''Rhytisma acerinum'' is a

plant pathogen Plant pathology (also phytopathology) is the scientific study of diseases in plants caused by pathogens (infectious organisms) and environmental conditions (physiological factors). Organisms that cause infectious disease include fungi, oomyc ...

that commonly affects

sycamore Sycamore is a name which has been applied to several types of trees, but with somewhat similar leaf forms. The name derives from the ancient Greek ' (''sūkomoros'') meaning "fig-mulberry". Species of trees known as sycamore: * ''Acer pseudoplata ...

s and

maple ''Acer'' () is a genus of trees and shrubs commonly known as maples. The genus is placed in the family Sapindaceae.Stevens, P. F. (2001 onwards). Angiosperm Phylogeny Website. Version 9, June 2008 nd more or less continuously updated since http ...

s in late summer and autumn, causing tar spot. Tar spot does not usually have an adverse effect on the trees' long-term health. ''R. acerinum'' is an

Ascomycete Ascomycota is a phylum of the kingdom Fungi that, together with the Basidiomycota, forms the subkingdom Dikarya. Its members are commonly known as the sac fungi or ascomycetes. It is the largest phylum of Fungi, with over 64,000 species. The defi ...

fungus that locally infects the leaves of trees and is a

biotrophic Symbiosis (from Ancient Greek, Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different Organism, biological organisms, be it Mutualism (biolog ...

parasite.Roland W.S. Weber and John Webster (2002). Teaching techniques for mycology: 18. ''Rhytisma acerinum'', cause of tar-spot disease of sycamore leaves. Mycologist, 16, pp 120-123 The disease is cosmetic and is therefore usually controlled only with sanitation methods.Hudelson, Brian. Wisconsin Horticulture UW-Extension Cooperative Extension. UW-Madison Plant Pathology, April 25, 2004. Web. Oct. 22, 2011.

Symptoms and signs

In late spring, chlorotic spots appear on tree leaves. These develop into brown-black lesions, retaining the yellow border. The lesions continue to grow, and by the end of summer form leaf spots that look like tar. The spot can grow up to in diameter. A microscopic sign of the pathogen are the stroma, mats of hyphae found in the lesions. These lesions can cause senescence of leaves but are mostly of cosmetic importance.

Affected species

''Rhytisma acerinum'' can occur in many tree species, with the most commonly affected genus being '' Acer''. The species most commonly affected by the disease are Norway maple (''

Acer platanoides ''Acer platanoides'', commonly known as the Norway maple, is a species of maple native to eastern and central Europe and western Asia, from Spain east to Russia, north to southern Scandinavia and southeast to northern Iran. It was introduced to ...

''), silver maple (''

Acer saccharinum ''Acer saccharinum'', commonly known as silver maple, creek maple, silverleaf maple, soft maple, large maple, water maple, swamp maple, or white maple, is a species of maple native to the eastern and central United States and southeastern Canad ...

''), and sycamore maple (''

Acer pseudoplatanus ''Acer pseudoplatanus'', known as the sycamore in the British Isles and as the sycamore maple in the United States, is a species of flowering plant in the Sapindus, soapberry and lychee family Sapindaceae. It is a large deciduous, broad-leaved ...

'').Hsiang Y and XL Tian. Sporulation and identity of tar spot of maple in Canada. ''Acta Silv. Lign. Hung., Spec. Edition '' (2007): 71-74.

Disease cycle

''Rhytisma acerinum'' is the

teleomorph In mycology, the terms teleomorph, anamorph, and holomorph apply to portions of the life cycles of fungi in the phyla Ascomycota and Basidiomycota: *Teleomorph: the sexual reproductive stage (morph), typically a fruiting body. *Anamorph: an ase ...

of tar spot. In the spring, needle-shaped

ascospores An ascus (; ) is the sexual spore-bearing cell produced in ascomycete fungi. Each ascus usually contains eight ascospores (or octad), produced by meiosis followed, in most species, by a mitotic cell division. However, asci in some genera or ...

are released from overwintering

apothecia An ascocarp, or ascoma (), is the fruiting body ( sporocarp) of an ascomycete phylum fungus. It consists of very tightly interwoven hyphae and millions of embedded asci, each of which typically contains four to eight ascospores. Ascocarps are mo ...

in fallen leaf debris.Hsiang, T, LX Tian, and C Sopher. "Non-native Hosts and Control of Rhytisma Acerinum Causing Tar Spot of Maple." Canadian Journal of Plant Pathology-revue Canadienne De Phytopathologie, 31.4 (2009): 488. These spores are disseminated by the wind and have a sticky coat to attach to new healthy leaves.Cornell University. Tar Spots of Maple. May 2011. Web. 22 Oct. 2011. . Once on the leaves, the spores germinate and penetrate through the

stoma In botany, a stoma (from Greek ''στόμα'', "mouth", plural "stomata"), also called a stomate (plural "stomates"), is a pore found in the epidermis of leaves, stems, and other organs, that controls the rate of gas exchange. The pore is bor ...

