Black spots on maple leaves, treatment

Tar like black spots on maple leaf, yellowing and brown around the rim.

This fungus impacts the leafage and forms black spots on the topside of maple tree leaves.

Black fruiting pods may appear on these leaves, which then cause leaf fall.

Read on to learn what this disease is and how to protect your maple trees against it.

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The solution to eliminating black spots on maple

Gather and burn all the leaves to eliminate any chance of having the fungus survive.

You can also put them in the compost.

It’s fine to wait for leaves to fall naturally. Removing live leaves from the tree would stress the maple if done too often (more than once a year).

What causes maple leaf black spot?

A fungus called Rhytisma acerinum is responsible for this. It’s common goeas by the name “tar spot” or “black tar spot” because it’s black as pitch.

The fungus doesn’t infect the inside of the tree, and it doesn’t have any severe parasitic action. The worst impact of this fungus is that it shuts down normal activity on leaves at the spot itself – that reduces the leave’s ability to photosynthesize and convert sap and sun to nutrients and energy for the tree.

The visual effect makes the tree look much sicker than it actually is.

Why raking up leaves under the maple is important for black tar spot

This particular fungus has three main phases in its life cycle –

  • in spring, microscopic spores are released which are airborne. The wind carries them over a distance and some of them land on maple tree leaves.
  • the sticky spores open up and start colonizing the leaf, and the imbalance resulting from this causes the maple leaves to form yellow spots.
  • the fungus keeps growing when leaves fall off for as long as the weather stays moist – which, for fallen leaves, is all winter long.

Over the winter, the fungus forms capsules which contain many new spores that will burst open when temperatures rise again after winter, starting the cycle all over again.

Removing all leaves before winter ensures that most of the hibernating spores are destroyed.

Composting leaves infected with maple black spot also works because the spores will be buried and will die off before being exposed to air again. To maximize this, make sure you turn your compost pile several times.

Be especially vigilant if you have a Japanese garden – maples are a key feature, planted for their beauty!

Won’t composting or moving leaves spread tar spot disease?

This particular fungus only infects hosts of the Acer family, the maple tree family. Other plants aren’t infected.

  • Only burn or evacuate infected leaves if you wish to protect nearby maples.
  • If there are more maples in the neighborhood, coordinate with neighbors to destroy leaves from all infected trees every two or three years. This will keep the disease in check for the entire community!

Smart tip about black maple spot

It isn’t necessary to treat the tree with chemicals. Let’s avoid contaminating our planet.

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Credits for images shared to Nature & Garden (all edits by Gaspard Lorthiois):
Black spots on maple leaf by Petra Karrasch ★ under Pixabay license