Dahlia smut, caused by Entyloma dahliae, affects the leaves of dahlias in areas of frequent late spring and summer rainfall or under frequent overhead irrigation. The overwintering spores of the fungus survive in the soil and on infected plant debris. Infection begins on the lower leaves when splashing rain or irrigation cause spores to germinate on the leaf surface. Infection advances as spores from lower leaves are splashed onto leaves higher in the plant. Lesions on the leaves can be numerous (shown below) and can coalesce eventually causing the leaves to die and drop from the plant. Management
Smut generally affects dahlias only in conditions where rainfall or overhead irrigation occur. Some areas experience the disease for a short period following late spring rains and see it decline if they have no rain during the summer. It may flare in the fall as the rainy season begins. Areas of frequent summer rainfall can be affected all season. Avoid splashing of spores from the soil surface by adding a layer of clean mulch at the base of the plants. Avoid overhead irrigation. Spores from infected leaves can survive in the soil for more than a year. Areas that have experienced the disease are likely to have annual recurrence. Remove and destroy infected leaves. Foliar applications of copper based fungicides have shown some effect as have preventive applications of Mancozeb.
The following photographs were taken by John Menzel.
Top: Early stage infection of White Smut on dahlia leaf.
Below: Bright yellow or tan lesions haloed by light green.
Top: Advanced White Smut infection on top of dahlia leaf. Below: Bottom of leaf view. Lesions tan to dark brown and dry surrounded by a light green halo.