By George Weigel/The Patriot-News
Q: My black-eyed susans have a nasty-looking black spot on most of the leaves. Is this the same black spot that affects roses? The plants are growing well and are blooming like crazy, but the foliage is a mess. Is there a cure or prevention?
A: That's probably septoria leaf spot - the main Achilles heel of black-eyed susans. 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 susan patch right after Labor Day. This disease tends to be worse in crowded conditions when poor air circulation encourages leaf dampness. If you divide off fist-sized pieces and space them 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 getting that on starting around early June and then re-spraying 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 neem oil.
Me? I'd just enjoy the flowers, ignore the ratty leaves and cut the whole patch down when it gets ugly enough. And divide.