This may seem like a minor crop to be concerned about, but the Plant Clinic has received many Rudebeckias in the last several weeks. This is not a host that usually has problems in our area, but this year we have seen leaf spot problems, in some cases covering entire leaves.
Both fungal and bacterial leaf diseases attack this host. All will cause spotting in some shade of brown or black, but the bacterial leaf spots are usually angular, with edges delineated by small veins, while the fungal leaf spots are more irregular rounded areas. Fungal pathogens usually include Cercospora, Phyllosticta, or Septoria, whereas the bacterial pathogens include Pseudomonas or Xanthomonas. The cultural control measures listed for anthracnose of hosta in the previous article will help to manage these diseases, regardless of pathogen. If you try a fungicide to control the disease, you will have to know whether a bacterium or a fungus is involved.