Leaf spot causes irregular spots which vary in size and may run together. The spots have grayish centers with dark brown borders. The disease is usually in the upper half of the leaves and is more serious after the plants have bloomed. Premature death of the leaves weakens the rhizomes. The causal organism is Didymellina macrospora and it can be controlled with sprays of chlorothalonil.
Plants infected with rhizome rot fail to grow in the spring.
The fungus develops in cool wet weather. No chemical control is listed for this disease caused by Botryotinia convoluta. Crown Rot is worse when plants are crowded. The leaf and flower stalk bases are attacked. The infection spreads to the rhizome causing a rapidly developing dry rot.