I noticed some dark splotches on some of the leaves of my black-eyed Susans. Some, but not all, have galls on them. I’ve been pulling off the affected leaves when I see them. Can you tell me the cause and any action to take?
— Susan Collins, Niles
Your photo and description indicate Septoria leaf spot, which is very common on Rudbeckia and is prevalent in Illinois. The disease begins on lower leaves and progresses up the plant but does not affect the flowers. The damage can become very unsightly as the leaf spots come together on the leaves. Conditions become conducive to this disease in late summer as the plantings fill in with decreased air circulation and when watering is done overhead. Septoria leaf spot is most severe when wet, humid weather persists for extended periods of time. Splashing water will spread the disease to unaffected leaves.
The plant in your photo is not being heavily affected by the disease now so you do not need to cut the plants back. Removing infected leaves is not worth the effort at this time in my opinion. Avoid overhead watering of your black-eyed Susans if possible. Established plants need very little supplemental water if any at all. Should the disease continue to affect your plants, try dividing the planting and spacing them out more to improve air circulation. The disease does overwinter on the foliage, so you should cut the plants back to the ground late this fall and remove the foliage from the garden. Good sanitation practices are an effective way to combat many diseases in the garden. There are fungicides that can be sprayed once every 14 days during the summer months to protect new foliage, but I would look for a disease-resistant variety or a different plant before starting a spray program.
There is a similar-looking bacterial disease called angular leaf spot that can affect black-eyed Susans. It begins as small, brown, angular spots on lower leaves, which may appear water-soaked but can quickly expand to affect the whole leaf. Maintenance recommendations are the same for this disease, except that a copper-based fungicide must be used.
The galls with purple blotches under them that you are seeing could be caused by a sap-sucking insect called rudbeckia psyllid. You can pick off the infected leaves to help control this insect. If more leaves continue to be affected, spray insecticidal soap under the leaves.
For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at [email protected]. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.
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