Pseudomonas syringae (Bacterial Blight) is a disease that attacks fruit trees and some other ornamental trees as well as lilacs. The photo is characteristic of what happens.
It is most often caused in woody plants that are too densely foliated for good air circulation in combination with very wet weather conditions. Our cold wet springs are perfect breeding grounds for this disease. And there is a similar disease that affects willows.
Pruning incorrectly causes dense foliage. And we see so much of that in our area — incorrect pruning.
In our area, I have most often seen this disease on ornamental cherry and almond trees, Callery pear, and lilacs. It is so prevalent in our region (west side of the Cascades), that I never recommend ornamental cherry trees, and rarely recommend lilacs even though I adore their aroma.
As far as I’m concerned, I don’t have use for a tree that is only spectacularly beautiful a handful of weeks of the year (cherry), regularly gets a really ugly disease (cherry), grows in a habit that requires a lot of pruning (cherry), suckers badly (cherry), and ordinarily is way too large for most small residential lots (cherry). But that’s just me.
What to Do If Your Tree Has Bacterial Blight
My sources say to prune out the damaged parts of the plant as soon as you notice it, and destroy the cuttings. Don’t compost them or the blight will get into the compost and affect things later on. When cutting out diseased wood, it’s a good practice to disinfect the cutting tools for EVERY cut. The disinfecting can be done by dipping blade into a bucket of water to which a small bit of bleach has been added — about 1 part bleach to 9 parts water. You can also use a spray bottle, but I find the over-spray damaging to whatever it touches, so out there in the garden, it’s not a good thing. Dry off the blade after dipping.
Some resources I’ve seen say that copper or other fungal sprays are useful. Others say they are provide minimal control.
Proper culture is the best way to avoid this disease. Prune for good circulation, and avoid wet conditions (particularly when chilly, which is hard to do here in the PNW), don’t over fertilize, choose disease-resistant varieties.
Bacterial blight will not usually kill a tree if limited to the new shoots. It’s just ugly. But if it should become systematic or affect the trunk, it will surely kill the plant. So removal of the plant when first discovered can help protect other plants.