Tar Spot of Maple - Maple Tree Tar Spot Disease

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.

Copyright (C) 2008 J.E. Partridge, University of Nebraska-Lincoln. All Rights Reserved.