Tar spot is a common fungal disease of maple trees. The fungus attacks maple leaves in the spring causing pale spots that continue to grow in size, turning yellow then eventually to raised black spots by fall. The black spots look like spots of tar on the leaves, and thus its name. There are several species of tar spot that affect maples, with the 3 most common species in North America being Rhytisma acerinum, Rhytisma americanum , and Rhytisma punctatum. Each species appears slightly different, occurs in different locals and have their preferred maple species hosts. I’ll go into a little more detail about these 3 species then switch over to a more general discussion about the disease.
The name acerinum comes from Latin and means “from Acers”. Acers being maples. It is a non-native species of North America and is often referred to as European tar spot.
Distinguishing Characteristics: are the tiny, black, raised, bumps that form in the first pale then yellowing spots on the leaves. These spots coalesce and form large, raised, black spots that measure up to about (4 cm (1 1/2″) in diameter. The disease is sometimes referred to as “giant tar spot”. Scientists have found this fungal species to be sensitive to air pollution containing high levels of Sulphur. (First Nature, (n.d.)).
Hosts: The main maple hosts are Norway maples, but many maple species may also be at risk such as Silver Maples, Field Maples, and Sycamore maples.
Geographical range: Rhytisma acerinum occurs mainly in the eastern half of the United States, the southern tips of Ontario and Quebec and extensively in Europe (iNaturalist.ca, (n.d)).
Distinguishing Characteristics: This species is native to North America and is pretty much confined to that region. The black spots that form from Rhytisma americanum differ from Rhytisma acerinum in that the spots are more highly raised and appear sculpted out with definite grooves and ridges, giving it a highly textured look. The spots are typically smaller than Rhytisma acerinum, although they can get quite large as well.
Hosts: silver maple, red maple, and sugar maple.
Geographical range: Mainly occurs in the eastern half of the United States and the very southern tips of Ontario and Quebec. (inaturalist.ca, (n.d.)).
The word punctatum is Latin and means “dotted”. Which well describes the appearance of this tar spot species.
Distinguishing Characteristics: This is another native of North America. It produces much smaller, black, pimple-like bumps that do not coalesce. Rather they form colonies of closely spaced bumps, each with only one perithecium (flask-shaped fruiting body). This species is often referred to as speckled tar spot. Scientists studying Rhytisma punctatum have found that leaves infected with Rhytisma punctatum tend to hold on to nitrogen and phosphorus (Liam S., (1993), rather than release it back to the tree in the fall, to be taken down to the roots in sap flow until spring for storage.
Hosts: mainly bigleaf maple, Manitoba maple, mountain maple, silver maple and striped maple, but can also infect red maples and sugar maples (Natural Resources Canada, (modified: 2015-08-04)).
Geographical range: is mainly the east and west coasts of the United States (with some scattered areas throughout U.S), the southern tips of British Columbia, Ontario, and Quebec as well as scattered throughout Europe (iNaturalist.ca, (n.d)).
Tar Spot General Information
Life Cycle of Rhytisma spp. Tar Spot
The fungus overwinters on fallen leaf debris. In the late spring the black coloured fungal fruiting bodies on these leaves mature and are forcibly ejected into the environment. The spores are carried on the wind, by wildlife, or by splashing water to susceptible maples. All of this begins taking place after the trees have fully put up their new canopies. The fungal spores that manage to land on their hosts leaves penetrate the leaves and slowly begin producing a mass of fungal tissue, called ‘stroma’, that causes chlorotic spots to begin appearing on leaf tissues. A layer of moisture on the leaf surface is required for infection to take place. New infections can continue taking place throughout the summer. Spore-bearing structures, called apothecia, form within these fungal masses and begin appearing as tiny, dark, raised spots within the chlorotic area. These fungal fruiting bodies continue their development over the summer and begin to coalesce forming what appears as large blackish, raised spots, usually with a yellow hallo, that become quite apparent by fall. An exception to this general description of life cycle is Rhytisma punctatum which only has one perithecium (flask-shaped fruiting body) rather than many, and thus appears as separate little spots. The cycle continues with the infected leaves dropping from the tree in the fall (often prematurely), where they overwinter and await the warmer moist weather to continue their development.
Management of the Disease
Damage caused by tar spot is considered mainly cosmetic. The overall health of the tree is usually not affected. The best way to manage this disease is to gather and dispose of infected leaves (do not put them in your home composter which does not usually get hot enough to kill the fungus). Any further action for homeowners is rarely warranted.
Other Plants That can be Infected by Tar Spot
You will most often find the term tar spots associated with maples because it commonly occurs there, but there are other types of plants that can also be infected with different species of tar spot, such as:
Rhytisma acerinum commonly infects sycamores.
Trabutia quercina infects oak trees.
Rhytisma salicinum infects willows.
Phyllachora maydis infect corn and other grasses. On corn it is quite destructive and quite an agricultural concern.
Coniothyrium ilicinum, Marcpderma curtisii, and Phytophthora ilicis infect hollies. Due to their evergreen nature tar spot can be quite destructive to hollies and cause extensive defoliation.
Rhytisma arbuti infects false azaleas, and Pacific madrone (Gibson I., (2020)).
Aulographina eucalypti infects eucalyptus.
Health Risk to Humans
During the course of my research for this article I stumbled upon an article by Dr. Iva Lloyd, ND on Naturopathic Foundation’s blog site. In the article she talks about tar spot and some of the symptom’s individuals with compromised immune systems or susceptibility to respiratory conditions might experience when exposed to tar spot. She went on to offer some recommendations for cleaning up leaves with tar spots so as to minimize their exposure to the airborne spores. If you suffer from respiratory problems, you can check out her article here Are your maple trees making you sick?
Photo Credits: all photos taken by the author.
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Kuo, Michael & Melissa Kuo (September 2017). Rhytisma americanum (tar spot of maple). Retrieved December 2, 2022, from the midwestnaturalist.com website: http://www.midwestnaturalist.com/rhytisma_americanum.html
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