Many rudbeckia species are available to gardeners. You may know them as black-eyed Susans or orange coneflowers. One particularly unappealing disease of this species is Septoria leaf spot, caused by a fungus, Septoria rudbeckiae. It is prevalent now in Illinois. The dark brown leaf spots are hard to miss, starting as 1/8-inch spots but quickly merging into large, brown areas on otherwise dark green leaves. The disease begins on lower leaves and progresses up the plant.
Another problem that might look similar is a bacterial disease, angular leaf spot. A lab can easily distinguish the two. Angular leaf spot produces bacterial streaming from sections observed with a microscope. Septoria leaf spot produces fruiting bodies embedded in the spots. The fuiting bodies produce diagnostic, long, narrow spores.
Disease spread depends on leaf moisture. With recent rainfall in Illinois, there is not much you can do to stop spore splashing. Still, you can help prevent further disease spread by watering the soil, as opposed to syringing the foliage. Also try to prevent overcrowding of plants and keep weeds under control.
Preventive fungicide applications protect new growth from Septoria leaf spot. Sprays should begin before symptoms appear, but there may be some benefit to newly forming leaves. Copper-based fungicides have some effect against both Septoria and the bacterium causing angular leaf spot. Copper products have protective-contact activity. A systemic product registered for this use by homeowners is Heritage. The active ingredient is azoxystrobin.