Maple Tree Tar Spot – Managing Tar Spot Of Maples

Your maple trees are absolutely gorgeous yellow, orange and red fireballs every fall – and you look forward to it with a great deal of anticipation. When you discover that your tree is suffering from tar spot of maples, you may start to fear that it spells the end to beautiful fall scenery forever. Never fear, maple tree tar spot is a very minor disease of maple trees and you’ll have plenty of fiery falls to come.

What is Maple Tar Spot Disease?

Maple tar spot is a very visible problem for maple trees. It starts with small yellow spots on growing leaves, and by late summer these yellow spots expand into large black blotches that look like tar has been dropped on the leaves. This is because a fungal pathogen in the genus Rhytisma has taken hold.

When the fungus initially infects a leaf, it causes a small 1/8-inch (1/3 cm.) wide yellow spot. As the season progresses, that spot spreads, eventually growing up to 3/4 (2 cm.) inches wide. The spreading yellow spot also changes colors as it grows, slowly turning from a yellow-green to a deep, tarry black.

The tar spots don’t emerge right away, but are typically obvious by mid to late summer. By the end of September, those black spots are at full size and may even appear to be rippled or deeply grooved like fingerprints. Don’t worry, though; the fungus only attacks the leaves, leaving the rest of your maple tree alone.

The black spots are fairly unsightly, but they don’t do any harm to your trees and will be shed when the leaves fall. Unfortunately, maple tree tar spot is spread on the wind, which means that your tree can get reinfected next year if spores happen to hitch a ride on the right breeze.

Maple Tar Spot Treatment

Because of the way maple tar spot disease is transmitted, complete control of maple tar spot is virtually impossible on mature trees. Prevention is the key with this disease, but if nearby trees are infected, you can’t reasonably expect to totally destroy this fungus without community support.

Start by raking all your maple’s fallen leaves and burning, bagging or composting them to eliminate the closest source of tar spot spores. If you leave the fallen leaves on the ground until spring, the spores on them will likely reinfect the new foliage and start the cycle again. Trees that have trouble with tar spots year after year may also be struggling with excessive moisture. You’ll do them a great favor if you increase the grade around them to eliminate standing water and prevent moisture build-up.

Young trees may require treatment, especially if other trees have had a lot of their leaf surfaces covered by tar spots in the recent past. If you’re planting a younger maple in an area prone to maple tar spot, though, applying a fungicide, like triadimefon and mancozeb, at bud break and twice again in 7- to 14-day intervals is recommended. Once your tree is well-established and too tall to easily spray, it should be able to fend for itself.

Every year about this time, I start receiving questions on the ugly black spots that homeowners are seeing on their maple trees. These spots are caused by a fungal disease aptly called Tar Spot. In many cases, these leaves with the black spots are dropping early, which adds to the concern. The black spots that look like big drops of tar look ugly, but the disease doesn’t hurt the tree or affect it’s over all health.
There are three different, but related fungi. All three produce the similar symptoms and can affect almost all species of maples, but mainly occur on Norway, silver and red maples.

How Does Maple Leave Tar Spot Develop?

The disease overwinters on the infected leaves that fall on the ground. As the new leaves begin to open in the spring, the spores on the fallen leaves begin to ripen, split open and are sent by wind currents into the atmosphere. The needle-like spores will land on the new leaves where they may germinate and start thee infection process.

The first symptoms are seen in the early summer as small yellow spots less than 1/8 of an inch in diameter. As the season progresses, a black spot develops within the yellow tissue and enlarges in size and thickness. By the end of August, the spots closely resemble a drop of tar. The spots may also show a pattern of wavy indentations or ripples.

Getting Rid of Maple Leave Tar Spot

Current research has shown that the black tar spot fungus does not cause any long-term damage to the tree. Universities recommend that infected leaves be raked up and removed from the lawn area to lessen the chance of reinfection to the tree.

I am a big advocate of mulching your leaves back into your lawn to add organic matter back into the soil. If Tar Spot is a problem in your trees, you can still mulch the leaves, but not back onto the lawn. They should be collected and place into a mulch pile or bin and then covered so that the leaves don’t produce spores the following spring.

The biggest problem with this recommendation is there are dozens of maples trees in most neighborhoods. Unless everyone is collecting the leaves off their trees, the spores will still be available to spread from lawn to lawn and from maple tree to maple tree. Even if you keep your lawn free of infected leaves, the chance of a tree from “down the block” infecting your tree is always present.

