The Curse of the Black Spot

 

The curse of the black spot! Tar spot fungus Rhytisma acerinum, that disfigures sycamore leaves in summer, continues to develop on its dead, decaying foliage in the leaf litter. The black spots become increasingly wrinkly as winter progress, then hundreds of asci - microscopically small flasks filled with needle-shaped spores - develop along the ridges. When spring arrives the spores are released into the air, just in time for them to infect a new crop of sycamore leaves, where they germinate and invade through the breathing pores (stomata) in the leaf surface.

Tar spot fungus is less common on urban sycamores that on trees in rural areas and it used to be thought that this was because the fungus was susceptible to sulphur dioxide in polluted city air. A more likely explanation could be that the fungus is less common on city sycamores because their fallen autumn leaves tend to be removed by councils from public places, so the annual cycle of reinfection from dead infected leaves is broken.