Tar Spot of Maple

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Tar Spot of Maple

Tar spot is one of the most readily visible and easiest diseases of maple to diagnose. At the same time, it is one of the least damaging ailments on its host. Tar spot is a little like anthracnose. The same name applies to similar diseases on a number of woody plants but different species of the fungus infect hosts other than maple. The disease can be found in North America and Europe. It could easily be called "tar spots" since two species of fungi are involved, producing two types of symptoms. The causal organisms are Rhytisma acerinum Pers. ex. Fr. (anamorph: Melasmia acerina ) and Rhytisma punctatum Pers. ex Fr. (anamorph ). In the U.S. Rhytisma acerinum is generally distributed throughout the country in moist environments while Rhytisma punctatum occurs in the northern two-thirds of the country. Hosts of both fungi include silver, red, sugar, mountain, Rocky Mountain, big leaf and sycamore maple as well as box elder. Additional maple hosts of Rhytisma acerinum are Norway, Amur and hedge. Strains of this species occur and are differentiated on the basis of host preference. The other hosts of Rhytisma punctatum are striped and vine maple. The fungi over season as stroma and developing apothecia and asci on fallen leaves that remain moist. Ascospores mature in the spring and are forcibly ejected through splits in the stroma. They travel a short distance (1mm) on their own and are then windblown to young, expanding foliage. Ascospores germinate and invade leaves through stomata. Symptoms develop 1-2 months after infection. Fungal stroma form in the upper leaf epidermis in the summer. The stroma is bonded to host tissue by a black gummy substance it produces. Large stroma are produced by Rhytisma acerinum while Rhytisma punctatum produces numerous small stroma. The stroma become convoluted as they mature. Conidia that are not infectious are produced in fertile ridges of this structure during the summer. Apothecia develop in ridges in the autumn and mature in the spring. Asci begin developing in the winter and also mature in the spring, completing the life cycle. Outbreaks of tar spot are sporadic, occurring during wet years. The disease tends to be more common on trees growing in moist, sheltered locations. Tar spot caused by Rhytisma acerinum seldom occurs in urban or industrial areas because the fungus is sensitive to sulfur dioxide, a common pollutant. This compound does not affect established infection sites but prevents the development of new ones.

Symptoms

Initial symptoms of tar spot consist of light green or yellowish green spots on foliage. Black, shiny, tar-like stroma develop in these spots. Rhytisma acerinum usually produces a few large spots. In some cases small stroma coalesce to form a single large one. Tissue on the lower leaf surface associated with the stroma turns brown. Rhytisma punctatum symptoms are sometimes referred to as speckled tar spot. Numerous small stroma are produced in yellowish green spots. They do not coalesce. Leaf tissue on the underside of these lesions remains yellow. Convolutions develop in the stroma produced by both species. The infected area is sometimes called wrinkled scab once the ridges become visible. Stroma retain their dark glossy appearance when leaves senesce and change color in the fall. Tissue infected by Rhytisma punctatum may also retain chlorophyll longer than healthy tissue. Heavy infection may lead to premature defoliation but it is not usually extensive enough to affect the health of the tree.

Plant Health Management

Sanitation.

Rake and destroy infected leaves in the fall.

Fungicide Application.

References

Hepting, G.H. 1971. Diseases of forest and shade trees of the United States. U.S. Dep. Agric. Agric. Handbk. 386. 658 pp.

Maple diseases and their control.1972. U.S. Dept. of Agric. Home and Garden Bull. No. 81. 8 pp.

Riffle, J.W., and Peterson, G.W., tech. coords. 1986. Diseases of trees in the Great Plains. U.S. For. Serv. Gen. Tech. Rep. RM-129. 149 pp.

Sinclair, W.A., Lyon, H.H., and Johnson, W.T. 1987. Diseases of trees and shrubs. Cornell University Press. Ithaca, NY. 574 pp.

Useful Links

Ohio State University

Caveat

This description is presented for information only and no endorsement is intended for products listed, nor criticism meant for products not mentioned. Always consult the product label before purchasing and using any pesticide.

Material contained on the Links from the page are the responsibility of the linked page's author(s).

This page was researched and drafted by: Jane Christensen, Department of Plant Pathology, University of Nebraska-Lincoln

Disease images were provided by: Dr. David Wysong, Department of Plant Pathology, University of Nebraska-Lincoln

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This page is authored and maintained by:
Dr. J.E. Partridge, Department of Plant Pathology, University of Nebraska-Lincoln

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