Treatment for Maple Leave Tar Spot

There are fungicides labelled to control Tar Spot on maple trees, but thorough coverage of all leaf surfaces is critical. These may be easy to do with a small tree, but when the tree reaches 20 feet or more, this is almost impossible, even for professional tree care companies.

Any fungicide that is applied only protects a leaf for about 14 to 28 days and then it must be applied again. If the leaves from trees that were infected the previous year are not raked up, the source of infection is still present. It is best to learn to live with a few leaves that have “dollops of tar” on them. There is not much you can do to correct the problem, so learn to live with it.

If you think your tree may have Tar Spot or if any of your landscape plants are showing signs of disease or inset activity, contact your neighborhood lawn care professional at Spring-Green.

What Are These Black Spots On The Leaves?

Walking to my garage this morning I happened to notice these yucky black spots on the fallen leaves from my maple tree.

What are they?

Is it the tree diseased?

Will it spread to the other trees around it?

It looks really gross.

According to Chicago Botanic, these spots are a fungus called “maple leaf tar spot, known scientifically as species of Rhytisma fungi.”

Will it hurt my tree?

Gardening Know How says that “the fungus only attacks the leaves, leaving the rest of your maple tree alone.”

Whew! That’s a relief.

What can be done about it?

Because the fungus is spread by the wind blowing the spores around, totally eliminating it is impossible. Well that stinks.

However, the article says to prevent it from spreading I can start by raking up the leaves and either burning them or bagging them up for compost.

Also, it stated that your trees suffer from this every year, they are quite possibly in an area of heavy moisture.

A way to combat it is to grade the area to get rid of extra moisture and standing water.

Hmm… we have had a lot of rain this past spring and basically all summer so I’m wondering if this is the culprit?

I’m glad to know it isn’t going to kill my beautiful maple tree. I love its shade in the spring and summer and of course its colors in the fall. I would hate to loose it from my yard.

If you see this on your trees leaves, now you know what it is and what to do about it.

As they say “the more you know, the more you grow.” Knowledge is Power.

Ok. Enough of my after school PSA, continue on with your day.

Your path towards ecological understanding

In 2007, Tom Hsiang and Xiuling Lynn Tian published a paper that closely looked into the genomes of the fungi associated with the different tree species. Together they extracted and sequenced DNA from Rhytisma species located on native and introduced maples. They found that the species growing on Norway maples was Rhytisma acerinum, a tar spot that evolved in Europe. On the contrary, the fungus growing on native maple trees were identified as R. americanum and R. punctatum, parasites that evolved in North America. So, I could finally say with confidence that these tar spots that I’ve taken several pictures of are indeed Rhytisma acerinum.

Although I mentioned earlier that this fungus is parasitic, it is only weakly parasitic. If this fungus reduced photosynthetic output during the peak of plant growing season, then it would have a greater effect on trees, and be considered a stronger parasite. However, R. acerinum only begins to negatively affect leaves at the end of the growing season, when photosynthesis is beginning to slow. It’s at this time in which trees stop producing costly substances that safeguard against parasites and pathogens, that the fungus takes full advantage. Because of the warm autumn we are having, trees like my parent’s maple are holding on to their leaves longer, and are still producing compounds that reduce parasitism. For this reason, leaves are still green with chlorophyll, and there are very few patches of tar spot fungus, even this late in the season.

Tar Spot on maples is an occasional tree disease that can cause concern for customers who have maple trees on their property, especially Norway, Red and Silver Maples. The black spots look like someone spilled tar in big drops on the leaves and can cause the leaves to fall early in August or September.

Tar Spot begins in the spring or early summer and develops light green to yellow green spots on the upper leaf surface. During the summer, black, tar-like structures form within the yellow spots. The spots can range in size from a quarter inch to 1 inch in diameter.

The disease can cause early leaf drop, but it has little detrimental effect on the trees. The best thing to do is to rake and clear away the diseased leaves to remove the source of infection for the following year. This may be the most difficult aspect when it comes to dealing with this disease. Usually, I recommend that people mulch their leaves in the fall to reincorporate the organic material back into the soil. If your tree has a problem with Tar Spot, especially if it occurs year after year, raking and disposing of the leaves will help reduce the incidence of the disease.

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