Rudbeckia Leaf Spot
The following question was sent to the P&PDL diagnosticians here at Purdue University:
Question: The leaves on our black-eyed Susans are covered with small, dark brown spots. This happens every summer. It happens to plants in full sun as well as those in partial shade. What causes it?
Answer: Almost every Rudbeckia 'Goldsturm' (a cultivar of black-eyed Susan) I see this time of year is infected with Septoria leaf spot, caused by the fungus Septoria rudbeckiae. Symptoms begin as small, dark brown lesions, which enlarge to one-eighth to one-fourth inch in diameter.
The fungus overwinters in infected plant residue. Spores are produced in late spring and early summer, causing leaf spots on the lower leaves. As the season progresses, lesions develop on upper leaves as well. The spores of the fungus are dispersed by splashing water (either irrigation or rainfall), and can cause lesions throughout the growing season. Like most fungal leaf spot diseases, the spores require moisture to germinate and cause infection.
It is important to remove the infected leaves at the end of the growing season to reduce the amount of spores available the following year. Proper plant spacing will increase air circulation around the foliage and allow leaves to dry off quickly after dew or rainfall events. Since Rudbeckia plants spread quickly, this will involve pulling volunteer plants. Avoid overhead irrigation, which will promote leaf wetness and also splash spores from plant to plant.
While Septoria leaf spot is unsightly, the damage is primarily cosmetic, and infected plants will bloom. Infected leaves may die a little earlier in the fall than uninfected leaves. A general-purpose garden fungicide may help reduce the spread of the disease, but these chemicals are protectants and do not cure infected leaves. Application in early to mid June may help reduce initial infection, and result in a slower onset of disease symptoms. For maximum control, application of a protectant fungicide should be made periodically throughout the growing season (check label for instructions on spray interval and rate). This is impractical for most gardeners, however. I try to plant something in front of them to hide the unsightly leaves, instead.
-Beverly Shaw, Advanced Master Gardener, Purdue University