Last month, I received an email message from a Lucas County resident that was noticing leaf drop on some maples in her neighborhood. Of course, the diagnostic process immediately begins, and my mind automatically goes to the OSU FactSheet, and I start going through the series of questions. If you aren't familiar with the FactSheet, or need a refresher, this resource is laid out in a order that takes you through the diagnostic process.
Here is a link to the FactSheet: https://ohioline.osu.edu/factsheet/plpath-gen-3
But back to the client's situation and the concern about the spots on the leaves and them dropping prematurely on the lawn below. I went out to the site and identified the maple as a red maple, and noticed in the neighborhood that every other condo had a red maple and all were exhibiting the same brown spots on the leaves, and were dropping leaves. While if you looked at the ground, there was a noticeable amount of leaves on the ground, but if you looked into the canopy, the trees were still full and dense. The photo below shows the full canopy. You will also noticed that there aren't many leaves on the ground as the grass had been recently mowed.
While at the site, I took some photos and collected some leaves that had fallen and some still attached above. My initial thought was anthracnose, a common foliage disease that we see in the spring when weather conditions are favorable. Once back to the office, I began an email response to the person that reached out and was concerned about the trees in her neighborhood. As I was doing some research and finding links to resources that I could provide to the client that could easily be shared with others in her neighborhood, I came across some enlightening information from Michigan State University that covered maple anthracnose, and something called maple leaf blister (see link below). As I read more, I thought that what I was seeing actually fit the description of the maple leaf blister, more than anthracnose, even though I am sure I had called it anthracnose in the past.
Maple leaf blister (Taphrinaspp.) is a disease of maple leaves that displays symptoms somewhat similar to maple anthracnose and can easily be confused. But with that said, there are some differences. The lesions caused by anthracnose are located between veins and along the leaf margins and often dark in color. Compared to maple anthracnose, the leaf spots of leaf blister are more rounded in shape and contain small, raised blisters. Lesions are a mix of lighter brown to black. The fungus that causes maple leaf blister overwinters in bud scales and attacks developing leaves early in spring. With the right weather conditions, the infection can spread rapidly and cover most of the leaves in the canopy.
Like anthracnose, maple leaf blister is unlikely to threaten a healthy tree, and cultural practices described for anthracnose can reduce spread of the fungus and the risk of serious damage to the tree. Ensuring sufficient water of approximately 1 inch per week will support the tree through the stress of disease and leaf regrowth. And like with many diseases, maintaining sufficient spacing for airflow between trees creates an environment that is less favorable for fungal growth and removing symptomatic leaves after they fall can help reduce the spread of the disease-causing fungi.
It is important to note that anthracnose and leaf blister can occur on a tree at the same time. The only way to be certain if a tree has one or the other or both is to send samples to a diagnostic laboratory and have it confirmed. That is what I did when I sent samples to the OSU's Plant and Pest Diagnostic Lab: https://ppdc.osu.edu/
Soon after the sample was submitted, I received an email confirmation that the sample had maple leaf blister. And as part of the detailed report sent from the lab, was links to two resources, one from the University of Illinois and one from Michigan State University. The MSU resource was the same source that I came across that got me questioning that actual cause of the leaf drop. We are grateful to have the clinic and the opportunity to send samples for diagnostics and confirmation as needed in the diagnostic process here in Ohio.
If you send samples to the PPDC, be sure that you are sending fresh samples that are shipped or delivered quickly. I have always said -the better the sample, the better the diagnosis will be. Links to forms and additional information can be found on the PPDC website. Additionally, the clinic is listing weekly summaries of what samples were received and the diagnosis made under the tab,weekly summaries. I did learn that a second sample was received from Franklin County with the maple leaf blister last week as well.
If you are seeing what appears to be maple leaf blister, send an email to firstname.lastname@example.org with a photo if possible. Happy diagnosing this season and don't forget to use the 20 Questions of Diagnostic FactSheet!