame when a plum tree has sap oozing from trunk. Hot, dry conditions in summer or sunscald in winter can stress the tree and may be the cause for a plum tree oozing sap.

Cytospora canker is a type of common fungal disease that often affects trees weakened by drought, severe weather, or injury caused by improper pruning or a lawnmower blade. If your plum tree is bleeding sap, it may be affected by canker, or by other forms of fungal or bacterial diseases, like pseudomonas.

Several types of borers, like peach tree borers, can infect plum trees. Borers are easy to distinguish from disease because the sap is mixed with frass (the debris and fecal matter left behind by boring insects). Borers can affect healthy trees, but they are more common on trees that are weakened by drought, sunscald or injury.

Aphids and other pests can also cause sap to drip from the branches.

Trees often ooze sap at a spot injured by lawn and garden equipment.

Once you determine the problem, the solution may involve improved maintenance, environmental modifications or other non-chemical approaches. Some pests may require chemical control.

To prevent mechanical damage, be careful when using mowers, weed trimmers, or other lawn equipment. Disease often enters the tree through the damaged bark.

Trim your tree properly in late winter/early spring for young trees and around mid-summer for older, established ones. Dispose of any damaged branches properly to prevent spread of disease – preferably by burning. Water your plum tree adequately too.

Here are a couple publication to check out:

https://catalog.extension.oregonstate.edu/sites/catalog/files/project/pdf/em8362.pdf   

https://pnwhandbooks.org/node/2448/print

Hope this helps!