Rudbeckia is a perennial plant also known as black-eyed Susan, yellow oxeye daisy, English bulls-eye, Gloriosa daisy, and coneflower. Diseases commonly found on Rudbeckia include downy mildew, rust, and powdery mildew. In the past few years, Rudbeckias have been bothered by fungal leaf spots. The fungal leaf spots have been making an appearance in Illinois, with recent conditions favorable for infection on young succulent growth.
Septoria leaf spot caused by a fungus named Septoria rudbeckiae, is one concern. The dark brown leaf spots are hard to miss, starting as 1/8-inch-diameter spots but quickly merging to cause large brown areas on otherwise dark green leaves. The disease begins on lower leaves and progresses up the plant.
A problem that might look like Septoria leaf spot is a bacterial disease called angular leaf spot. A plant lab can easily distinguish between the two. Angular leaf spot produces bacterial streaming, easily viewed in tissue sections observed with a microscope. On the other hand, Septoria leaf spot produces fruiting bodies that are embedded in the spots. These fruiting bodies
are smaller than the head of a pin. Long, narrow spores are produced in the fruiting bodies, but these can only be viewed with a compound microscope. To see two images of Septoria leaf spot on Rudbeckia, visit the Web site at http://www.ppdl.org/dd/id/septoria-rudbeckia.html.
Other fungal leaf diseases we might see on Rudbeckia could involve these fungal species: Ramularia, Cercospora, Alternaria, Colletotrichum, and Phyllosticta. The leaf spots appear similar to Septoria leaf spot; but spores are different, and they are formed in different types of fruiting structures that can be discerned in a lab. In Illinois, we most frequently see Septoria leaf spot, Cercospora leaf spot, and angular leaf spot..
Disease spread of the fungal leaf spots 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. Infection is more likely if leaves remain wet for long periods. When conditions are dry, remove fallen or badly infected foliage to reduce inoculum. Also try to prevent overcrowding of plants and keep weeds under control. Preventive fungicide applications will protect new growth from fungal leaf spots. These sprays should begin before symptoms begin, with the intent to protect newly forming leaves. Thiophanate-methyl can be used for this purpose. It is sold to homeowners under a general ornamental label available as Bonide Bonomyl, Dragon 3336, or Ferti-lome Halt. Read and follow label directions carefully. Heritage is a systemic product available to commercial growers. It contains azoxystrobin and is upwardly systemic (toward new growth). The fungal leaf spots can be unsightly but do not kill plants.