Black-eyed Susan (Rudbeckia) is a beautiful native wildflower and a popular perennial garden plant. Many different cultivars are available for gardeners. While these flowering plants are hardy and do well in gardens, providing a reliable showy display of color by mid summer, there are a few pest and disease problems that can affect them. Two of these, one a disease and one an insect, can cause similar symptoms to appear during mid summer. If the upper surfaces of the leaves develop small to large, angular shaped brown or purple lesions, the problem could be Rudbeckia downy mildew or Rudbeckia psyllid.
If the problem is downy mildew caused by the water mold pathogen Plasmopara halstedii, the lesions will tend to be more brown but can have a purplish color too.
To confirm downy mildew, flip the leaf over and look for sporulation of the pathogen on the lower leaf surface. Whitish sporulation will be visible between the veins directly below the lesions that are present on the upper leaf surface, especially during or following wet or humid weather.
These spores can be rain splashed or wind-borne to new infection sites. A film of water must be present on the leaf for the spores to germinate so keeping the leaves as dry as possible will help minimize disease. This can be accomplished in a couple of ways. First, avoid the use of overhead irrigation if possible. If that’s your preferred method of watering, the best timing is afternoon for this problem. Studies have shown that the greatest spore germination activity for this pathogen occurs during the morning. Also, space plants to allow for good air circulation that will hasten drying after a rain or dew formation.
Other control practices include sanitation and, if necessary, the use of fungicides. Sanitation involves the thorough removal of plant debris from infected plants because that’s where the pathogen will be planning to overwinter. If there is a summer with frequent periods of wet weather favorable for disease, and you’ve had a previous problem with downy mildew, protective/preventive fungicides may be a good choice. There are a variety of products available including biological controls. Biological control products may have active ingredients such as the bacteria Bacillus subtilis or Streptomyces lydicus. Other options include potassium bicarbonate and copper products. Always look for the plant type and downy mildews on the product label and apply as directed.
The Rudbeckia psyllid (Bactericera antennata) has a nymph stage whose feeding on the undersides of the leaves causes striking purple lesions on the leaf as well as purple discoloration of the veins, most notable on the lower leaf surfaces. The nymph is also quite striking in appearance when viewed with magnification as it is multi-colored and fringed with hairs.
The adult psyllid is a very tiny insect that holds its wings over its back like a cicada. References say either that the adult overwinters in protected spots like crevices or leaf litter or that the overwintering stage is not yet confirmed. In a Michigan report of this insect on hibiscus, it stated that there was probably one generation per year so that would likely be similar here in Connecticut. There is a tiny wasp that parasitizes the nymph stage and this is what happened when you find a nymph that looks like this:
Products including Neem, insecticidal soaps and horticultural oils will control this pest. Thorough coverage is important for success. These products are kinder to beneficial insects that will eat or parasitize the psyllids. Other reported host plants besides Rudbeckia are Echinacea and Hibiscus.
I hope your Rudbeckias are looking great and have no problems, but if they have a bit of discoloration on the leaves, look for evidence of these two culprits on the lower leaf surfaces.