A leaf of sycamore, Acer pseudoplatanus, infested with the tar spot fungus, Rhytisma acerinum, photographed in Norfolk UK in August. R. acerinum is sensitive to sulphur dioxide in the atmosphere, and is becoming more widespread even in cities. due to clean air legislation and the steep decline in the use of coal for domestic heating. In the late summer, as here, the fungus appears as melanised lesions with shiny raised spots resembling tar, surrounded by yellowing areas of leaf tissue. The presence of the fungus is not fatal to the leaf, though heavy infestations may cause premature leaf fall. The fungus overwinters on the fallen leaves. In the following Spring, the tar spots split open to reveal a green apothecium, a saucer shaped ascocarp bearing asci on its surface. These spores are carried on the wind to young leaves, where the infection begins again.