How can I manage tar spot on my maple trees?

Asked September 30, 2015, 1:08 PM EDT

HELP! The two maple trees in my backyard have a heavy case of tar spot! I didn't notice any last year. What can I do to prevent this from happening next year?

Hennepin County Minnesota

3 Responses

This has been a good year for tar spot and other fungal diseases. Here is a link that will help you understand the causes, and some control methods for many of our common leaf spot diseases:
Management of leaf spot diseases of trees and shrubs.

I read the info on tar spot and should have given you more detail in my first request. We did rake and dispose of leaves last year. We do have mulch around the trees. The trees are large, mature trees and we didn't water them this year. One tree has no branches/leaves within 12 ft of the ground and the other tree has no branches/leaves within 30 ft of the ground. Is this an air-borne problem? Did this blow in from a neighbor yard? The neighbor on south has an uncovered yard waste pile (not a working compost pile) in the corner of his lot with a maple sapling growing in the center of it. That sapling also has tar spot. Is it possible that is where it came from? Neighbors to the north and west have no evidence of disease in their large, mature maples (larger than ours). We bought our house 2 years ago and the previous owner also had a large (8ft x 5 ft, and 3 ft deep) uncovered yard-waste pile in the back corner. We have worked to make ours a working compost pile. It is weed-free now and most areas are yielding good compost. Some areas are still just old packed leaves. Do we need to get rid of our pile?

It can be problematic if there are other trees that are also infected. If their owners do not rake up and destroy the leaves the disease is less likely to be controlled.

Here is some further information from Cornell University - the advise is the same as for Minnesota:

Current research has shown that the tar spot fungus does not cause long term damage to the host. The most effective management practice in a home lawn situation is to rake and destroy leaves in the fall. This will reduce the number of overwintering "spots" (containing the fungal reproductive structures) which can produce spores the following spring. However, where other infected trees are growing nearby, those leaves should also be raked and destroyed. Mulching leaves will suffice to destroy many of the spots before they mature, but the mulch pile should be covered or turned before new leaves begin to emerge in the spring. Application of fungicides are possible when high levels of infection become unacceptable but control of the disease is difficult. Complete coverage of leaf tissue is needed for success and this can be difficult on mature maples. Also the appearance of the disease has become wide spread across much of New York State, and if others in a neighborhood setting are not managing the disease on their trees with fungicides or proper sanitation, the act of spraying may be a waste of time and money. We cannot recommend that homeowners attempt to treat large trees. If pesticide application is desirable, consult a licensed tree care professional. If fungicides are used, applications may be needed most years.