. The subsequent infection causes

chlorosis In botany, chlorosis is a condition in which leaves produce insufficient chlorophyll. As chlorophyll is responsible for the green color of leaves, chlorotic leaves are pale, yellow, or yellow-white. The affected plant has little or no ability to ...

of the leaves in localized yellow spots. As the season continues into summer, apothecia begin to form, giving rise to brown-black leaf lesions that resemble spots of tar. Leaves retain their yellow border from the initial chlorosis. Apothecia survive in the fallen plant debris over winter, releasing spores when the temperature is warm again. The infection of Tar Spot is localized to the chlorotic areas on the leaves and is mostly a cosmetic issue, rather than an economically detrimental disease. The

anamorph In mycology, the terms teleomorph, anamorph, and holomorph apply to portions of the life cycles of fungi in the phyla Ascomycota and Basidiomycota: *Teleomorph: the sexual reproductive stage (morph), typically a fruiting body. *Anamorph: an ase ...

of tar spot is ''Melasmia acerina''. In late summer,

conidiophores A conidium ( ; ), sometimes termed an asexual chlamydospore or chlamydoconidium (), is an Asexual reproduction, asexual, non-motility, motile spore of a fungus. The word ''conidium'' comes from the Ancient Greek word for dust, ('). They are al ...

are formed in the mass of fungal tissue called the stroma. Stroma is located in the black lesions of the infected leaves. Conidiophores form non-infectious

conidia A conidium ( ; ), sometimes termed an asexual chlamydospore or chlamydoconidium (), is an asexual, non-motile spore of a fungus. The word ''conidium'' comes from the Ancient Greek word for dust, ('). They are also called mitospores due to the ...

that are released both in conditions of wetness and drought. The most favorable environment for the pathogen is when there is an extended period of moisture such as fog or rain, which prevents the leaves from drying out.US. Maine Forest Service. Maine Dept of Conservation. Tar Leaf Spot of Norway Maple. 2009. Web. 22 Oct. 2011. . Young trees growing in shade are therefore more susceptible. Under these conditions, conidiophores excrete a milky substance of conidia. In times of drought, the conidia stick together as one unit and form yellowish tendrils. Because the conidia are not infectious, this stage is not seen as often as the teleomorph, and it is not certain why the spores are produced.

Geographical occurrence

Tar spot is most commonly found in Europe and North America. It was thought to be a useful pollution indicator because it is not found in areas with high amounts of sulfur dioxide."Tar Spot of Maple Key Words: Plant Disease, Acer, 'Rhytisma acerinum'', ''Melasmia acerina'', ''Rhytisma punctatum',." UNIVERSITY OF NEBRASKA - NU-Distance Classroom. Web. 22 Oct. 2011. <>.Urban Distribution of ''Rhytisma acerinum'' (Pers.) Fries (Tar Spot) on Sycamore. I. D. Leith, D. Fowler. New Phytologist. Vol. 108, No. 2 (Feb., 1988), pp. 175-181. However, while sulfur dioxide is toxic to the fungus in laboratory studies (sulfur dioxide prevents stroma from causing subsequent infections),"Healy, Rosanne. "Minor Leaf Spot Diseases of Maple: Tar Spot and Leaf Blister , Horticulture and Home Pest News." Home , Integrated Pest Management at Iowa State University. Web. 05 Dec. 2011. . no correlation was found between pollution levels and the disease. Instead, it was found that in urban areas fallen (infected) leaves were more likely to be swept up and removed, thus removing a source of infection for the plants. As mentioned previously, the disease appears in the summer especially in times of wetness, but it can also develop through a drier season.

Disease control

Tar spot is a localized disease that causes mostly cosmetic symptoms and is therefore not a highly controlled disease. One of the best ways to manage the pathogen is through proper sanitation techniques. Because the fungus overwinters in diseased leaf debris, removing the debris in fall can help reduce the occurrence of the disease. In certain severe cases, fungicides can be implemented to help with control."Ozarks Fruit and Garden Review: Tar Spot? Not a Problem!" US Fed News Service, Including US State News: n/a. ProQuest Research Library. Jun 30 2010. Web. 5 Dec. 2011. Copper fungicides sprayed in early spring when leaves are budding and twice more throughout the season help reduce the disease. However, chemical control is not normally used because this disease is mostly cosmetic. Resistant varieties are unknown. Also, because the affected tree species are of low economic value, yield reduction is of little concern to growers. By far the most important practice is to keep a clean yard and remove as much debris as possible.

Significance

Tar spot has been recognized since 1794, when it was discovered in Europe by

Christiaan Hendrik Persoon Christiaan Hendrik Persoon (1 February 1761 – 16 November 1836) was a German mycologist who made additions to Linnaeus' mushroom taxonomy. Early life Persoon was born in South Africa at the Cape of Good Hope, the third child of an immig ...

, a prominent mycologist of that time."Cornell Plant Pathology Herbarium." Cornell University - Department of Plant Pathology. Web. 22 Oct. 2011. . Tar spot has little historical importance because of its cosmetic nature. However,

equine atypical myopathy Equine atypical myopathy, also known as atypical myoglobinuria, or EAM is a fatal disease of horses (equidae) characterized by acute rhabdomyolysis. It is caused by the ingestion of the seeds or young shoots of sycamore tree. EAM has a high fatality ...

has been associated with the ingestion of tar spot infected maple leaves."Gluck Center." Learning, Discovery, Service , in the College of Agriculture. Web. 05 Dec. 2011. . This disease causes the complete degeneration of muscle fibers and subsequent death in horses.

References

External links

Commentary and video of Tar Spot
{{Taxonbar, from=Q163458 Fungal tree pathogens and diseases Leotiomycetes Fungi described in 1794