Black-Eyed Susan Leaf Diseases

Septoria Leaf Spot

Photo by Michelle Grabowski, University of Minnesota Extension

August 20, 2015 – Kenosha News

Question: Am I the only person who is especially plagued with fungal-type diseases this year? I have powdery mildew on lots of plants, but also a disease or diseases that I noticed last year on my black-eyed susans. I dislike using chemicals in the yard as we have many birds and butterflies. What can I do about my black-eyed susans? M.S.

Answer: Plant diseases are rampant this summer. The damp, drizzly weather in late May and June created a perfect environment for fungal, and some bacterial, pathogens to flourish.

Black-eyed susans (Rudbeckia) are susceptible to several leaf diseases. In southern Wisconsin Septoria leaf spot and angular leaf spot seem to be the most common leaf diseases of Rudbeckia.

Septoria leaf spot is caused by the fungus Septoria rudbeckiae and infects yellow coneflowers (Ratibida) in addition to Rudbeckia. Although it is related to other types of Septoria, Septoria rudbeckiae is not the same fungi so does not cause Septoria leaf disease in tomato or other plants.  Septoria leaf spot of Rudbeckia begins as small (1/8 to 1/4 inch) chocolate brown spots. The spots are rounded but may be defined on the edges by leaf veins. Leaf spots usually appear first on the lower leaves and gradually move up the plant. With a 10x magnifier you might be able to see very tiny black spore bearing structures called pycnidia in the center of the spots on the underside of infected leaves.

Angular leaf spot is primarily a problem on Rudbeckia ‘Goldstrum’. In this disease the pathogen is a bacteria. The disease begins as small angular brown leaf spots that can enlarge to encompass the entire leaf. Bacterial infections are often described as “water soaked.” Leaf spotting and browning travels from the bottom to the top of the infected plant. Close inspection of leaves under a microscope to look for bacterial streaming is needed to distinguish angular leaf spot from Septoria or other fungal leaf spots.

Prevention is recommended to reduce or eliminate both diseases. This fall completely remove from the garden all leaves, stems, and flowers of infected plants. Dispose of infected plant materials by burning or burying. Next year provide good air circulation around plants. Avoid overhead watering and mulch around plants to help prevent rain splash. Remove and destroy infected leaves early. Organic copper-based fungicides and bactericides may help prevent infection if applied according to label directions.

 

Barb Larson is horticulture educator for Kenosha County University of Wisconsin Extension. Gardening questions can be emailed to barbara.larson@kenoshacounty.org or call Master Gardener Volunteers at 262-857-1942.