Chondrostereum purpureum

Chondrostereum purpureum is a fungal plant pathogen which causes silver leaf disease of trees. It attacks most species of the rose family Rosaceae, particularly the genus Prunus. The disease is progressive and often fatal. The common name is taken from the progressive silvering of leaves on affected branches. It is spread by airborne spores landing on freshly exposed sapwood. For this reason cherries and plums are pruned in summer, when spores are least likely to be present and when disease is visible. Silver leaf can also occur on poming fruits like apples and pears; plums are especially vulnerable.

from Wikipedia