Spots on maple leaves

Asked September 7, 2015, 12:03 PM EDT

I just noticed these spots on a maple tree. Anything we can do now to treat it (early Sept.)? What are the chances the tree will survive winter?

Oakland County Michigan

1 Response

What you are seeing is a very common fungal problem called Tar Spot. It is considered a "cosmetic" disease. It looks bad but is harmless. It happens to many maple trees when we have humid air from the time you turn off your furnace in the spring to when you turn it back on in the fall.

Once any fungal problem is seen, it's too late to treat it. Earlier, the tar spot fungus got into the leaf, incubated and then manifested itself. It is literally part of the leaf. That's why it cannot be scratched off. It is the leaf.

For tar spot, fallen leaves from the year before that had the tar fungus are the inoculum for the new year. In May, as leaves are expanding, the fungus gets into the leaf. Nothing is seen until June when yellowed round spots are seen on the leaf surface. By July, small black pimple-like dots are visible. The spots grow larger and by sometime in August, they coalesce. Now there is one big black spot with a wide yellow ring around the edge. Eventually, the yellow ring is overtaken by the black, raised spot.

Leaves do not fall off any earlier and the tree's health is not harmed. Leaves continue with their business of photosynthesis except for the spot area. The spots do not decrease the food stored by the tree. Eventually, the leaves fall off at the regular time and the leaves break apart. But the fugus remains. Next spring in May, the cycle begins again.

You can pick upand destroy every fallen leaf on your property in an attempt to lessen tar spot. But your neighbors for miles around you did not. Tar spot is there, floating happily in the humid air, looking for tender, young maple leaves to land on.

Attempting to spray trees with a fungicide at the right moment is difficult, if not impossible, because of the size of the tree and the timing. With large maples, it would be hard to boom enough fungicide into the trees and have it remain long enough to protect the leaves from a harmless disease.

Unless there is another health issue with this maple, it will be there next spring and many springs to come. Just water the tree if the soil is dry so it can grow and set leaf buds for 2016. Millions of maples all over Michigan and surrounding states get tar spot. Think of it like pimples: they aren't cute but it won't kill you or even make you sick.