A Maple With Leaves White as Snow

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 Norway maple leaves suffering from powdery mildew. Photo: City of Toronto

Question: I have a large red maple that, over the last few years, has started developing some sort of white fungus on its leaves. Since I like to shred fall leaves and use them as mulch, is it all right to do so with these diseased leaves or would it be better to destroy the infected leaves to prevent this fungus from spreading to other plants and also from infecting the leaves of our maple in the years to come? Is there anything to do to get rid of this annoying fungus?

Lisette B.

Answer: First of all, I think your maple is more likely a red-leaved variety of Norway maple (Acer platanoides), possibly ‘Crimson King’, rather than the tree known commonly as red maple (A. rubra), because the latter is not subject to this kind of fungus.

Your maple appears to be suffering from powdery mildew. Until recently, Norway maples in North America were free of this disease, but over the last ten years or so, it has become very common. It’s believed that the fungus Sawadaea bicornis, common on this species in its native Europe, was accidentally introduced into North America and has now spread to much of the continent.

Not Much You Can Do

You’ll probably have to get used to the appearance of white powder on your maple’s leaves in late summer, as the disease is wind-borne and thus very difficult to prevent. Plus it’s already present pretty much everywhere Norway maples are grown.

Powdery mildew is generally considered an essentially aesthetic disease and does no harm to your tree’s health. If this disease really disturbs you, the only treatment would be to spray regularly all summer with fungicides, not something easy to do with trees because of their height. The only logical “cure” would be to remove the tree and to replace it with a variety not subject to powdery mildew.

You’ll find the incidence of the disease varies from year to year and is most prevalent when the soil is dry at the end of the growing season, but the air is still very humid.

Destroy the Leaves or Not?

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Powdery mildew on ‘Crimson King’ maple leaves. Photo: City of Toronto

Since powdery mildew is carried by the wind and is already present in city plantings all around you, you’ll find no particular advantage to destroying affected leaves rather than using them as mulch. Moreover, shredding the leaves will destroy many of the spores and most of others will die when the chopped leaves decompose as just they do in contact with the soil. Composting too will destroy most spores.

Also, there is no need to fear that this disease will spread to other plants: there are hundreds of strains of powdery mildew caused by different fungi, but most, such as the one that affects your maple, are very host-specific. Thus Norway maple strain of powdery mildew (a specific clone of Sawadaea bicornis) will only affect other Norway maples.

Note that ‘Crimson King’ and ‘Schwedleri’, two varieties with dark purple foliage, are the two cultivars considered to be the most susceptible to this disease, so other readers might want to avoid them when choosing trees to plant in their gardens.20170928A City of Toronto

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