Rhytisma acerinum and Rhytisma punctatum, two causes of Tar Spot of maple

Tom Volk's Fungus of the Month for October 2007

by Heather Hallen Adams and Tom Volk

Please click TomVolkFungi.net for the rest of Tom Volk's pages on fungi

Rhytisma acerinum (green leaves) and R. punctatum (brown leaf), tar spot of maple

This month, we have two fungi for the price of one: Rhytisma acerinum and R. punctatum, the causal agents of tar spot of maple.

Both of these Rhytisma species form black spots on maple leaves late in the season (September and October until leaf fall is a good time to observe tar spot in the northern latitudes). R. acerinum forms comparatively few, large spots on a given leaf, while R. punctatum forms clusters of many, small ("punctate") spots. While the aesthetic value of spotted leaves may be open to dispute, neither tar spot organism causes serious damage to the maple tree.

In fact, tar spot is an example of a widespread ecological niche in fungi, the endophytes ("endo-" = "within", "-phyte" = "plant"). Nearly all plants are or can be hosts to endophytic fungi, which live quietly and unobtrusively within their chosen plant tissue (leaves, twigs, etc.) for the vast majority of the time. It's only when the plant material senesces that the fungi make their presence known with fruiting bodies bursting out of the host. Another example of an endophytic fungus is Claviceps purpurea.

Rhytisma acerinum, tar spot of mapleRhytisma species are members of the Ascomycota. Sterile fungal tissue - the stroma - forms within the maple leaf and the fungus eventually bursts out of, . In Rhytisma acerinum and most other Rhytisma species, multiple apothecia are embedded within this stroma, while Rhytisma punctatum forms much smaller stroma, each bearing a single apothecium. You may recall the term "apothecium" from Sarcoscypha coccinea, Scutellinia scutellata, morels, fairy cups, and other so-called cup fungi; if so, you remember that "apothecium" refers to an open, cup-shaped fruiting body in which the asci are exposed. Not quite the case in Rhytisma. Proper apothecia are formed within the stroma, but these are covered over by a layer of tissue with fissures that open up above the apothecia, allowing the ascospores to find their way into the world. In acknowledgement of this somewhat atypical apothecium, the fruiting structures of Rhytisma and related fungi in the Rhytismatales are sometimes give their own, special name: the hysterothecium. "Hystero-", from the Greek, means "womb," and refers in this case to the enclosed, protected nature of the fruiting body.

Rhytisma andromedae asciThe tar spots we see on our maple leaves at this time of year do not yet possess mature hysterothecia. Think about it: you life's goal is to find a nice, secure home in a maple leaf. If you produce your spores now and send them on their way, where will they settle? All the maple leaves are dead or dying. So, the fungi sit patiently on their fallen leaves throughout the winter. In the spring, when the maples are sending out fresh new leaves, the Rhytisma on last year's tattered, decayed leaves mature and shoot off their long, filamentous ascospores. While the majority of ascospores will not find themselves on a maple leaf, enough will to continue the life of the species. Looking at the maples in Michigan and Wisconsin, we clearly are not suffering for lack of Rhytisma acerinum and R. punctatum!

To the left you can see some mature asci of Rhytisma andromedae, a European species very closely related to R. acerinum. Notable in this slide are the asci containing needle-like ascospores, which stain red. Note another yellower ascus in the center with immature asci. These asci are still below the tar-like covering but will be shed as soon as the tar breaks up due to weather or outside microbial action.

Rhytisma punctatum, tar spot of maple closeupSome other Rhytisma species inhabit different plant hosts (e.g. R. ulmi on elm, and R. arbuti on certain Ericaceaous plants such as Arbutus and Rhododendron). Other members of the family Rhytismataceae are not quite as innocuous as R. acerinum and R. punctatum. Several of these cousins, such as Lophodermium and Cyclaneusma, cause needlecast diseases of conifers that are of particular concern to Christmas tree growers.

We hope you have enjoyed learning something about all those black blotches on your maple leaves. There are many interesting fungi you can find in your yard. Keep looking!

Heather Hallen Adams in HawaiiThis month's co-author is Heather Hallen Adams, my friend who is a mycologist at Michigan State University. Heather works mostly on Amanita toxins, as well as genomics in Amanita and Gibberella zeae / Fusarium graminearum. Heather has already co-authored FotM pages that fungus and on Gastrocybe lateritia.

If you have anything to add, or if you have corrections, comments, or recommendations for future FotM's (or maybe you'd like to be co-author of a FotM?), please write to me at my email address

This page and other pages are © Copyright 2007 by Thomas J. Volk, University of Wisconsin-La Crosse.

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