Leaves, fruits or shoots with whitish, later darker, mealy large spots or coatings; in the spots yellow, orange and later black dots (fruiting bodies); often, but not always premature shedding of the leaves; maybe mistaken for dust layers.
Affected tree species
All tree species; Apple; Ash; Beech; Elder; European mountain ash; Horse chestnut; Maple; Oak; Rose;
Powdery mildews are true parasitic ascomycetes and mostly highly host specific. The spores infect young leaves entering them by germination tubes; subsequently they produce superficial mycelia, quickly developing spores in high numbers; other than leaf and needle attacking microfungi powdery mildews are favoured by long lasting warm and dry periods in spring and a wide amplitude of day and night temperatures. Further information (in German):Gehölzkrankheiten in Wort und Bild (TUM Weihenstephan): Acer; Corylus; Quercus; Ribes grossularia
Curative measures Adult trees commonly withstand mildew infestations quite well, young trees may need sometimes control of powdery mildews by fungicides (see official register of Plant Protection).
Powdery mildew: fruiting bodies in various stages of development
Leaf of Fraxinus with Phyllactinia fraxini
Phyllactinia fraxini: fruiting bodies
Leaf of Aesculus hippocastanum with the American Horse chestnut powdery mildew Erysiphe flexuosa (recently spreading over Europe)
Fruiting bodies of Erysiphe flexuosa with characteristic appendices