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.
Also related:
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.
Read also:
- Black spots, but not on maple? Treating black spot disease on leaves
- Organic ways of dealing with fungus in the garden
Credits for images shared to Nature & Garden (all edits by Gaspard Lorthiois):
Black spots on maple leaf by Petra Karrasch ★ under Pixabay license
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.