I am asked a number of times, as to what the sticky gum is exuding from plum trees. Most often it is called ‘Gummosis’. This can brought on by stresses to a tree.
In our orchard this only occurs with the Prunus species of Almonds ‘Prunus armeniaca’ and Plums ‘Prunus Domestica’. We do not have any on Cherries ‘Prunus Avium’, Peaches ‘Prunus Persica’ or on Japanese Plums ‘Prunus Salicina’. This may be just good luck, as it does occur in other countries.
The cause is normally due from uneven watering causing the bark to split, and resin to exude. Which can happen at any time of the year, but normally in the spring, when growth is rapid and key minerals are not available in the soil. This split allows the disease organism ‘Pseudomonas syringae’ or ‘Pseudomonas morsprunorum’ to enter through the wound.
This split or crack in the bark can occur due to a freeze during the intial spring growing phase.
The unnatural stresses are where the bark has been cut when pruning, or by insects penetrating the bark, as with Peach Twig Borers (not normal in the UK). This can allow Phytophthora infections to enter and gradually kill a branch or the tree.
For further reading and more technical details please refer to https://www.fusion360inc.com/bark-necrosis-and-gummosis-diagnosis-and-treatment/
This gum, can be initially relatively clear, turning very hard and dark brown after a month.
At present there is no known cure. But there are numerous new chemicals coming onto the market as a prevention. But once a tree has it, there is no cure.