Sooty blotch and flyspeck

Sooty blotch and flyspeck (SBFS) or apple summer disease is a plant disease caused by a complex of saprophytic fungi which colonize the epicuticular wax layer of apple (Malus x domestica Borkh.). It is found worldwide in regions with moist growing seasons.

Sooty blotch and flyspeck is a descriptive term for a condition of darkly pigmented blemishes and smudges caused by a number of different fungi affecting fruit including apples, pear, persimmon, banana, papaya, and several other cultivated tree and vine crops. The greenish black coating resembling soot or flyspeck-like dots grow into irregular stains and blotches during the summer or growing season. They can grow into each other and may cover the entire fruit surface. Frequently blotches run down in a track resembling tears (German: "Regenfleckenkrankheit"). The blotches can be removed by vigorous rubbing or be scratched off revealing a normal peel. Symptoms can be seen particularly well when apples are light or green colored. Late varieties are more susceptible, because the disease builds over a longer growing period.

Moist and cool weather favors the infection. Orchards with shade or regions near bodies of water with frequent morning dew are especially prone to disease. Infections can occur from June until autumn. Secondary infections are caused by conidia, which are transported by wind and rain onto other developing fruit. The first visible signs of SBFS colonies appear about 3 weeks after infection.

Microscopically, the flyspeck-like spots and sooty blemishes are fungal mycelium adhering to fruit. The fungi live as saprophytes on the wax layer surface of apples and do not invade the peel. The hyphae, fruiting bodies, and survival structures of these fungi become melanized over time. SBFS fungi also grow on the surfaces of stems, twigs, leaves, and fruit of a wide range of wild plants.

...
Wikipedia