Have you been noticing large spots on your Norway maple leaves this summer and are concerned that they could be a serious issue? Here’s an easy-to-understand review of what the culprit is, and what it means for your beautiful trees.
Usually by the time late August rolls around, we start getting calls from Beautiful Trees clients about their Norway maple trees. Clients report the presence of large, scary-looking, black leaf spots, giving the foliage the appearance that it’s been sprayed with road tar. Those leaf spots are called “Tar Spot” (Rhytisma acerinum), a fungal leaf disease found exclusively on Norway maple (Acer platanoides). But let’s back up a minute, and go through the life cycle of this disease.
At first, this fungal leaf disease presents itself as large, yellow spots that start to become noticeable in July.
Over the weeks of August and early September, the yellow leaf spots gradually fill in with tiny, black fruiting structures.
Now you can see how the large “tar” like spots on the leaf surface are actually several tiny black fruiting structures of the fungus. The leaf spot will remain that size and the black fungal fruiting structures within it will lay in dormancy for the remainder of the year.
When we receive hot, dry conditions in August and early September, heavily infested Norway maple trees may drop some of their leaves in order to compensate for the stress of the weather (leaves are expensive for a tree to maintain if it doesn’t have enough resources). The early shedding of leaves that are covered in black Tar spots may be alarming, but most Norway maple trees tolerate this fungal foliar foe and recover nicely.
Tar spot infected leaves will eventually drop to the ground in autumn, along with all the other leaves on the tree. Over fall and winter, the fungal fruiting structures on the infected leaves will remain dormant, waiting for next spring. Once the April showers and mild temperatures return, Tar spot fruiting structures will rupture and shoot their fungal spores into the air in hopes of landing on new spring leaves of their host, the Norway maple.
This fungus requires mild temperatures and leaf wetness periods to successfully infect and colonize new spring leaves. There is a high level of Tar spot disease in 2024 because of the frequent rains we experienced during leaf emergence in early spring. In years where the weather is dryer during leaf emergence, we notice that leaves remain healthy and largely unblemished for the rest of the year.
Although it may look unsightly, Tar spot rarely compromises plant health because:
Since Tar spot fungus overwinters in fallen, infected leaves and sporulates in the spring when the new foliage is emerging, we can then use this knowledge to reduce the risk of disease next spring. By collecting and removing fallen, diseased leaves, we can reduce the amount of disease inoculum in our yards laying in wait for next year. This sanitation method works better if neighbouring property owners with Norway maples participate as well.
Regular mowing of infected leaves in late summer and autumn can also reduce disease incidence by accelerating the breakdown of leaves, and the fruiting structures found within them.
For young trees that are susceptible to foliar loss, some property owners may want to spray new foliage to protect it from potential leaf drop and reduced photosynthesis, but spraying is not usually warranted. It may be more advantageous to boost the root zone with organic amendments and correct fertilizer deficiencies that are present on the site.
Call us for a Free Estimate to inspect your beautiful trees for any plant health concerns and provide effective treatment services that boost the health of your landscape trees and shrubs.
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