mycosphærella

from The Century Dictionary.

• noun A genus of pyreno-mycetous fungi having minute membranous perithecia buried in the tissue of the host. The spores are hyaline or greenish and uniseptate. About 500 species have been described, mostly under the untenable name Sphærella. They occur chiefly on fallen leaves. M. maculiformis is common on oak-leaves.