What is afflicting my plum tree?

I've had this plum tree for at least six years or more. It's always been very happy and has been producing for about five years. About a month ago I noticed some of the leaves dying but it was hot so I thought it was the heat. Then about a week ago I saw quite a lot of branches were dying and noticed a lot of sap dripping from the base of those dying branches.

I cannot see any insect frass with the sap. But I do see fine web-like strands on some of the leaves.

Any advice? I've had bad luck with plums (all the other varieties I've planted die the same year I plant them) but this one was doing so well for so long.

It looks like your plum tree has some form of gummosis or canker. This problem is extremely common on stone fruits in the Pacific Northwest. It can be caused by a number of factors, including sunburn/heat stress ("southwest injury"), bacteria, fungi, borers, etc.:

https://en.wikipedia.org/wiki/Gummosis

I see this problem all the time. This year I've seen it on a couple of peach trees. The heat wave we had earlier in July stressed out a lot of trees and made them more susceptible to this problem. Too much rain and poor drainage can also be a factor in some cases, particularly in the spring.

First, I remove the gummy deposits with a clean knife and throw them in the trash. (Do not compost this stuff or leave it in the orchard/garden.)  Then, using sterilized tools and alcohol, I remove all the dead branches or ones that have a lot of damage, burning the diseased wood and sterilizing the saw and pruners after each cut with alcohol. If there are gummy deposits on the main trunk, I clean the area down to clean bark or wood and then a few days later I paint over the affected area and the entire trunk up to the first branch with white latex paint. Then I mulch the tree with compost/manure/chips and give the tree a foliar feeding by spraying a seaweed/fish product on the leaves. Compost tea is excellent, too. This strengthens the tree and helps it resist the fungus or bacteria that caused the disease. Hope this helps...

Good luck!

Here's a video by Edge of Nowhere Farm with their take on how to treat gummosis using a special whitewash product instead of latex paint. I've never used this stuff, but it sounds like it might be a good substitute for latex paint:

Thank you so much , M. K., for the clear directions! It feels so much better to have some action to take!

We have this weird wet and cool weather this week. Do you think it best to take care of it right away or should I wait until it dries up this weekend or next week?

I would wait until things dry out this weekend. The tree should be dry before starting treatment.