Septoria leaf spot on Rudbeckia is caused by the fungus Septoria rudbeckiae. Look for small, dark brown spots about 1/8 of an inch in diameter starting on the lower leaves. The fungus overwinters in infected plant debris. Fortunately, flowers are not affected, but damage can be unsightly as the leaf spots coalesce on the leaves.
This disease is most severe where wet, humid weather persists for extended periods. Cultural control measures include rotation to non-susceptible crops for two years, control of susceptible weeds, and removal of crop debris from the planting area or prompt incorporation of refuse after harvest. Careful attention to the timing of irrigation (avoid late afternoon or evening) and proper plant spacing to reduce the amount of time during which the plants remain wet can reduce disease spread. Treat with protective fungicides at regular intervals. Among the compounds registered for use on ornamentals are chlorothalonil, thiophanate methyl, copper compounds, and mancozeb. The effectiveness and number of sprays required will vary with weather conditions.