Answer: What you describe is a common fungal leaf spot disease on Rudbeckia, commonly known as Blackeyed Susan. This disease is caused by the fungus Septoria rudbeckiae. Symptoms begin as small dark lesions, usually angular in shape, which enlarge. The fungus overwinters in infected plant residue and spores are released in late spring and early summer, causing leaf spots first on the lower leaves. The spores of the fungus are dispersed by splashing water, require moisture to germinate, and can cause infection throughout the growing season.
To manage the disease, remove plant debris and infected leaves at the end of the growing season to reduce the number of spores that will start infections the following year. Proper plant spacing and thinning of volunteer plants will increase air circulation around foliage and allow leaves to dry quickly. Avoid overhead irrigation. The leaf spot is primarily cosmetic, and infected plants will bloom despite unsightly leaves. A protectant fungicide application in mid June will help reduce initial infection and slow the spread of the disease, but it will not cure infected leaves. Applications may be made periodically throughout the growing season (check fungicide labels for instructions on spray intervals and rate).
For even more information on the disease, visit the following website:
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Best wishes with your garden!
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