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Septoria Leaf Spot of Rudbeckia
Rudbeckia sp. with Septoria leaf spot, caused by the fungus Septoria rudbeckiae. This is a common leaf spot disease of Rudbeckia. Symptoms begin as small dark brown lesions which enlarge to 1/8 - 1/4 inch in diameter. The lesions may be angular in shape, or have rounded edges. There is a bacterial leaf spot disease of Rudbeckia that causes similar symptoms, but the fungal disease is more common. The two diseases can be distinguished when the lesions are examined under the microscope. Bacterial streaming is absent in lesions caused by Septoria.
The fungus produces small, pinpoint-size black flask-shaped structures called pycnidia, which contain the threadlike spores typical of Septoria. 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. Removal of infected leaves at the end of the growing season is important 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 alittle 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.
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Last updated: 16 July 2001/tlm.
The Plant & Pest Diagnostic Laboratory at Purdue University.