P&PDL Picture of the Week for
September 30, 2013

Septoria leaf spot gives Rudbeckia 'Goldsturm' a black-eye

Janna Beckerman, Associate Professor, Botany & Plant Pathology, Purdue University

Septoria leaf spot, caused by the fungus Septoria rudbeckiae is a common leaf spot disease of black-eyed Susan (Rudbeckia spp.). The variety ‘Goldsturm’ seems particularly susceptible to it, which is unfortunate, as it is an otherwise great Rudbeckia!

Symptoms of this disease first appear as small, brown lesions that expand to 1/8 - 1/4 inch in diameter around the time of flowering. The lesions may be angular in shape, or have rounded edges (Figure 2). Over the summer, the fungus produces small pimples called pycnidia, which contain spores that spread the pathogen (Figure 3). The spores of the fungus are dispersed by splashing water (either irrigation or rainfall), and can cause lesions throughout the growing season. Like most fungal leaf spot diseases, the spores require moisture to germinate and cause infection. Avoid overhead irrigation that promotes leaf wetness and also splash spores from plant to plant. In unusually wet weather, or heavy irrigation, plants can appear killed by this disease (Figure 1).

The fungus overwinters in infected plant residue so removing infected leaves at the end of the growing season is important to reduce the amount of spores available the following year. Proper plant spacing will increase air circulation around the foliage and allow leaves to dry off quickly after dew or rainfall events. Since black-eyed Susans spread quickly (thanks to the help of goldfinches and native sparrows!), this will involve pulling the volunteer seedlings that sprout up next year.

While Septoria leaf spot is unsightly, the damage is primarily cosmetic, and infected plants will return and bloom again and again—the ones in Figure 1 did!  A general-purpose garden fungicide like Daconil may help reduce the spread of the disease, but must be applied before symptoms can be seen (in early to mid June) through bloom, on an every other week basis (or more in wet weather). As always, check label for instructions on spray interval and rate. This is impractical for most gardeners, however. A more effective approach would be to replace ‘Goldsturm’ with other, more resistant varieties of black-eyed Susans, or use annual black-eyed Susans (which reseed and act like perennials)—the goldfinches love them all. Other, more resistant perennial black-eyed Susans include ‘Becky’, ‘Cherokee Sunset’, ‘Irish Eyes’ and ‘Prairie Sun’. Or, stick with ‘Goldsturm’. If it doesn’t bother the finches, why should it bother you?

Click image to enlarge

Figure 1

Figure 2

Figure 3

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service