Speckled tar spot

Written by J.B. Tanney Revision 2025

French disease name: Tache goudronneuse ponctuée
Other disease names: Tar spot
Pathogen name: Rhytisma punctatum (Pers.) Fr.
Kingdom: Fungi
Phylum: Ascomycota
Class: Leotiomycetes
Order: Rhytismatales
Family: Rhytismataceae
Partial list of synonyms:
- Melasmia punctata Sacc. & Roum.
- Placuntium punctatum (Pers.) Ehrenb.
- Xyloma punctatum Pers.

General information and importance

Speckled tar spot is caused by the fungus Rhytisma punctatum, which is native to Canada. The main symptom of this disease is the formation of yellow spots with tar-like growths on the leaves of maple (Acer) trees. Speckled tar spot primarily affects the leaves of maple trees, causing aesthetic damage without significantly affecting the overall health of the tree. Speckled tar spot is not a serious health issue and is mainly a cosmetic concern in urban trees.

Distribution and hosts

In Canada, R. punctatum is endemic and commonly found wherever suitable hosts occur naturally or are planted as ornamental trees. Rhytisma punctatum occurs on the leaves of maple, including the native bigleaf maple (A. macrophyllum), Manitoba maple (A. negundo), striped maple (A. pensylvanicum), red maple (A. rubrum), silver maple (A. saccharinum), sugar maple (A. saccharum), and mountain maple (A. spicatum), and on the non-native sycamore maple (A. pseudoplatanus). In North America, it is also reported in the western, northern Midwest, northeastern, and northern South Atlantic United States on the species listed above, as well as on vine maple (A. circinatum), Douglas maple (A. glabrum), and bigtooth maple (A. grandidentatum). Worldwide, R. punctatum is reported from Asia (China, Japan, Korea, Taiwan) and Europe (Denmark, Germany, Sweden, Ukraine, and the United Kingdom).

Tree parts affected

Speckled tar spot infects leaves of various maples.

Symptoms and signs

Speckled tar spot symptoms first appear in early to mid-summer as light green to pale-yellow circular spots on infected leaves. As summer progresses, the leaf spots increase in size, and small (1–2-millimetre diameter), shiny, black, raised, tar-like structures (stromata) develop in groups of 20 to 30, giving rise to the common name “speckled tar spot.” The stromata contain individual sexual reproductive structures (hysterothecia) that mature on fallen leaves during the following spring. In humid or moist conditions, the hysterothecia open and forcibly eject colourless, filiform, aseptate ascospores that are surrounded by a gelatinous sheath and measure 30 to 36 micrometres × 1.5 to 2 micrometres. Ascospores are produced from eight-spored, clavate asci measuring 70 to 80 micrometres × 9 to 10 micrometres and are surrounded by filiform paraphyses. Asexual spores (conidia) are also produced by pycnidia within the stromata. The hyaline, allantoid conidia are 4 to 6 micrometres × 1 micrometre, and are produced by cylindrical, simple, or branched conidiophores.

Disease cycle

When new host leaves emerge during spring, ascospores of R. punctatum are forcibly ejected from hysterothecia that are embedded within stromata on overwintered leaves. Airborne ascospores land on, germinate, and invade host leaf cells both intercellularly and intracellularly. Leaf infections may be asymptomatic until mid- to late-summer when pale, chlorotic leaf spots begin to appear. Stromata develop within these expanding leaf spots and become evident as small, black, shiny, raised structures that resemble flecks of tar. Leaves naturally senesce in autumn, and the leaf spots may remain green for some time while the rest of the leaf tissue turns brown (the green island effect). The hysterothecia within the stromata overwinter and mature in the following spring. Asexual spores may function as spermatia rather than infective propagules.

Damage

Although severe R. punctatum infections may reduce photosynthetic capability of its host to some extent, speckled tar spot is primarily a cosmetic issue in ornamental and urban trees.

Prevention and management

Pest management strategies for a particular pest vary depending on several factors. These include:

the population level of the pest (i.e., how numerous the pest is on the affected host[s]);
the expected damage or other negative consequences of the pest’s activity and population level (either to the host, property, or the environment);
an understanding of the pest’s life cycle, its various life stages, and the various natural or abiotic agents that affect population levels;
how many individual host specimens are affected (an individual tree, small groups of trees, plantations, forests);
the value of the host(s) versus the costs of pest management approaches; and
consideration of the various silvicultural, mechanical, chemical, biological, and natural control approaches available and their various advantages and disadvantages.

Decisions about pest management strategies require information about each of these factors for informed decision-making. These various factors should then be weighed carefully in terms of costs and benefits before action is taken against any particular pest.

In urban settings, speckled tar spot can be reduced through basic sanitation measures such as raking and disposing of fallen leaves in autumn (e.g., off-site disposal or burning) to reduce inoculum sources.

Photos

Stromata of Rhytisma punctatum, the causal pathogen of speckled tar spot, on bigleaf maple.

CFS

Close-up of the distinctive “specked” tar spots of <em>Rhytisma punctatum </em>on an infected maple leaf.

Close-up of the distinctive “specked” tar spots of Rhytisma punctatum on an infected maple leaf.

Selected references

Allen, E.A; Morrison, D.J.; Wallis, G.W. 1996. Common tree diseases of British Columbia. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre. Victoria, British Columbia. 178 p. https://ostrnrcan-dostrncan.canada.ca/entities/publication/0ba1e470-4f14-41b2-a035-d27dd2e38372?fromSearchPage=true

Funk, A. 1985. Foliar fungi of western trees. Canadian Forestry Service, Pacific Forest Research Centre. Victoria, British Columbia. BC-X-265. 159 p. https://ostrnrcan-dostrncan.canada.ca/entities/publication/f31475f4-c426-46c2-9456-21025c2c419c?fromSearchPage=true

Hsiang, T.; Tian, X.L. 2007. Sporulation and identity of tar spot of maple in Canada. Acta Silvatica et Lignaria Hungarica, Special Edition: 71–74.

Renault, T.R.; Magasi, L.P.; Marks, D.B. 1975. Common pest problems of sugar maple in the Maritimes, with particular reference to sugar bushes. Department of the Environment, Canadian Forestry Service, Maritimes Forest Research Centre. Fredericton, New Brunswick. M-X-58. 75 p. https://ostrnrcan-dostrncan.canada.ca/entities/publication/30ce4cbc-593b-49d1-b4e4-c897222d9a3b?fromSearchPage=true

Wolfe, E.R.; Younginger, B.S.; LeRoy, C.J. 2019. Fungal endophyte-infected leaf litter alters in-stream microbial communities and negatively influences aquatic fungal sporulation. Oikos 128(3): 405–415. https://doi.org/10.1111/oik.05619

Woo, J.Y.; Partridge, A.D. 1969. The life history and cytology of Rhytisma punctatum on bigleaf maple. Mycologia 61(6): 1085–1095. https://doi.org/10.1080/00275514.1969.12018838

Cite this fact sheet

Tanney, J.B. 2025. Speckled tar spot. In J.P. Brandt, B.I. Daigle, J.-L. St-Germain, A.C. Skinner, B.C. Callan, and V.G. Nealis, editors. Trees, insects, mites, and diseases of Canada’s forests. Natural Resources Canada, Canadian Forest Service, Headquarters. Ottawa, Ontario.