Rudbeckia are known for being real work horses in the mid-summer to mid-fall garden. They are also known for being low maintenance and fairly pest and disease free. However, there are a few leaf spot diseases that can greatly mar the appearance of Rudbeckia, especially certain species, varieties or cultivars. Following are the leaf spot diseases to watch for on your Rudbeckia plants.
Septoria Leaf Spot
Septoria is one of the most common Leaf spot diseases of rudbeckia. While there are several species of the fungus Septoria, (most of which are host specific), Septoria rudbeckiae is the species that infects Rudbeckia.
Signs and Symptoms: in late spring/early summer (June or July) dark brown to purplish spots, that measure about 1/8″ to 1/4″ (3-6 mm), begin appearing on the lower leaves during periods of moisture. Black fungal fruiting bodies may be found in the center of these spots. When the leaves get wet, the spores contained within these fungal structures (pycnidium) are splashed to uninfected leaf surfaces. As the disease progresses it spreads further up the plant and the spots grow larger and begin to coalesce, even to the point of entirely covering the leaf surface. The spots may be rounded or angular and can cross the veins (unlike bacterial infections that are contained by the veins). Fortunately, the flowers are not affected, and the plants overall health is usually not seriously affected.
Managing the disease:
- Plant a different species or cultivars of rudbeckia, that is resistant to Septoria leaf spot such as Rudbeckia x ‘American Gold Rush’.
- When purchasing or planting new rudbeckia plants examine them carefully for symptoms of the disease to avoid introducing it to your garden.
- Remove and destroy affected plant parts to reduce the spread of the disease.
- Sterilize anything that has come in contact with infected plant tissue, such as tools, hands, gloves, even clothing should be washed.
- Do not work in the garden when the leaves are wet.
- Provide good air circulation around plants to ensure leaves dry as quickly as possible.
- Avoid overhead watering.
- Copper based fungicides can be applied before the onset of the disease, beginning about mid-June and continuing throughout the summer. These act as a protectant against the disease rather than kill the fungi.
- Cut back the plants and remove the plant debris in the fall, as the fungi overwinter in the plant debris.
Other Fungal Leaf Spot Diseases of Rudbecia
There are several other fungal leaf spot diseases that cause similar damage to rudbeckias such as: Alternaria, Cercospora rudbeckiae, Cylindrocladium Colletotrichum, Phyllosticta rudbeckiae, Ramularia rudbeckiae and others. Their occurrence is less common than Septoria and tends to be less severe. A lab is usually required to do a culture to accurately distinguish between these fungal diseases. However, the treatment and management of these fungal diseases is the same as Septoria.
Angular Leaf Spot of Rudbeckia
Angular leaf spot is a bacterial infection caused by either Pseudomonas or Xanthomonas. This is a more serious disease than Septoria and most commonly infects the cultivar ‘Goldsturm’.
Signs and Symptoms:
Its appearance is very similar to Septoria, with brown to purplish spots appearing first towards the base of the plant then working their way up. The spots tend to be more angular, being bound by the veins, and may appear water soaked. The spots quickly enlarge and coalesce, killing the whole leaf. Rather than black, fungal fruiting bodies, bacteria oozing out of the spots may be seen under a microscope.
Managing the disease:
Non-chemical management of angular leaf spot is the same as for Septoria leaf spot.
Rusts
Rusts, such as Aecidum batesii, Puccinia dioicae, Uromyces perigynius and Uromyces rudbeckiae are another type of fungi called Basidiomycetes. This disease can potentially be very serious.
Signs and Symptoms:
The appearance is quite different from the aforementioned fungal leaf spot diseases. Rust appears as rust coloured, blister-like spots filled with rust coloured powdery spores, on the undersides of leaves. The upper leaf surfaces may start off with a flat tan or yellow coloured spot that turns to a rust colour.
Managing the disease:
The management of this disease is the similar to the previous fungal diseases with the addition of not growing true sedges (Carex Spp.) near rudbeckias. Some of these sedges act as alternative hosts for the disease. The plants can be dusted with sulfur in spring to help protect them against developing the disease.
Photo credits: all photos taken by the author.
References:
Jeffers A. D., 2017. RUDBECKIA. https://hgic.clemson.edu/factsheet/rudbeckia/
Michigan State University, (n.d.). Septoria leaf spot https://www.canr.msu.edu/resources/septoria-leaf-spot
Missouri Botanical Garden, (n.d.). Leaf Spots of Rudbeckia. https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/diseases/fungal-spots/leaf-spots-of-rudbeckia
Moorman G. W., (Updated: July 31, 2016). Rudbeckia Diseases. https://extension.psu.edu/rudbeckia-diseases
Pettis S., Jr., (Updated JUL 20, 2020). Pest Alert: Bacterial Leaf Spot. https://henderson.ces.ncsu.edu/2020/07/pest-alert-bacterial-leaf-spot/
Reimer S. (Baltimore Sun) Aug 06, 2009. Disease of the week: septoria leaf spot of rudbeckia. https://www.baltimoresun.com/bs-mtblog-2009-08-disease_of_the_week_septoria_l-story.html
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