We have a plum tree that usually gives yellow plums, but this year many of them are turning purple and looking rotten beneath the purple area while ripening. What could be the cause of this? There also seems to be some resin on the purple area.
It could be scab. This article has more information that may help:
I have a yellow plum tree that won't do anything. What should I do?
Assuming you mean it is not fruiting, this article will help: https://www.gardeningknowhow.com/edible/fruits/plum/plum-trees-not-fruiting.htm