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Black-eyed Susan

Leaf spot disease

By: Sandy Feather ©2009
Penn State Extension


Q. My black-eyed Susans came up fine last spring, but once they started blooming, the leaves turned black and the plants just seemed to dry up. I water periodically when the weather is dry, but know that I am not overwatering. Is this some kind of blight on Black-eye Susan, and is there anything I can do about it?

A. Black-eyed Susans (Rudbeckia spp.) are susceptible to a number of leaf spot diseases that could cause the symptoms you describe. Septoria leaf spot is one of the most common fungal diseases, characterized by small, dark brown leaf spots that range in size from one-eighth to one-quarter inch in diameter. These spots generally are round, but may take on an angular shape where they are limited by the leaf veins. The disease progresses rapidly during wet weather or with regular overhead irrigation, resulting in plants that seem to turn black and die overnight. There are other fungal leaf spot diseases than afflict black-eyed Susans, but they are not generally as severe as septoria leaf spot.


IDENTIFYING LEAF SPOT

If you examine an individual leaf spot with a 10x hand lens or under a microscope, you will see tiny black dots within the spot. These are the fruiting bodies of the causal fungus, waiting for favorable environmental conditions to release thousands of spores. Plant pathologists often use the phrase “spots with dots, fungus ya’ gots” as a mnemonic device to separate fungal diseases from those caused by other pathogens. It would require microscopic examination by a plant pathologist to tell exactly which fungus is causing the leaf spot, but fortunately, treatment is the same for all.

Black-Eyed Susan and Deer


PLANT CULTURE

Cut back the plants at the end of the season, and remove the trimmings from the garden. They should be destroyed or sent out with the trash, rather than thrown on the compost pile because few of us compost intensively enough to destroy pathogens. Make sure plants are spaced properly to allow for good air circulation. Don’t forget to remove volunteer seedlings to prevent overcrowding. Drip irrigation or soaker hoses are preferred methods of watering because they put the water on the soil rather than on the plants.

 

Since these fungal diseases are spread by water splashing the spores around – and because the spores require moisture to germinate and infect susceptible plants – keeping the foliage as dry as possible is desirable. Obviously the plants get wet when it rains, but you do not have to add to the problem by wetting the foliage when you water. If drip irrigation or soaker hoses are not in your plants’ future, at least use a hose with a watering wand and direct the water onto the soil as much as possible. Water the first thing in the morning so that plants dry quickly. Remove and destroy spotted leaves as they appear. Finally, you can use fungicide applications to protect new growth from infection. Ortho Garden Disease Control (chlorothalonil) and copper-based fungicides such as Soap Shield are labeled to control fungal leaf spot diseases on rudbeckia.


STILL SEEING SPOTS?

Angular leaf spot is prevalent on Rudbeckia ‘Goldsturm.’ This bacterial disease starts with angular spots that appear water-soaked. They soon turn brown and may grow to cover entire leaves. Angular leaf spot starts at the bottom of the plant and works its way up the stem. Unlike fungal leaf spots, those caused by bacteria do not have dark dots inside. Cut back plants in fall after the first hard freeze, and destroy infected trimmings. Sterilize tools by dipping them in 70 percent alcohol and allowing them to air dry before using them on healthy plants. In spring, treat emerging plants with a copper-based fungicide such as Bordeaux mix, and make repeated applications according to label directions. This will reduce the likelihood of infection, and reduce the severity of any infection that does occur. Avoid overhead irrigation because splashing water spreads the bacteria. Replacing susceptible cultivars like ‘Goldsturm’ with more resistant ones is an excellent alternative to spraying.


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