Fungal diseases, frosts hurt leaves on trees, shrubs

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What is wrong with the leaves on my big maple? They have brown discolorations on many of the big veins on some of the leaves. There are few areas where there are brownish spots next to the veins. The leaves are still on the tree. What do I spray?

Pesticides are not an option. Any time that you find spots on leaves, it is too late to fix. Fungal diseases can be prevented but not cured. This problem is called anthracnose.

There are several different kinds of fungi that can cause anthracnose. The “signature” of anthracnose on maples is that the discoloration appears along the large leaf veins. Portions turn brown, sometimes with a tan center. This happens to be a springtime fungal problem that is difficult to predict. But, in maples, it is rarely serious.

For some kinds of trees it could be considered cosmetic. They have some leaves that just don’t look cute. Other kinds of trees, like sycamore, can also get another form of anthracnose. For those trees, it can be devastating. They end up with twigs and branches dying back after several years of being infected.

Anthracnose is triggered by the weather. Moderate temperatures and rainy or humid weather is needed. But how many springs do we see this weather but not anthracnose? This is not a disease that you can prevent because it only shows up sporadically. And it does not hurt your maple.

For trees that have frequent problems, timing preventative spraying may be difficult. It may be easier to use a straight nitrogen fertilizer to stimulate new leaf growth. You are encouraging the tree to grow beyond its early spring trouble and produce new leaves. That would be products like 21-0-0 or 46-0-0 which are only nitrogen.

Just do a light application.

Studies have shown that when a balanced fertilizer like 12-12-12 that also has phosphorus and potassium included does not help the trees. Stay with a straight nitrogen product.

I have some kind of disease that has hit some of the trees and shrubs in my yard. The leaves have all turned black and mostly fallen off my walnut, honey locust, mulberry, Japanese maple and smoke bush. I don’t know how to stop it and I can hardly sleep at night. I have other trees but it has not spread to them yet. What can I do?

Your first job: understanding that this is not a disease or an insect problem. This is freezing damage from the end of April. The leaves became flash-frozen when the temperature dropped below 25 degrees the night of April 28. When the leaves thawed out, they were dead. All their little cell walls in the leaf tissue were exploded. You can’t recover from that. They are currently drying up and falling off. This is to make way for the new buds that are expanding now.

Within several weeks, those new leaves will be opening and enlarging. For trees and shrubs, whether leaves lived or died depended on several things. The kind of woody ornamental it was, how developed the leaves were and where it was in the yard it was located all made a difference.

We had very little rain and some of these were stressed from lack of water and this can increase damage. The only way that the tree or shrub will not produce more leaves is if you cut off the twigs or the twigs froze with the leaves.

For now, your job is simple. Do not fertilize any of these damaged plants. It could hurt them. Water them if the soil is dry and it is currently dry in many locations. Do not prune them. And now wait.

If one late freeze was enough to end the life of wood ornamentals, there would be none left in Michigan.

About Gretchen Voyle 51 Articles
Gretchen Voyle is the MSU Extension-Livingston County Horticulture Educator. She can be reached at (517) 546-3950.