Tar Spot

Welcome back to Ask a Naturalist, your own personal Google for information on all things natural on PEI. My inbox is overflowing with the question “why are so many Maple trees losing their leaves early this year”? In the trees I’ve seen, it’s due to Tar Spot fungus (Rhytisma acerinum).

Tar Spot is the common name for several species of fungi in our region. There are a couple of native versions that infect native maples and willows, but the type that’s most prevalent now is non-native and prefers the introduced Norway Maple, a tree that has been extensively planted as an ornamental on PEI.

Tar Spot releases spores in spring, allowing them to infect newly emerging leaves. Cool, wet weather is ideal for this fungus, and that’s exactly what we had back in May. As Tar Spot develops, pale circular spots appear on the leaf, followed by development of black apothecia (reproductive structures; you can see these starting in the photo). Eventually, the leaves sport unsightly black spots, and heavy infections can cause early leaf fall.

A second fungus – Anthracnose (Aureobasidium apocryptum) – also infects Maple leaves, likes cool, wet spring weather, and is prevalent right now. It creates irregular brown patches rather than circular spots on leaves and causes them to wither and fall early. A good look at your leaves should tell you whether the issue is Tar Spot, Anthracnose, or both.

Losing leaves in August isn’t a serious issue: by now, those leaves have served their purpose, photosynthesizing all summer to provide food for the tree. An otherwise healthy tree infected by Tar Spot or Anthracnose will leaf out again next spring.

These fungi overwinter in fallen, infected leaves, and so raking the leaves and sending them out with your compost can reduce the spread. However, Tar Spot is well established and widespread, and Anthracnose is native, making elimination of either fungus unlikely. Planting native trees rather than Norway Maple is the best defense against Tar Spot in particular.

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