How to clear up this fungus(if that’s what it is) - Ask Extension
Every year my black eyed susan’s come back but with ugly black spots covering the leaves. Eventually the leaves shrivel up and die. I’ve been told...
Knowledgebase
How to clear up this fungus(if that’s what it is) #846283
Asked August 18, 2023, 1:47 PM EDT
Every year my black eyed susan’s come back but with ugly black spots covering the leaves. Eventually the leaves shrivel up and die. I’ve been told it’s a fungus but how to get rid of it?? I’m willing to invest the time and $$ to solve this!! Thank you
Baltimore County Maryland
Expert Response
This appears to be Septoria Leaf Spot or possibly Angular Leaf Spot, common infections that can afflict several species and varieties of Black-eyed Susan (Rudbeckia). Some cultivars are more susceptible to infection than others; for example, we have observed that 'Goldsturm,' a widely-grown variety, contracts this spotting often, whereas more wild-type selections seem more resistant. (There are also some more recent cultivar introductions that are purportedly disease-resistant.) We receive questions about this symptom on Black-eyed Susan regularly. Although unsightly, mild to moderate leaf spot infections tend not to impact long-term plant health too badly, though severe infections recurring year to year could weaken plants because it reduces how much energy they can store in roots due to the premature leaf loss.
We rarely recommend fungicide use for leaf spot diseases like this because they require relatively precise timing of application (well before symptoms develop, usually) to kill spores before they enter the leaf tissue, they often require multiple applications per year to keep infections suppressed, and they don't always work well, even when used properly (such as if weather interrupts a spray schedule). Additionally, some fungicide ingredients may risk harm to pollinators or other organisms, especially if used on flowering and native plants attractive to wildlife. Unless a planting of a single species/variety is extensive, it's often just cheaper and easier to replace a few plants that routinely become infected to the point of undesirability than to attempt yearly treatments with fungicide, and in that case, resistant cultivars (if available) or species not as prone to that particular pathogen should be chosen. Nothing is immune to any and all leaf spot diseases, but at least different species tend to have different vulnerabilities.
For now, it is too late in the season to treat for either condition. Fungicides cannot cure existing infection (and few treat bacterial pathogens); they only help to suppress infection at the start of a disease cycle. Cool, wet weather tends to favor leaf infections, and if you happen to wet foliage when irrigating plants during drought, try to do so only early in the day so leaves can dry by nightfall, or just water the soil surface instead to keep leaves dry. It's easier for many pathogen spores to infect leaf surfaces that stay wet for long periods. You can trim off the worst of the leaves, including removing any that are more-or-less fully blackened, and once freezes kill the top growth by mid autumn or so, remove all dead Black-eyed Susan debris and discard it (don't compost). This isn't a foolproof approach, but it may reduce the number of spores overwintering in that debris that would easily begin next year's infection cycle.
Miri
We rarely recommend fungicide use for leaf spot diseases like this because they require relatively precise timing of application (well before symptoms develop, usually) to kill spores before they enter the leaf tissue, they often require multiple applications per year to keep infections suppressed, and they don't always work well, even when used properly (such as if weather interrupts a spray schedule). Additionally, some fungicide ingredients may risk harm to pollinators or other organisms, especially if used on flowering and native plants attractive to wildlife. Unless a planting of a single species/variety is extensive, it's often just cheaper and easier to replace a few plants that routinely become infected to the point of undesirability than to attempt yearly treatments with fungicide, and in that case, resistant cultivars (if available) or species not as prone to that particular pathogen should be chosen. Nothing is immune to any and all leaf spot diseases, but at least different species tend to have different vulnerabilities.
For now, it is too late in the season to treat for either condition. Fungicides cannot cure existing infection (and few treat bacterial pathogens); they only help to suppress infection at the start of a disease cycle. Cool, wet weather tends to favor leaf infections, and if you happen to wet foliage when irrigating plants during drought, try to do so only early in the day so leaves can dry by nightfall, or just water the soil surface instead to keep leaves dry. It's easier for many pathogen spores to infect leaf surfaces that stay wet for long periods. You can trim off the worst of the leaves, including removing any that are more-or-less fully blackened, and once freezes kill the top growth by mid autumn or so, remove all dead Black-eyed Susan debris and discard it (don't compost). This isn't a foolproof approach, but it may reduce the number of spores overwintering in that debris that would easily begin next year's infection cycle.
Miri
