The Black Spots on Your Black-Eyed Susans Are Not a Fungus!

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.

12 thoughts on “The Black Spots on Your Black-Eyed Susans Are Not a Fungus!

  1. camper hire May 18, 2012 / 9:57 pm

    Very great post. I simply stumbled upon your weblog and wished to mention that I have really enjoyed surfing around your weblog posts. In any case I will be subscribing to your rss feed and I hope you write again very soon!

    • gardendaze May 19, 2012 / 9:40 am

      Great to have you and thanks so much for reading!

  2. Lee McLean June 18, 2012 / 6:45 pm

    thanks for your advice for my Black Eyed Susans

    • gardendaze June 18, 2012 / 6:57 pm

      You’re very welcome–thanks for reading!

      karla

  3. rigglez August 15, 2012 / 11:26 am

    I’m doing some research for my mother about her black eyed susans. Just like your article, they have black spots all over their leaves. You mentioned the beetles as the problem but that they only last for a week. The spots on my mother’s plants last most of the summer.
    Could this till be a beetle? Is there anything that she can do for them?

    • gardendaze August 15, 2012 / 12:00 pm

      I’m sorry–I guess I was unclear in my writing. The beetles life cycle only lasts for a week. The black spots do indeed last for the remainder of the growing season because its damaged tissue.

      Mine have gotten so ugly this past season that I just went and selectively pruned out the worst of the leaves. As it so happened, they were with flower stalks that were just about finished blooming anyway so cutting them back meant I wasn’t losing any bloom.

      In many cases, I had new rosettes of fresh growth at the bottom and lovely green leaves–quite a relief and so pleasant to look at after all that ugly brown on the leaves.

      Try that and see if it works.

      I still maintain that we are seeing insect damage and not fungal damage–which is what all the garden centers tell us–when we have this much damage on our black eyd susan leaves. But even if it is fungal damage, cutting it off and taking it away will certainly help. Fungicides do not cure–they just keep the problem from spreading–as will removing the leaves, which every good garden center worth its salt will tell you to do in the case of a fungal infestation.

      So as soon as those flower stalks stop blooming, cut them back hard and I think your Mom will be happy with the result!

      Thanks for reading!

      Karla

  4. Aron June 12, 2014 / 6:09 pm

    You have no clue what you are talking about. It is fungus and if left untreated it will destroy the entire plant.

    • gardendaze June 12, 2014 / 6:27 pm

      Thanks for your thought. How long would you suggest this takes since they’ve looked like this for at least 8 years? Thanks for reading and commenting.

      Karla

      • Ken September 29, 2014 / 2:35 am

        I’m just browsing through the web to look up info regarding black spot issue in Black Eyed Susan and I stumble upon this page. GARDENDAZE might not be wrong for her case. But Aron might not be wrong either. For my case, I doubt it was the beetles that caused it. My Black Eyed Susans got black spots, then the whole plant would dry up and died. It was probably my fault with the way I normally splash water at my plants with the fire hose thing and I start the whole ‘fungus’ issue :)) but I got it spread from one plant to the next one in line. Some of them stay healthy if they are not planted close the one with issue. So I think it is actually fungus and not beetles for my case. And I normally spray my garden with pesticide stuff but not fungicide stuff. So there, next year, I’ll do both. Aron probably won’t object if the title of this writing is something like ‘The Black Spots on Your Black-Eyed Susans MIGHT Not be a Fungus!’ instead of ‘The Black Spots on Your Black-Eyed Susans ARE Not a Fungus!’ … Because .. Oh yeah, it is ‘fungus’ for some of the folks, heehee … And there go my 2 cents on the subject 🙂 Cheers!

  5. barbara markland August 14, 2015 / 2:02 am

    ..I enjoyed you article. had beauitful susans for over 12 years. last 2 all black leaves & stunted. other flowers not affected.

    • gardendaze August 14, 2015 / 8:45 am

      Thanks, Barbara. I’ve gotten more pushback on this article than any other–primarily because black eyed susans do get a number of difference fungal diseases so it can be difficult to tell, once you have the black spots, what’s going on.

      Early this spring I posted photos of the actual chewed holes in my leaves–before the blackening started–so that the folks could see that yes, if fact, I was having insect damage and then blackening. It’s much like what happens with the rose sawfly larva on rose leaves if you’re familiar with that.

      And the only reason I even caught on is because the same thing was happening to my golden oregano, another favorite of the four lined plant bug.

      They now seem to be gnawing on my echinacea, but the leaves are just getting chewed a bit at this point. Of course, it could be be because I removed almost all the black eyed susans and they needed another host plant. Sometimes you can’t win for trying. Sigh.

      Thanks for reading and commenting!

      Karla

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