what is afflicting my Mexican sunflower plant after a decade? - Ask Extension
I've had for a decade a sun and drought-tolerant plant that I believe is a helianthus but I don't know its scientific name. The person who gave it to ...
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what is afflicting my Mexican sunflower plant after a decade? #875916
Asked July 04, 2024, 12:14 PM EDT
I've had for a decade a sun and drought-tolerant plant that I believe is a helianthus but I don't know its scientific name. The person who gave it to me said it was some kind of Mexican sunflower. It's sited in a very hot, sunny, south-facing spot along the driveway. For 10 days it thrived, then last year, but especially this year (see photos) it got this mildew-looking film on the leaves. I can wipe it off, but the bottom leaves are already yellowing and since I don't know what this is, I'm not sure wiping will help. Please advise. Thanks!
Multnomah County Oregon
Expert Response
Thank you for your question. Your plant has powdery mildew, described here: https://pnwhandbooks.org/plantdisease/host-disease/sunflower-helianthus-spp-powdery-mildew
You can read that providing more air circulation helps, as well as removing any leaves that have been affected. Once the fungus has established itself, fungicides do no good. Neither does wiping it off, because the fungus has penetrated cell walls. Next year, apply one of the fungicides marked "H" (available to homeowners) before you see any signs of the mildew.
Good luck!
You can read that providing more air circulation helps, as well as removing any leaves that have been affected. Once the fungus has established itself, fungicides do no good. Neither does wiping it off, because the fungus has penetrated cell walls. Next year, apply one of the fungicides marked "H" (available to homeowners) before you see any signs of the mildew.
Good luck!
Hi:
Because this is a very, very sunny spot, with good air circulation, I'm wondering how it could be subject to mildew. Since I won't use chemicals in my landscaping (I'm assuming the "H" fungicides you refer to are not an organic, natural product?), I may need to remove this plant and replace with with something that isn't vulnerable to this fungicide. Especially since it sounds like it's now in the soil. Do I understand this correctly? Thanks so much!
Kyna Rubin
We live in the fungus capitol of the world, and powdery mildew is an airborne fungus, not a soil borne one. Our humidity is high, and our cool temperatures are perfect for most fungi. Soil fungi cause “root rot”, not mildew.
The fungicides on the list labelled “O” are organic, but may be difficult to obtain.
The fungicides on the list labelled “O” are organic, but may be difficult to obtain.
Welcome!
