Master Gardener: Tar spot likes to target maples

Are you seeing spots on your maple tree leaves?

It may be tar spot, a fungal disease frequently found on maple trees.

It is easily identifiable as the large, roundish black spots look like someone came along and splashed tar on the leaves. While the first symptoms usually show up in mid-June, people generally don’t notice the disease until late summer, when leaves may start to fall prematurely.

Tar spot is caused by several different fungi in the genus Rhytisma. Symptoms develop one to two months after infection.

The first symptoms are tiny, pale yellow spots which appear in spring or early summer. The yellow spots grow to an inch or more over the summer.

On red and silver maple, a black spot usually develops in each yellow spot by mid-July to early August. By late summer the black spot has developed into the noticeable tar spot.

Beneath the black spot the leaf surface turns brown. Individual leaves can have multiple tar spots.

While tar spot is most commonly seen on silver, red and Norway maples, it can also occur on other varieties of maple. Maple seeds may also become infected by the fungus.

A different form of tar spot, sometimes called “speckled tar spot”, can affect striped and Norway maples. Twenty to 50 small spots appear on leaves in late July or early August.

On striped maple, the spots do not get much bigger, but on Norway maple the spots grow and eventually join to form a large black area. Boxelder, tulip tree, willow and holly can also get tar spot caused by a Rhytisma fungi.

Tar spot fungi overwinter on fallen infected leaves. They produce spores the following spring.

As trees start to leaf out, the needlelike spores are discharged from the black spots. Spores are then dispersed by the wind.

If spores land on young maple leaves they can germinate, enter the leaf tissue and the disease starts over.

Moisture is necessary for the spores to infect the leaves. Outbreaks of tar spot are more severe in years when we have a wet May and June.

Tar spot is unlikely to damage a tree, but a heavy infection can cause leaves to fall off early, which can be alarming to the homeowner. Fortunately, this usually happens late enough in the season so that the growth of the tree is not affected.

Keeping your maple trees healthy will help minimize the impact tar spot can have on a tree and help it to withstand losing its leaves.

Cultural methods such as mulching, proper watering and fertilizing help. Trees need an inch of water per week, so watering may be necessary during dry periods.

If tar spot is a recurring problem the tree may need some pruning to thin out the canopy. This will help promote good air circulation which can help reduce the disease severity.

If your maple tree has tar spot the best thing to do is to rake up the leaves and destroy them or get rid of them. Encourage your neighbors with maples to do the same.

Good sanitation will reduce the number of overwintering tar spots which produce the spores. Fewer spores next spring means less chance trees will become infected.

Leaves can be mulched as this will destroy many of the spots before they mature. An active, hot (140-degree F) compost pile should break down the leaves and the fungi.

To be on the safe side, in the spring cover or turn the mulch pile before new leaves begin to appear.

Since tar spot is mainly an aesthetic issue using a fungicide is usually not recommended. Leaves need to be completely covered with the fungicide and this can be hard to do on large maples.

If your neighbors do not treat their trees with fungicides or clean up their leaves, spraying your trees will be a waste of time and money. As the spores are wind born, they can easily travel throughout the neighborhood.

If you decide to use fungicides, you may need to spray almost every year depending on the spring weather conditions.

However, in the case of younger, smaller trees, they may need some fungicide protection to prevent defoliation which would be detrimental to the tree. Available fungicides are preventive, not curative, and must be applied before infection occurs.

Fungicide application should begin at bud break to protect the new growth, a second application would be needed when leaves are half expanded and a third once leaves are fully expanded. During cool, wet weather you may need to reapply frequently so follow the fungicide directions carefully.

If you do use a fungicide be sure to read and follow the label directions as well as the safety precautions. Select a fungicide that is labeled for tar spot and the plant that you are using it on (i.e., maple).

Fortunately, this is one tree disease that homeowners don’t have to worry over as it is not considered to be detrimental to the life of the tree.

Master Gardeners

Have a gardening question? The Master Gardener office is open.

Please wear a mask when visiting the CCE office and check in at the reception window.

Master Gardener volunteers are normally in the office from 10 a.m. to noon, Monday through Friday. You can stop in at the CCE office on 420 East Main St. in Batavia, call (585) 343-3040, ext. 127, or e-mail them at: [email protected]

Visit our CCE web site at genesee.cce.cornell.edu or like us on Facebook.

Next Garden Talk

Garden Talk on Sept. 3 will be held on Zoom and will start at noon. The topic will be “Pet Friendly Plants”

There is nothing nicer than the scent of flowers in our garden and home, but many plants can be dangerous for our cats and dogs. Learn what is safe for our furry companions, as well as what to avoid for their safety and well-being.

Those interested may register at www.genesee.cce.cornell.edu/.

Latest posts