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London Planetree

Weather conditions can cause leaf drop in late summer

By: Sandy Feather ©2015
Penn State Extension


Q. There are a number of young London planetrees that have been installed as street trees in my neighborhood, and I am very concerned about them. They have a lot of yellow leaves, and many leaves have turned completely brown and fallen off. It almost looks like fall with all of the leaves laying on the street and sidewalks. Is there some kind of blight affecting them? 

A. If you look at the London planetrees (Platanus x acerifolia) that are planted as street trees around the city, you will notice that all of them look like the ones on your street. Young trees as well as old, established ones are suffering from a combination of issues. Sycamore anthracnose is a very common disease on our native sycamores (Platanus occidentalis), which are closely related to London planetrees.

Sycamore trees
Native Sycamore trees in winter

London planetrees are much more resistant to this disease, which is why they are planted more often than sycamores. But resistant does mean immune, and the extremely wet weather in June and early July created very favorable conditions for anthracnose.


London Plane Diseases

Fortunately, it was hot and relatively dry in May when these trees were leafing out; otherwise, anthracnose would have been more severe, because new leaves are much more susceptible. The rains held off until leaves were mature, so anthracnose is not as bad as it could be.

 

London planetrees are also susceptible to powdery mildew, and we have had favorable conditions for that disease to develop. Fortunately, London planetrees can live with both of these diseases, and only very high value trees in situations that cannot tolerate any blemish require treatment.


Extreme Summer Weather Conditions

The biggest issue, however, is drought and heat stress. The symptoms you describe appeared very suddenly, when the weather turned hot and dry in the second half of July. The overabundant rains of June and early July created a situation that kept soil saturated, which probably caused a portion of the trees’ fine feeder roots to rot and die. Fine feeder roots are responsible for absorbing water and nutrients from the soil, and represent the largest mass of a tree’s root system.

The plentiful rain also pushed more new growth on many trees than is normal, and such succulent growth uses more water than hardened off, mature foliage does. When the weather suddenly turned hot and dry, the trees had no time to adapt to the changed circumstances. Shedding of leaves is one way the trees are coping with the change in weather. At this point, regular, moderate rainfall or supplemental irrigation that provides about an inch of water weekly would be the most beneficial treatment.


The good news

It certainly is not ideal that these trees are defoliating prematurely, but their buds for next year are already set. They should leaf out and grow normally next spring – at least until Mother Nature throws another curve ball at them.

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