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 (including all Acer species) against it.
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 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!
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.
- Black spots, but not on maple? Treating black spot disease on leaves
- Organic ways of dealing with fungus in the garden
Smart tip about black maple spot
It isn’t necessary to treat the tree with chemicals. Let’s avoid contaminating our planet.
Is there a way to treat the spores when they’re airborne or a way to treat the leaves in the spring?
I have a huge maple in my backyard but even though I get rid of every single leaf in the fall, the black spots come back in July the following year. It’s not aesthetically pleasing but as long as it doesn’t hurt the tree I don’t really mind… EXCEPT that they fall off so early. Mid summer I’m raking every day just so I can see the grass and make my yard look nice for summer activities. And I always have to pick them out of my garden which can be difficult given the fencing & it makes me not want to get a pool or hot tub cuz it will just be filled with leaves. It’s just really annoying having to rake all the time & getting rid of fallen leaves has not helped prevent their return. None of my neighbors have the black spots and neither does my other maple tree which is only 10 feet away.
Hi Shannon, a solution that might help control the tar spots in your case is Bordeaux mix. It’s a fungicide typically sprayed on trees and plants before infections occur. I reworked the article to explain how to use it in this situation. Hope it helps!
Could mulch around the tree cause this leaf disease?
Hello Irina, that’s a good question. Most mulches won’t “cause” the problem. Bark mulch, specific plant mulches, rock mulch, etc are rarely infected. In itself, the fact of mulching won’t create the disease, nor will it worsen it if the disease is already there. Quite the opposite: if you mulch a tree that’s already infected with a thick layer of mulch, you’ll actually be burying the spores from this fungus deep enough that it won’t be able to emerge and spread. Scavenger critters like thrips, worms, and other bugs will destroy the spores when buried underground.
The only case where there’s a risk is if you’re using dead leaf mulch which comes from trees that are contaminated themselves. That’s why it’s often best to burn contaminated leaves.
So, I was reading this with a friend, and they brought up how you’re supposed to collect the fallen leaves. They mentioned- because they were confused, and now I am too. “What if you just cleared the tree of all its leaves in the summertime, and burned ’em all then?” I am also starting to think on this. . . what if you DID manage to clear the tree of is leaves and burn them? Would the spots come back? (I consider this tree my child as I am the only one stopping my father from uprooting her. I still want her, and I want her to be healthy!)
Hi Alex, it’s a good point. It would definitely clear most of the infestation, admitting it were feasible on larger trees, but it wouldn’t be absolute. The reason is that spores from this fungus not only land on leaves, but on branches and twigs as well. Every time it rains, drops dislodge the spores and splash them around, onto fresh leaves if they’d had the time to grow back.
It’s possible to remove all leaves from a tree once a year without much risk (it’s called defoliation). Depending on the season it would try so send out new leaves immediately (spring-summer) or wait for the next season (fall). Doing it more often starts getting risky, since a tree has limited reserves to draw from. However, you can, after the first massive defoliation, selectively remove up to half the leaves again the moment you notice they’re infected.
More effective would be to spray bordeaux mixture (copper-based) just as buds are opening and then again 6 and 12 weeks later. A totally natural option is one of the various fermented teas.
Sadly, this fungus is very volatile and travels easily, so there’s always a risk of re-contamination. All in all, simply raking the leaves up and discarding them reduces infections by nearly 80-90%, so most people do that and consider the few spots as a concession to biodiversity… After all, who knows what cure might someday arise from this pitch-black fungus?!
So if I understand you correctly, gathering all leaves that fall from the tree & destroying them away from the tree itself will prevent future tar spots on the tree?
Thus you are saying that the fungus actually reproduces in the fallen leaves that stay at the base or around the tree over winter like a breeding ground that eventually gets to its root system. Or does the fungus crawl or grow & spread up its trunk?
Dear Elaine, those are great questions and it’s a good opportunity to look into how these fungus spread. The particular types of black spot fungus that make tar-like spots on leaves expel air-borne spores when temperatures rise in spring. Since they overwinter on the fallen leaves that they’ve contaminated, that’s why burning them is so effective. Theoretically, tiny amounts of spores grow and develop on bark, but that usually isn’t enough to trigger a large-scale infection. This type of fungus doesn’t grow a root system in the ground or soil.