Growths (or worms?) on plum tree leaves
We've taken over the care of my husband's family's centennial farm in the Valley City area. In the shelterbelt is a plum tree -- what my father called a wild plum tree. The tree was in full bloom last weekend and when I checked it again this week, I found growths or worms on some of the leaves. What are they, and are they harmful to the tree? Also, what care should we give the tree? We've been cutting out the dead wood and had a bowl full of plums 2 years ago, but only a couple plums last year. Do they need another variety to produce? We believe the shelterbelt is at least 60 years old. Thank you for your help!
Barnes County North Dakota
The damage you're seeing are galls that the tree created in response to feeding by tiny mites. I don't know which specific mites caused the damage, but they're in a broader group called eriophyid mites. Eriophyid mites emerge extremely early in the growing season and the damage is created just as the leaves are emerging - several weeks ago. And they're already done causing damage.
On a positive note, the damage looks relatively small to me - a tree can lose up to 25% of its leaf cover before it begins to get stressed. So even though there's nothing that can be done at this point, there is really no effect on tree health. The damage is aesthetic.
Plums usually require a second tree to set fruit. Adding another plum, whether another wild tree or perhaps a named variety will help with fruit production. More information can be found at: https://www.ag.ndsu.edu/publications/lawns-gardens-trees/tree-fruit-culture-and-cultivars-in-north-d....
Best wishes with your tree,
Thank you so much for your response. It's so helpful! We'll plan to get another plum or two planted to try to improve the tree's production.
I have another question about our plum tree.
As the fruit began to develop, I noticed that several were deformed. This past weekend, my husband saw that those deformed plums now look rotten while still on the tree. We didn't pull any off to see if there were worms or insects inside.
As you can see, there are a number of the rotten plums, but others that look like they're developing normally nearby.
Is this damage also the result of the mites? Or something else?
Should we remove all of these bad plums and destroy them? I also forgot to ask if there's any treatment or remedy for the mite problem?
This is a separate issue, a fungal disease called 'plum pockets'. These deformed fruit won't be edible. Though they can be removed from the trees right now, it might not be the best strategy for managing the disease.
Instead, two things need to be done. First, rake up and remove/destroy the infected fruits in the fall. Then, next spring, apply a preventative spray treatment of Bordeaux mix or lime sulfur when temperatures are above freezing, but before the buds have begun to swell.
Regarding the mites, a preventative treatment in the spring, using a miticide, could help to reduce the issue. However, usually the damage caused by the mites is minimal and no treatment is needed. Trees can lose up to 25% of their leaf surface without feeling any stress. Based on the earlier photos, it looks like there's plenty of healthy/unaffected leaf tissue to support this tree.
Excellent information. Thank you again for your help!