Maple tree leaves rusty - Knowledgebase Question

Altamont, Mi
Question by asch58
July 15, 2010
I planted a new Autumn Blaze Maple tree this spring and the leaves look like they have rust spots on them. Our soil is clay, but I amended the soil with compost. It gets plenty of sun. It has produced new leaves but they get rusty too.


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Answer from NGA
July 15, 2010

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Several fungal leaf spot diseases can infect maple, including anthracnose, Phyllosticta leaf spot, and tar spot. Extended periods of cool, moist weather, as new leaves are emerging, favor these diseases. Your description leads me to believe the problem is Phyllosticta leaf spot. This disease is characterized by orangy/tan or brown spots with darker margins. Sometimes the diseased centers dry and fall out leaving holes in the leaf. Later, fruiting bodies of the fungus (black pepper-like dots) develop within the leaf spots.

The fungi overwinters on leaves and twigs that were infected the preceding year.
The spotting may appear unsightly, but rarely causes more than minor damage to the tree so chemical controls are not usually recommended. Raking the leaves in the fall and pruning dead or dying branches remove fungal material will help reduce new infections next spring. Because your tree has developed this problem two years in a row, I'd consider using a fungicide early next spring to protect the new growth. Fungicide application should begin at bud break to protect the young, succulent growth. Repeat applications, according to directions on the fungicide label. Read the label carefully and apply only as directed. Fungicides containing copper such as Microcop are currently labeled for fungal leaf spots of Maple. Best wishes with your tree.

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