Black spots on maple leaves, treatment

Blotches as black as pitch spread across the topside of maple leaves, making the whole tree look diseased. This is tar spot (Rhytisma acerinum), a fungus that never enters the wood and rarely harms the tree; gather up the fallen leaves before winter and the spores that would start the cycle again go with them.

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 (including all Acer species) against it.

Also related:

• Black spot disease on other plants

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).

• Rake up fallen maple leaves infected with tar spot or anthracnose and compost or remove them rather than leaving them as mulch under the tree, since this reduces overwintering fungal spores.
• Rake again once the maple has finished dropping its leaves, since removing fallen leaves is the standard way to reduce the tar spot fungus that overwinters in leaf litter.
• Running the mower over fallen leaves before composting shreds them into smaller pieces, increasing the surface area available to microbes and thereby speeding up their breakdown in the heap.
• Check your council’s rules on garden bonfires before burning, since many areas restrict or forbid them.

What causes maple leaf black spot?

A fungus called Rhytisma acerinum is responsible for this. It’s common and goes 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 and then black spots.
• the fungus keeps growing even as 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. These 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!

Ways to control maple tar spot, compared

Tar spot is managed by breaking the fungus life cycle rather than by curing infected leaves. Here is how the options described below compare.

In most gardens, raking and destroying the fallen leaves is enough, and spraying is reserved for cases where that has failed.

Can I treat black spots on leaves with anything?

The leaves themselves, once contaminated, won’t heal. In special cases where removing leaves hasn’t lessened the infection in the following year, it’s worth looking into other options to control the spores.

One of these is a semi-natural option that is used for fungus control in orchards, vineyards and in the vegetable patch: bordeaux mix. This mix contains copper sulfate in minute amounts, a metallic compound that interferes with fungus cells like spores and destroys them.

Healthy plants aren’t affected unless too much product is used, or if it’s used repeatedly on the same plot for years on end. It’s important to follow dosage requirements, and to look for even more natural alternatives whenever possible.

Applying Bordeaux mix to control maple tar spot disease

When maple leaves first show symptoms of tar spot:

• Spray a first dose of Bordeaux mix on the entire tree to hopefully neutralize any spores that haven’t yet opened.
• Wait until fall (Autumn) or until all the leaves of the tree have fallen off to spray a second dose. Rake fallen leaves beforehand.
• In Spring, just as the very first leaf buds appear, spray a third dose on all the branches and bark to eliminate spores that may have overwintered.

The first dose simply helps limit the spread of the disease. It won’t cure the tree immediately, but that way there will be less damage. Removing contaminated leaves from the ground before spraying the second dose will help focus the effect on what’s left. And the third dose aims to clear the tree before leaves appear.

Read also:

• Black spots, but not on maple? Treating black spot disease on leaves
• Organic ways of dealing with fungus in the garden

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