Tar spot

Description:

In late spring, yellow 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 a leaf spots that look like tar. The spot can grow up to 1.5 inches, 3.81 centimeters, 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.

Habitat:

Tar Spot is most commonly found in Europe and North America. It known as a pollution indicator because it is not found in areas with high amounts of sulfur dioxide. Sulfur dioxide prevents stroma from causing subsequent infections.Therefore disease tends to be localized in more rural than urban areas. As mentioned previously, the disease appears in the summer especially in times of wetness, but can also develop through a dryer season

Notes:

Rhytisma acerinum is a plant pathogen that commonly affects sycamores and maples 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 fungus that locally infects the leaves of trees and is a biotrophic parasite. The disease is cosmetic and is therefore usually controlled only with sanitation methods