Answer: Rudbeckia is sometimes subject to diseases that cause spotting on the foliage. The best prevention is to make sure the plants are growing in full sun and in a location with good air circulation, space the plants and divide as needed to avoid crowding.
(Plants that have been in place for three years or more will bloom less due to needed to be divided. This can be done in fall or spring.)
Then keep them healthy by top dressing with compost once or twice a year, fertilizing if needed in early spring and early summer with a general purpose granular fertilizer such as 10-10-10 per the label directions, and using an organic mulch. The mulch will help reduce watering needs and also feed the soil slowly as it breaks down over time.
Water deeply (avoid wetting the foliage) during extended dry spells. In the fall, do a thorough clean up off all fallen foliage and old stems to limit carryover of problems from one year to the next.
If the spotting is minor, remove the affected leaves. If it is severe, you may need to use a fungicide containing sulfur or potassium bicarbonate. Read and follow the label directions.
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