Why have the Linden boulevard trees been dropping thick black sap?

Asked November 26, 2017, 5:06 PM EST

Hi, Over the last summer and fall, the two American Linden trees on our boulevard in Minneapolis have been excreting a terrible, messy black sap. The sap covered all of our hostas, azaleas and other plants nearby. Most concerning is that the black sap covered our front screen porch woodwork and siding (painted white and pale yellow) and filtered through the screens and got inside the porch. Now, even after cleaning, our white paint is permanently stained. Could you please tell me if the tree might be infested by some kind of pest? If so, do you have any advice on how we can stop it from happening again next summer? The trees are technically the City of Minneapolis's responsibility since they are on the boulevard, but I am wondering if there are any steps we can take to mitigate the problem. Thank you!

Hennepin County Minnesota

1 Response

What you have described was a common occurrence this past summer. The black sap-like substance is honeydew secreted by aphids or other sucking insects. The honeydew, which is originally clear, turns black when it's overgrown by fungus called sooty mold.

The problem may occur again or not depending upon the presence and relative abundance of aphids in future growing seasons.

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