You raked your lawn last weekend and you're going to have to rake it again this weekend. It's August.
The leaves coming down are covered in spots — black ones, or brown papery ones, or both — and the tree they came off looks thinner than it did in June. If you've got a Norway maple in the front yard, you are almost certainly seeing this right now, because half of Durham Region is.
This is the number one call we've taken this month, so let's go through it properly: what the spots actually are, which ones matter, and why the answer to almost all of it is "wait until the leaves are down and then rake."
Why This Year Is So Bad
Two things stacked on top of each other, and the order matters.
A wet spring. The fungi that cause leaf spot diseases overwinter in last year's fallen leaves. In spring, rain splashes and wind carry the spores up onto the tender new leaves that are just unfurling. That infection window is short and it is entirely weather-dependent — a dry spring means very little infection, and a wet one means a lot. Southern Ontario got a wet one.
Then heat and dry soil. The infections took hold in May and June, and then the trees spent July and August dealing with heat and low soil moisture on top of a compromised set of leaves. A tree carrying a heavy leaf-spot load in a dry August does the sensible thing: it drops the leaves that are costing it more water than they're producing in sugar.
So the dramatic part — the August leaf fall — is mostly the tree triaging, not the tree dying. The fungus made the leaves inefficient and the drought made the tree unwilling to carry them. Both had to happen.
The Three Suspects
Nearly everything I'm asked to look at right now is one of three things, and they're genuinely easy to tell apart once someone shows you the difference.
Tar Spot — the dramatic one that doesn't matter
Caused by fungi in the Rhytisma genus, and it is unmistakable once you know it: raised, glossy, coal-black spots with clearly defined edges, sitting on the leaf like a drop of tar that dried there. They often start as pale yellow-green blotches in early summer and blacken as the season goes on, sometimes with a yellow halo around the edge.
Tar spot looks like the worst thing you've ever seen on a tree. It is essentially cosmetic. It does not kill maples, it does not cause twig dieback, and a healthy tree carrying a heavy tar spot load will leaf out normally next spring. Norway maples get it worst, which is unfortunate, because Norway maples are everywhere in this region.
Anthracnose — the one worth paying attention to
Caused by several different fungi depending on host species, most often Kabatiella and Discula species on maples. The look is completely different from tar spot: irregular, papery, tan-to-brown dead patches, and critically, the damage tends to follow the leaf veins rather than sitting in tidy round spots. Leaves may curl or shrivel. The affected tissue is dead and dry — it's a lesion, not a deposit.
The reason anthracnose earns more attention than tar spot is that it doesn't necessarily stop at the leaves. Anthracnose can move into twigs and cause dieback, and it tends to be worst on the lower and interior branches where air moves least and leaves stay wet longest. One bad year on a mature, otherwise healthy tree is not a crisis. Several consecutive bad years, on a young tree, or on a tree already dealing with something else — that's a plant health care problem.
Powdery Mildew — the one that looks like dust
A white or grey powdery coating on the leaf surface, as though someone dusted the tree with flour. The giveaway is that it sits on top of the leaf. You can rub it off with your thumb, which you cannot do with tar spot or anthracnose.
It shows up in July and August when we get warm days and cool, humid nights, which is exactly the pattern we've had. It's also mostly cosmetic on established trees and shrubs. There's more detail in our post on powdery mildew, including why the mildew on your lilac isn't the same fungus as the one on your oak.
The Quick Field Test
Pick up five or six fallen leaves and look at them in decent light:
- Raised, shiny, jet-black, sharply defined, won't rub off — tar spot. Cosmetic.
- Flat, dull, tan or brown, irregular, papery, spreading along the veins — anthracnose. Worth monitoring.
- White or grey, dusty, rubs off with your thumb — powdery mildew. Cosmetic.
- Whole leaf uniformly scorched brown from the edges inward, no discrete spots — that's not a leaf spot disease at all. That's drought or root stress, and it's a different conversation.
That last one matters more than people expect. A meaningful share of the "spots on my leaves" calls I go out to turn out to be leaf scorch on a tree with a compacted or girdled root system, where the leaf symptoms are a downstream symptom of a root problem.
