The following is information on Septoria leaf spot of Rudbeckia in the landscape.
Septoria leaf spot on brown-eyed Susan, Rudbeckia 'Goldstrum' can be found on some very small plants in the landscape. Infection this early could develop into some serious defoliation. To keep some leaves on heavily spotted plants (irregular brown spots on the leaves with small black fruiting bodies on the oldest spots), make a fungicide application with Daconil or other labeled fungicide.
Symptoms begin as small, dark brown lesions that enlarge from 1/8 to 1/4 inch in diameter. Although the lesions are usually rounded, there may be angles where leaf veins limit the spread of the fungus. Symptoms of this disease may be difficult to distinguish from those of angular leaf spot of Rudbeckia. Microscopic examination of the lesion will reveal black, flask-shaped structures called pycnidia that contain thousands of thread-like spores. Spores are produced in late spring and early summer, causing leaf spots on the lower leaves. The spores of the fungus are dispersed by splashing water, with lesions first appearing on lower leaves and later developing on upper leaves as the season progresses.
To manage this disease, remove the infected leaves at the end of the growing season to reduce inoculum levels. Because leaf moisture is essential for infection to occur, increase air circulation around the foliage by properly spacing plants (and removing volunteer seedlings) to prevent over-crowding. As with other foliar disease problems, avoid overhead watering. Fungicides containing chlorothalonil or copper may protect new growth and reduce the spread of the disease. Preventative applications of fungicides should begin in late May to early June prior to the onset of symptoms.
Information from Bob Mulrooney, Extension Plant Pathologist, UD and "Diseases of Rudbeckia" by Janna Beckerman, Extension Plant Pathologist, University of Minnesota.