There are two primary diseases that cause Black-eyed Susan (Rudbeckia) leaves to get spots that progress to blotches and sometimes to totally-black leaves: Septoria Leaf Spot (fungal) and Angular Leaf Spot (bacterial). The most common of the two is Septoria. Symptoms manifest as irregular black spots on the leaves, and it can disfigure the plant significantly if the weather is favorable for disease spread (wet, hot and humid). Once established, it is difficult to control.

Management for both diseases is the same and is the approach used for suppressing most plant leaf infections. Adjusting growing conditions can help reduce plant vulnerability, such as improving air circulation by reducing crowding (dividing dense plants or not planting them too close together) or eliminating poor drainage to reduce humidity levels. Wet leaf surfaces tend to be easier for spores to infect, so if you can’t avoid wetting leaves when watering, irrigate early enough in the day so the foliage can dry by nightfall. Keep mulch thin and away from the base of the plants. Inspect foliage periodically to catch symptoms early; remove and discard (don’t compost) infected leaves.

At the end of the season after frosts have killed top growth, remove dead plant debris so fewer spores overwinter around the base of the plant. Plants with Septoria are not usually killed outright, but can die back to the base, and may be weakened by successive years of heavy infection and the resulting defoliation. If disease pressure is heavy, you may want to consider growing something else. Other perennials might be more practical to grow, but there are species and cultivars of Rudbeckia that are much more resistant to developing leaf spot infections. The often-grown cultivar ‘Goldsturm’, for example, is quite susceptible to these diseases.

While fungicides containing either copper or chlorothalonil can work to suppress Septoria, they won’t necessarily work well (or at all) on bacterial pathogens like Angular Leaf Spot (copper ingredients might). Additionally, regular reapplications tend to be needed for most fungicide formulations, and their residues might have negative impacts on pollinators or other organisms, so we generally do not recommend fungicide use.

Any fungal lawn issues would not be associated with the leaf spots on the Rudbeckia and it would not be from what was used 30 years ago. 

Let us know if you have further questions. 

Emily