Rhytisma acerinum is a plant pathogen that commonly affects sycamores and maples in late summer and autumn, causing tar spot.

[4] The lesions continue to grow, and by the end of summer form leaf spots that look like tar.

As the season continues into summer, apothecia begin to form, giving rise to brown-black leaf lesions that resemble spots of tar.

Apothecia survive in the fallen plant debris over winter, releasing spores when the temperature is warm again.

[5] 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.

[2][7] 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.

[11] As mentioned previously, the disease appears in the summer especially in times of wetness, but it can also develop through a drier season.

[7][12] Copper fungicides sprayed in early spring when leaves are budding and twice more throughout the season help reduce the disease.

[3][5] Tar spot has been recognized since 1794, when it was discovered in Europe by Christiaan Hendrik Persoon, a prominent mycologist of that time.