For years, I’ve listened as speakers, other gardeners and even nursery and landscape folks all advised homeowners that the black they were seeing on their black eyed susans (rudbeckia species)was a fungal disease and that they should spray them with a fungicide. This advice was given in wet seasons and in dry, without seeing the plant and without any further diagnostic inquiry.
So a few years ago, when black spots started showing up on my black eyed susans, I decided to take a closer look. I knew darn well I wasn’t going to spray with a fungicide for 2 reasons: first, what I was seeing looked nothing like a fungus, and next, fungicides only work if you begin spraying before you have a problem; they sure don’t solve the problem once you have it. So no point in spraying once the black leaves are already there.
So the first season I watched. The leaves started out in a small patch, got a little worse, and then stopped. Next season, same thing: Small patch of black leaves that gradually spread for about a week or so and then stopped. This is not fungal behavior, folks. Fungi do not stop on their own. They just keep going until the leaves are completely covered and the leaves fall off. Think of powdery mildew on lilacs or, worse yet, on phlox leaves or think of black spot on roses.
Last year I discovered the culprit. It’s a beetle. And beetles being what they are, it wouldn’t stay around for a photo. I was lucky that I got a good enough look at him that I was able to make an ID. It looked enough like a cucumber beetle (elongated body, and stripes, but larger head and dissimilar enough that I knew it wasn’t a cucumber beetle) that I was able to pin it down to the general family of leaf beetles–but that’s about all.
And in true leaf beetle fashion, it sucks the underside of the leaf–much like that caterpiller-like rose sawfly larva. Then, when the sun comes out, it burns the top of the leaf, the leaf turns black, the gardener goes to the garden center and says, “My black eyed susan leaves are all black” and voila! He or she gets sold a fungicide! Amazing how that happens.
This damage, by the way, is totally consistent with insect damage–it has a definite cycle: in other words, begins slowly, gets worse, and then ends when the insect life cycle is over.
So don’t be the next victim! Since this is only going to occur for about a week, try to live with it. Since this is a beetle, my guess is that an insecticide with spinosad would kill it–or, you can do as I do. Once the worst of the infestation has passed, pull off the affected leaves and get on with your gardening without harming any good bugs in the process.