black mould on maple trees


I have raked up some leaves that have black mould on them. I want to keep them in a paper leaf bag until spring and let them decompose and dig them into the garden in the spring. Would thismould be harmful to the soil if I were to do so?


Your question is very apt at this time of year as the season ends for the maple cycle. The botanical name for the fungus is Rhytisma acerinum.

In southern Ontario this fungus appears most frequently in urban areas, because many cities opted to plant Norway maples along city streets and boulevards, for their shade value. Many opinons have been expressed about managing the blemished leaves, and some writers have suggested gathering the leaves for municipal disposal management. We have found no advantage to what could be a laborious task of separating yard waste this way.

I am including a passage from University of Guelph’s Arboretum site, as it answers your question succinctly.

“The black spots are a fungus disease called Tar Spot. It infects Norway maple (including all of the horticultural forms such as “Crimson King”), Silver maple and Freeman’s maple. The disease infects leaves late in the season and has little impact on the overall health of the tree. The disease is already widespread and sanitation (leaf removal) is not effective in reducing the infestation level for the following year, so let the leaves decay under the trees to at least improve the soil health. Norway maples are generally short lived along city streets, but the seeds move through storm sewers into natural areas where, once they germinate and grow, they are able to shade out native plants. Consider planting a replacement for your Norway maple.”

Good composting and happy gardening!

December 4, 2020