It looks life-threatening. Ugly black spots are showing on our fallen maple leaves. They’re called tar spots and they are one diabolical tree fungus.
First thing, don’t panic. Tar spots only damage leaves and not the tree itself. Once they are collected, the fungus disappears on the tree.
Here’s the problem: It will return in springtime if the infected leaves stay near the trees. The fungi in May reactivate and infect any maples (except Japanese) it touches.
Now’s the time to be vigilant about eradicating this disease. The worst you can do is to not rake the leaves. The fungi will fly on the wind, attaching to any maples nearby.
So rake the leaves, bag them and set out on the street for collection. Hopefully, they’ll be going to a landfill and not a compost station. The virus may remain active in compost.
You probably won’t see many results next spring, but in two to three years of removing the leaves and virus, you should win the battle.
Our spring weather will influence the fungi. It loves long periods of rain, fog and mist. Long stretches of hot, dry weather stops the disease. Maples planted close together in continual shade are the most susceptible.
The disease begins with small, yellow spots on leaves. By summer, two or more large, black spots appear on each leaf. This interrupts the circulatory system and causes browning. The leaves often prematurely fall.
The fungus is hard to kill. Multiple applications of a fungicide must begin in early May before the yellow spots appear.
The best practice is to regularly rake the killed leaves and dispose of them. Do not chip for mulch.
You can compost them, but be careful. Bury infected leaves with other materials such as grass clippings and soil. Be sure to dig the compost into the soil when you spread it.
Remember: Tar spots are cosmetic and will not harm the tree. The most effective weapon against them is your leaf rake.