Dear Frederic:

Thanks for contacting the UVM Extension Master Gardener Helpline about the problem with your plum tree. We believe this is a fungal disease called plum pockets. It can be common after mild winters with wet springs. It is caused by a fungus and spreads through air and water from infected trees. We attach a link to a resource from the University of Minnesota on this disease: https://extension.umn.edu/plant-diseases/plum-pockets

While this disease can ruin the fruit, it does not affect the tree’s health. Unfortunately, there is not much you can do now to save this season’s fruit with sprays. There are a few things you can do now to manage the plum pocket for the next season. As the Minnesota factsheet notes, you should remove infected plums before they produce spores and bury them in a compost pile or put them in the trash. Good sanitation is also key to management including raking up and disposing fruit and leaves on the ground, and removing infected branches and any witches’ brooms (dense clusters of live and dead twigs) in the tree.

We also attach links to resources about the fungicides mentioned in the Minnesota factsheet to be used if plum pocket has destroyed most of the fruit for several years in a row. The fungicides can be applied before bloom to manage the disease:

https://ipm.ucanr.edu/PMG/PESTNOTES/pn7481.html
https://www.canr.msu.edu/news/lime_sulfur_sprays_can_improve_spring_disease_control#:~:text=Lime%20sulfur%20is%20also%20used,with%20your%20certifier%20for%20confirmation.

We hope you find this information to be helpful.

Pat