Little blighters - Ask Extension
Dear extension this past summer most of the flowering plants developed a Brown black dying of the leaves and stunted growth especially in the Dahlias,...
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Little blighters #892904
Asked February 20, 2025, 7:44 PM EST
Dear extension this past summer most of the flowering plants developed a Brown black dying of the leaves and stunted growth especially in the Dahlias, blackeyed susans, Shasta Daiseys and sunflowers. This is the first time I have seen this kind of wilt and brown black color in the raised boxes. The plants where removed by high-school kids under my supervision. So so of the perennials came out of the old rotting wood boxes with more roots than others. But the blight also occurred on newly planted sunflowers. In the new raised boxes.which are built out of Doug Fir 2 X 8 s stacked 3 high with a 2 X 6 laid flat on top to sit on. Attached is a picture of a picture of the brown black blight on blackeyed susans.
Multnomah County Oregon
Expert Response
Hi David and thanks for your question and picture of your mysterious brown/black disease. I'm going to assume a couple of things in my answer, and if any of these were present, they are pretty self explanatory.
I'm assuming:
1. The plants got the correct amount of water (ie. the soil wasn't soggy and they got extra water during the heat)
2. No herbicides were used in the area which could have drifted over the plants
Now for my best guesses. All of the plants you mentioned are in the same family of plants: Asteraceae. Because of this, a disease would easily affect all of the plants as would a soil problem.
Tackling the disease possibility first, it looks the most like a soil borne fungus, to which Asteraceae plants are fairly susceptible.
If this fungus is in the soil of your raised beds, you might have to replace the soil. You can also spread out the plants so they get more air, water and fertilize less frequently or apply a fungicide to the soil near the plants. I would wait and see if the same problem occurs next year and then take a leaf sample in to an Extension office to check for the microscopic spores before I replaced all the soil.
As to the soil issue, it is definitely time for a soil test. You don't say where you got the soil for the beds, but there are several micronutrients that, if missing, can cause the blackening symptoms. I'll attach a local lab that does soil testing and be sure to tell them what the symptoms of the plants were and that you are looking to check micronutrients.
https://al-labs-west.com/soil-analysis/
And finally, there is a tiny insect called a thrips that can spread a disease to this family. Yellow sticky cards near the plants can help identify this tiny critter but there are not many controls other than hosing off the underside of the plants and using chemicals. Taking a leaf into an Extension office may help identify this problem using a microscope. Call first to be sure someone is there. I know, this also involves waiting until next year to see if the problem recurs.
So... start with a soil test this winter and with fingers crossed, see if the problem happens again. Or, you could just plant flowers from a different family...
I'm assuming:
1. The plants got the correct amount of water (ie. the soil wasn't soggy and they got extra water during the heat)
2. No herbicides were used in the area which could have drifted over the plants
Now for my best guesses. All of the plants you mentioned are in the same family of plants: Asteraceae. Because of this, a disease would easily affect all of the plants as would a soil problem.
Tackling the disease possibility first, it looks the most like a soil borne fungus, to which Asteraceae plants are fairly susceptible.
If this fungus is in the soil of your raised beds, you might have to replace the soil. You can also spread out the plants so they get more air, water and fertilize less frequently or apply a fungicide to the soil near the plants. I would wait and see if the same problem occurs next year and then take a leaf sample in to an Extension office to check for the microscopic spores before I replaced all the soil.
As to the soil issue, it is definitely time for a soil test. You don't say where you got the soil for the beds, but there are several micronutrients that, if missing, can cause the blackening symptoms. I'll attach a local lab that does soil testing and be sure to tell them what the symptoms of the plants were and that you are looking to check micronutrients.
https://al-labs-west.com/soil-analysis/
And finally, there is a tiny insect called a thrips that can spread a disease to this family. Yellow sticky cards near the plants can help identify this tiny critter but there are not many controls other than hosing off the underside of the plants and using chemicals. Taking a leaf into an Extension office may help identify this problem using a microscope. Call first to be sure someone is there. I know, this also involves waiting until next year to see if the problem recurs.
So... start with a soil test this winter and with fingers crossed, see if the problem happens again. Or, you could just plant flowers from a different family...
