Black spots/shriveled leaves on parking strip trees

Asked May 16, 2020, 10:19 PM EDT

I bought an older home in Mt. Angel last year. There are two mature trees in the parking strip on the south side of my home which leaf out beautifully, but as the season goes on, some of the leaves develop black sections and/or shrivel and fall to the ground. I end up with lots of detritus coming down (leaves + little branches) into the grass to the north. I use a push mower on the grass, so the small twigs and leaves that fall are cut and remain on the ground. I do clean up leaves ASAP when they drop in the fall, but this "dropping" of detritus that occurs from spring to summer is spread over time, and the leaves are two small to pick up - thus ending up as part of the mulch when I mow. Within the grass area are some Japanese maples; they do not _seem_ to be impacted at this point, but I worry about them! Wondering what kind of trees these are, what their issue is, and what I can do to support them. Also would like to know if I should be concerned about the Japanese maples below. Attaching three images: 1) My trees (left side of the street), and my neighbor's (right side, topped) which have the same issue; 2) a picture looking up into one of my trees; 3) a neighborhood tree that I _think_ is the same type of tree with the same problem (including this pic as I was able to get closer to a leaf to photograph) Any information/suggestions would be much appreciated! Ideally, I would like to save these trees!

Marion County Oregon

1 Response

Hello: these look like some cultivar of Red Maple (Acer rubrum). These are very common street trees, but are susceptible to the fungal disease anthracnose. This is a foliar leaf-spotting disease which seems to be primarily a problem on Red Maple. If it occurs on other Maples, including Japanese Maple, it is not a significant problem by comparison. The disease occurs when leaves get infected in wet weather. The wetter the weather is during early shoot and leaf growth, the more of this disease we tend to see. Conversely, if it is very dry in spring, there is less disease.

As you have observed, leaves which have a number of spots have a tendency to fall off the tree as summer goes on. The little twigs that you see may simply be shaded out as part of canopy development, or it's possible that they could be associated with twig cankers of the disease. The good news is, because our weather does dry out in summer, the wetness that causes the disease to occur is no longer present and that essentially bring a halt to further disease progression.

The link below is to the discussion of this disease in the PNW Disease Management Handbook. As you can see, cultivars vary in their susceptibility. If you have a susceptible cultivar (yours do show at least some susceptibility) there are some cultural methods to reduce disease, as well as a number of sprays. Generally speaking, most homeowners find the thought of applying fungicides as shoots grow t protect them quite unappealing. I think the thing to keep in mind is that the trees tend to tolerate this disease quite well and are not at risk for any serious symptoms beyond some defoliation, so sprays are not considered necessary. In general, they retain their aesthetic and functional appeal year in and year out, even while suffering some damage.
Feel free to write with further questions.