How to - Pests & Diseases

What to do about tar spots

David Hobson

As you're raking, find out the steps that need to be taken if you encounter this blight

They’re piling up and it’s time 
to begin raking them, especially those sporting tattoos gone wrong. They’re the ones infected with tar spot, a fungal disease that mainly attacks Norway maples, resulting in black splotches that can be up to two centimetres across. The same fungal spores, however, will happily alight on and infect other species of maple trees, too.

Caused by the fungus Rhystisma acerinum, an import from Europe, tar spot becomes noticeable in late summer and into fall, but actually begins in spring when the fungal spores are released after over-wintering on infected leaves.

Realistically, little can be done to prevent the spread of tar spot, particularly as there are more maple trees than people in Ontario and it appears most trees across the northeastern part of the continent have become infected after exposure year after year.

Fungicides are somewhat effective, but they’re impractical since almost every leaf on every afflicted tree would have to be sprayed. The best hope is that weather conditions in coming years will be such that the problem is less severe. Fortunately tar spot, though unsightly, is not likely to kill the tree.

The question is what to do 
with all those fallen leaves blanketing the yard? It’s typically recommended that all infected leaves be raked up and burned, buried or sent for municipal composting. Leaves that are collected by municipalities are subject to higher temperatures 
in the composting process, which should destroy the spores, while 
a backyard compost pile may not heat up enough to be effective. But if the neighbours up the street don’t dispose of their leaves in an appropriate way, then any attempt on your behalf to prevent the spread becomes futile.

If you should decide to continue composting your own leaves, but are still concerned about contributing to the spread of tar spot, cover your pile with a tarp or other material to contain the spores, which are released mainly between May and the end of June.

Thankfully, there is no evidence that compost produced from infected maple leaves will trouble other garden plants in any way.


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