Why Spraying Right Now Does Nothing
This is the part that saves people money, so I'll be direct.
Fungicides for leaf spot diseases are protective, not curative. They work by putting a protective film on healthy tissue before spores arrive. They cannot undo an infection that's already established, and they cannot repair a dead lesion. The infection window for this year closed back in spring. Spraying a spotted tree in late August is spending money to accomplish nothing — the leaves are already infected and about to fall off anyway.
There's also a legal reality in Ontario that most homeowners have never been told about. Under Ontario Regulation 63/09, the cosmetic pesticide rules mean a licensed applicator can only use a conventional pesticide on your tree if a professional — an ISA Certified Arborist, a registered forester, or a qualified utility arborist — first provides a written opinion that the treatment is necessary to maintain the health of that tree. We're required to apply integrated pest management principles when we decide, and to refuse when the treatment isn't necessary.
For a purely cosmetic problem like tar spot, that opinion legitimately cannot be written. The only products available for cosmetic use are the low-toxicity ones — insecticidal soap, horticultural oil, copper spray and similar. So if someone quotes you a conventional fungicide program for tar spot on a healthy maple, they are either not going to do what they said, or they're operating outside the rules.
Anthracnose that's genuinely causing twig dieback on a valuable tree is a different assessment, and that's a case where the written-opinion path exists for a reason. But the honest answer for the large majority of what I'm seeing this August is that no spray is warranted.
What Actually Works
Every one of these fungi overwinters in the fallen leaves under the tree. That's the whole life cycle, and it's also the whole intervention.
Rake and remove the leaves — properly. Not raked into a corner of the yard. Off the property, into municipal yard waste. Home compost piles rarely run hot enough to reliably kill fungal spores, so a backyard pile of infected maple leaves is a spore reservoir sitting fifteen feet from the tree that made it. Do this in fall, when the leaves are actually down.
Accept that it's a neighbourhood problem. Here's the frustrating part, and I'd rather you hear it from me than discover it next June. If you do a perfect cleanup and the three houses around you leave their infected leaves down all winter, spores blow back onto your tree in spring. On streets with mature overlapping canopies — which describes a lot of Ajax and Whitby — this is genuinely a shared problem. It's worth an actual conversation with your neighbours, awkward as that sounds.
Water deeply through dry spells. The drought stress is doing more real damage to your tree this summer than the fungus is. Deep, infrequent watering out at the drip line beats frequent shallow sprinkling.
Mulch the root zone. A proper mulch ring — wide, a few inches deep, and not piled against the trunk — moderates soil temperature and moisture, which addresses the stress half of the equation.
Don't reach for fertilizer. Pushing a stressed tree into soft new growth in late summer is counterproductive. If the soil genuinely needs work, that's a decision to make from evidence, and root zone treatment in the right season is a different thing from dumping nitrogen on a struggling tree in August.
Consider thinning a dense crown. Anthracnose and powdery mildew both thrive where air doesn't move and leaves stay wet. Norway maples in particular grow dense enough to hold humidity inside the canopy all day. A proper crown thinning improves airflow and light penetration and genuinely lowers disease pressure the following year — and unlike a spray, it's a one-time investment that also improves the tree's structure.
When It Actually Is Serious
Leaf spot on a mature maple in a bad weather year is normal and the tree will be fine. Come find me if:
- The same tree has defoliated early three or more years running — repeated early defoliation drains the reserves a tree needs to push next spring's leaves
- You can see twig and branch dieback, not just leaf damage — bare twig tips, a thinning branch structure that persists into next season
- It's a young or recently planted tree, which has no reserves to spare
- The decline is lopsided — one side of the canopy failing while the other looks fine. That's not a leaf spot pattern at all, and it's the signature of something like Verticillium wilt
- It's an oak showing rapid whole-crown wilting rather than discrete leaf spots — worth reading what a sick oak in Ontario usually has
Those five are the difference between a cosmetic year and a tree health care problem. Everything else is a rake and a hose.
Not sure whether the spots on your maple are the harmless kind? Send us a photo or book a free assessment — we cover Ajax, Whitby, Pickering, and the rest of Durham Region, and we'll happily tell you when the answer is "your tree is fine, save your money."
