Black spot on Black eye Susan Goldstrum.
what can I do to prevent the black spots that occur on my Blackeye Susans Goldstrum every summer? The plant leaves eventually turns all black and have to be cut back.
There are several fungal diseases that often cause problems in these plants. These fungal diseases overwinter in infected plants and they're difficult to control effectively with fungicides.
There are several practices we recommend to help minimize disease symptoms:
1) 'Goldsturm' variety of Rudbeckia are highly susceptible to diseases like septoria. Consider replacing them with resistant varieties.
2) Don't mulch around the plants. This minimizes moisture retention and makes the area less hospitable to the fungus
3) Allow more spacing between your plants to increase air circulation.
4) Avoid overhead watering to minimize leaf wetness.
5) Do not overfertilize. Over-fertilization may increase leafhopper populations.
6) Clean out plant debris at the end of the season. Symptomatic foliage should be discarded in the trash, not compost.
The following blog has information on disease-resistant black-eyed Susan.