Black raised spots on Maple leaves are most commonly the result of a fungal disease known as Tar Spot (Rhytisma acerinum).
The issue is largely cosmetic as Tar Spot doesn’t have a long-term effect on the trees’ health with affected leaves.
Unfortunately, because fungal spores are carried on the wind, it is virtually impossible to completely control and prevent Tar Spot in Maples.
Treatment of the condition is limited to younger Maple trees, but there are a number of preventative measures you can take to try to minimize the chance of re-infection the following year, including the removal and disposal of the fallen leaves from the ground beneath an afflicted tree.
Black spots on Maple leaves
Maple leaves with black spots have been infected by the fungal pathogen Rhytisma, or ‘Tar Spot’. The fungal spores, which are carried from tree to tree with the wind, do not affect the health of a tree. The best way to prevent Tar Spot from returning is to rake up the fallen leaves and dispose of them.
How to Identify Tar Spot on Maple leaves
It is fairly easy to identify Tar Spot on Maple trees, and most people will be familiar with the black splotches that result from the fungal infection.
Tar Spots can affect Norway Maple, Silver Maple, Box Elder Maple, Oregon Maple, Red Maple, Sugar Maple, Japanese Maple, Freeman’s Maple, and Sycamore trees.
The first signs of infection generally begin to show as minuscule light green dots on leaves in early to mid-summer.
They then transform into yellow dots with a diameter of approximately 1/3 cm in mid to late summer before growing to become larger tar-like black circles of up to 2 cm in diameter in the autumn.
The black spots may also become slightly raised and can also show up on maple seeds.
Trees affected by Tar Spot will often drop their leaves prematurely, and the leaves will be marked with tar-like splotches of black, often surrounded by a thin yellow ring.
In some cases, the leaves of an affected tree will also turn yellow prematurely.
Causes of Black Spots on Maple leaves
Fungal rust spots on leaves develop when fungal spores are carried from one infected tree to another through air currents or a shared water source.
The fungus thrives in a moist environment and is more likely to take hold if the area you live in a rainy area, or if you have had a particularly rainy summer and autumn.
How to Treat Black Spots on Maple Leaves
It is difficult to treat Tar Spot in a mature Maple and your time is better spent working on preventing the condition from returning the following year.
If your Maple trees are still young and are developing Tar Spots, or if you know that you are growing maples in a Tar-Spot-prone area, it may be worth applying a fungicide when the buds begin to break open.
You can then continue to apply the solution weekly for 3 weeks.
However, because Tar Spot doesn’t affect the inside of the tree and is not life-threatening. Once a tree is too large to spray, there’s barely anything you can do to treat the condition.
How to Prevent Black Spots on Maple Leaves
If you are really serious about preventing the return of Maple Tar Spot and live in a particularly Tar-Spot-prone area, you can hire a professional arborist to come and spray your trees with a fungicide.
This is quite the job! The arborist will need to make sure they have applied the anti-fungal solution to every single leaf of the affected tree, something that can be very labor-intensive on a larger Maple.
Usually, because black spots of Maple leaves are almost entirely a cosmetic issue, there is no need to go to the trouble of using a fungicide on a mature tree.
If you are looking to deal with the problem yourself, you can rake and dispose of the leaves as soon as they fall from an afflicted tree in the late summer and throughout the autumn.
If Tar Spot is something that affects your whole area, it is worth attempting a community approach and getting others involved.
If everyone in your neighborhood pledges to rake up and dispose of the leaves from the affected trees on their property, you stand a better chance of eliminating or at least minimizing the problem.
If the tree is still small and is in an area with poor soil drainage, you might consider transplanting it to a location with better water drainage.
Frequently Asked Questions about Black Spots on Maple Leaves
Can Maple Tar Spot spread to other trees?
The Rhytisma acerinum fungus only affects trees from the Maple tree family, so it will not spread to other types of trees in the surrounding area. However, it can spread to other trees within the Maple family, also known as the Acer family.
What should I do with my Tar-Spot-affected leaves once I have raked them up?
The quickest way to dispose of your leaves is to burn them or to evacuate them from the area. Do not leave them lying around nearby, as the spores may be spread by the wind and take hold on a nearby tree. If you decide to compost your leaves, make sure you have a composting system in place that reaches temperatures high enough to decompose the fungal spores–at least 140 degrees Fahrenheit (60 degrees Celcius).
Does Maple Tar Spot have any effect on tree growth?
Maple Tar Spot does not have any real effect on tree growth. Because it occurs so late in the growing season, the tree will already have formed next year’s buds by the time it loses the afflicted leaves. Make sure you are taking optimal care of your Maple in order to ensure that it is as unaffected as possible by Tar Spot.
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