What is causing the leaves to drop off my crabapple tree this summer?
Question:
I have a flowering crabapple tree that has been losing its leaves all summer. I am wondering if it has a disease that I need to treat?
Answer:
I believe what you’re looking at on your flowering crabapple is a disease called apple scab. You can read more about it in these resources:
Iowa State Univ Extension page on Apple Scab
Univ of MN Extension page on Apple scab
As you’ll see in the links, scab is an extremely common fungal pathogen affecting apple and crabapple trees. It isn’t typically fatal to the infected plant, but it does cause leaf drop and the “feather” blotching you see on the infected leaves still on the tree. Recommended cultural practices for combatting scab are promptly removing (and destroying, not composting) fallen leaves throughout the season and in the fall, keeping the tree healthy and properly pruned to promote air flow through the canopy (read how in our UMaine Cooperative Extension Bulletin #2058, Flowering Crabapples in Maine), and removing any suckers or watersprouts (new stems sprouting from the base or the trunk). There are fungicidal treatments available if the tree becomes too unsightly to tolerate, but keep in mind that most of the time this condition is cosmetic. If you do choose to go the chemical intervention route, please read carefully and follow conscientiously all instructions on the label, or consult a license arborist with pesticide credentials to apply treatments. More is not better.
All of that said, visual diagnoses are never as definitive as lab testing, so if you remain concerned after receiving my reply or if the condition worsens even after implementing the above recommendations, you can always submit a sample to the UMaine Cooperative Extension Plant Disease Diagnostic Laboratory. Their page has instructions on preparing and submitting the sample, and they typically get back to you with results in 2-3 weeks.