Black Eyed Susans - Knowledgebase Question

Center Barnstead, Ne
Avatar for esquivel
Question by esquivel
August 23, 2010
How can I tackle black spots on my Black eyed Susans.Every single leaf is infected with black spots. Thankyou in advance.

Answer from NGA
August 23, 2010
What you describe is probably septoria leaf spot which is common on Black Eyed Susan plants. It's a fungal leaf disease similar to black spot on roses, but the trouble-making organism (Septoria rudbeckiae) is different.

Leaf spot happens a lot this time of year, especially when the weather has been damp and rainy or flat-out humid. Although it can look pretty bad, septoria is primarily a cosmetic issue. It's usually not bad enough to kill plants. What usually happens is that the disease picks up steam while the flowers are blooming, then causes an early demise of the foliage. That weakens the plants for sure, but black-eyed susans are usually vigorous enough that they come up the following spring.

A good way to start dealing with this is to plan on digging and dividing your plant right after Labor Day. This disease tends to be worse in crowded conditions when poor air circulation encourages leaf dampness. If you divide your plant and set divisions about 2 feet apart, that should help next year.

Another good move is cutting off and removing all the infected foliage from the garden this fall. Septoria spores overwinter on plant residue, so the more of these you can get rid of, the fewer spores you'll have to reinfect next spring.

The third prong of attack, if you need/want it, is spraying a protective fungicide. The key is to apply in early June and respray every 7 to 10 days while the conditions are ripe for infection. Daconil (chlorothalonil) is a common chemical fungicide that usually does the job, but good organic options are copper, one of the new bicarbonate fungicides (i.e. GreenCure, Remedy, Bi-Carb) and possibly a neem oil based fungicide.

Good luck with your garden!

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