Dahlia smut is caused by the fungus Entyloma dahliae. Symptoms appear as pale spots usually beginning on the lower leaves during cool, wet conditions due to rainfall or frequent overhead irrigation. Light to dark brown spots can be seen in the center of these white spots. This disease can infect plants grown in greenhouses or cut flowers in the field. Other species of Entyloma infect other plants such as sunflower, gaillardia and calendula. As the spots mature, spores appear on the surface of each spot. This fungus requires very moist conditions to infect and spread. Fungicides effective against rust should be effective to prevent further infections. Reduce the density of the plant canopy, improve air circulation and light penetration, and reduce leaf wetness periods. Remove and destroy affected plants and all symptomatic leaves, and treat with a fungicide. Dahlia smut can over-winter in soil attached to stored tubers. Treat tubers overwintered indoors with a fungicidal dust before planting.
