Why are maple leaves turning yellow and dropping early? A Penn State forester explains

This past month I began to observe that the leaves on some of my maple trees located in the forested part of the property began to yellow and drop. I also observed this phenomenon while traveling up Route 26 into State College.

I thought that I knew what was occurring but thought I better check in with the Penn State foresters. Since my friend Joe Harding, who was the director of forest lands for Penn State, retired, I went to his replacement with the same last name but no relation — Brent Harding, the current director of forest lands for Penn State. I told him that some of my trees looked like October foliage in August.

Brent said that he was indeed getting inquiries about premature leaf drop from several states. He went on to say that leaf drop is an adaptation that allows the trees to shed leaves in summer to reduce the potential for even greater water loss. The fewer leaves, the less water needed to keep them happy and results in less water escaping from the soft leaf tissue. This adaptation essentially sacrifices the least productive leaves, those that are shaded by the leaves above and at the outside of the canopy in favor of those that can still capture some sun and make sugar. It is really a simple and elegant system designed to protect the trees.

If there’s not enough water to keep all the leaves happy, shed some and send their products to those that remain. The reason the leaves often turn yellow before they drop is that the tree harvests many of the nutrients from the leaves before it sheds them, which is a simple measure of conservation of resources.

Now while it’s always better to avoid conditions that result in drought-induced leaf drop, don’t worry, it’s not the end of the world. It is a system that has evolved over millions of years to deal with exactly the kind of weather that we have been experiencing this year. Of course, if we had some good, deep and consistent rainfall, that would reduce the leaf drop and your tree would be much happier. Recently we had some heavy rains, but they came down fast and resulted in some runoff instead of soaking into the soil.

Still, there could be something else going on. This time of year, there’s another leaf drop problem that could rear its ugly head. If you see trees showing some premature fall color in the top of the crown, an odd branch or chunk of the canopy top showing beautiful reds and oranges right now, that’s something to worry about.

Maples are susceptible to a fungal disease known as verticillium wilt. Caused by a collection of fungal species, it acts by infecting the vascular tissue of trees and clogging the plumbing. The result is that you can water all day and night and you’ll never get enough water from the roots to the leaves. This causes whole branches to color prematurely and eventually die. But the difference is that rather than shedding interior or basal leaves, verticillium infected trees show dieback that starts at the top of the tree.

Verticillium wilt is caused by a collection of disease organisms that are always present in the soil. They just tend to cause problems when trees become stressed like when they put on a ton of growth during a nice, cool, wet spring followed by a particularly hot and stressful drought like we’re experiencing right now.

Unfortunately, once verticillium wilt symptoms set in, it usually spells a gradual and irreversible decline. Not much can be done about a tree during this march to permanent decline, but at least now you’ll know the difference between normal leaf drop and disease caused leaf drop. Another disease that I have noticed on my red maples is anthracnose, which can be a problem each year.

Thanks, Brent, for confirming my thoughts on the cause of my turning leaves and premature drop.

Bill Lamont is a professor emeritus in the department of plant science at Penn State and can be reached by e-mail: wlamont@psu.edu.