PlantLife Online

Answer

Jorunn, sounds as though your Norway Maple is getting anthracnose fungi. Anthracnose fungi occur primarily on the leaves and twigs of deciduous trees. The fungi overwinter in infected twigs or dead leaf litter. In spring the fungi produces numerous microscopic spores that spread from splashing rain or irrigation to the new growth where they germinate causing the white powdery appearance and eventually leaf drop. Anthracnose is often worse during wet humid springs and unfortunately hard to control and generally not worth the cost. The best defense is to plant resistant species. The other more serious possibility is a type of root fungi, like verticillium wilt in which case the tree will slowly continue to decline. Try making a few cuts in the area that has been defoliated if the tissue looks green the limbs are still alive, if the tissue looks brown or gray its is dead and should be removed. I would give the maple sometime, if defoliation continues up the tree this year or next season the tree should be removed.