Crown gall causes large abnormal growths at the base of the infected plant. The plants are stunted and the shoots spindly. Infected plants are destroyed. No chemical control is listed.
Bacterial wilt causes stem drooping and wilting. Infected stems have a wet soft rot near the soil. Destroy infected plants and rotate plantings. No chemical control is listed.
Bacteriosis causes browning and softening of the stem. The pith is moist and blackened with the rot extending into the bark. A foul odor is associated with the disease. The roots may be infected. No chemical control is listed.
Botrytis blight is worse in dull cloudy weather. The buds rot and infected parts may be covered with grayish powdery mold. Remove infected plant parts. Spray with benomyl.
Powdery mildew causes white or grayish mold. The disease is worse late in the season and badly infected leaves are distorted and drop. Spray with sulfur or benomyl.
Verticillium wilt causes brown or black streaking in the conductive tissue of the stem. Plants wilt and die. Stored roots rot if infected. Destroy infected plants rotate plantings. No chemical control is listed.
Dahlias growing in heavy wet soil, and infected with stem rot, wilt and die. A white mold encircles the stem at the soil line. Improve soil drainage and do not crowd the plants. Rotate plantings. No chemical control is listed.
Dahlia mosaic virus causes leaf mottling. Pale green bands develop along the midribs and leaves are dwarfed and show general mosaic or yellow spotting. Destroy infected plants and control the aphids that spread the disease. No chemical control is listed.