This time of year some Black-eyed Susan (Rudbeckia sp.) plants may turn up a bit spotty. Have you noticed any of this at nurseries or in your landscapes? These leaf spots are often caused by a fungus named Septoria rudbeckiae. The leaf spots enlarge to 1/8 to 1/4 inch in diameter and are black in color. In severe infections, the leaf spots can cover most of the leaf surface.
Another pathogen, Pseudomonas, is a bacterial leaf spot that can look very similar to Septoria. To determine which pathogen you are seeing, the plant can be submitted to a diagnostic clinic like those provided by state university extension offices for testing.
Septoria can be avoided by providing conditions where the plant's leaves are able to dry out between waterings. Avoid overhead irrigation during cloudy weather or during the early morning or late day when the leaves are likely to remain wet for long periods of time. Instead, water Rudbeckia in the mid-morning to early afternoon when sunlight and air circulation are more conducive to evaporation.
Source: Perennial Pulse, Ball Publishing
