Diplocarpon mespili

About

Diplocarpon mespili is a pathogenic fungus which causes quince leaf blight, a leaf disease affecting chiefly common quince. It occurs in wet summers, causing severe leaf spotting and early defoliation, also affecting fruit to a lesser extent. It may also affect other Rosaceae such as hawthorn and medlar, but is typically less damaging than on quince.

Summary from Wikipedia