SUMMARYGnomonia platani, the fungus that causes sycamore anthracnose, produces asexual spores in thin-walled pycnidia in young twigs. A pressure cushion is formed prior to formation of the pycnidium. It originates at the phellogen. Pressure cushion enlargement occurs through growth and swelling of hyphal cells. The pressure cushion ruptures the phellem. The pycnidium forms in the cortical cells beneath the pressure cushion or at the base or margin of the pressure cushion. Conidia produced in the pycnidium are released through the ruptured phellem.