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Is this orange powder on my euphorbias rust? Do I need to destroy? #752121
Asked May 25, 2021, 6:01 PM EDT
Hello! I just noticed this bright orange powder all over my Euphorbia characias 'Wilcott' (Euphorbia 'Silver Swan'). The powder is on two of the three Silver Swans that I have. They are about 4 feet apart from each other; the third plant is about 7 ft away.
The plant pictured has powder all over the back (wall facing) side, from top to bottom of stems. It is a 2 year old plant (2.5 ft high, 2.5 ft wide) that has been very healthy until now. The other infected plant was planted last month. (It has just a few orange powder spots.)
I have two other varieties of euphorbias in the same area, 1 specimen of each, and neither of them has any orange so far.
All these plants are on tiered flower beds on a compact, west-facing backyard hill with full sun and intense wind exposure. Drainage is good--planted in a mix of compost, sand, dirt. I would guess that poor air circulation is a factor, since the wall-side is worst, but I'm not convinced, as it's such an exposed site.
We've had very variable weather in Seattle recently, but showers every few days, or frequent watering for nearby new plantings. It was in the 60s for several days, and then two days ago the temperature went down again. That is the only remarkable change recently.
Diagnosis: I'm guessing this is a type of rust fungus. Do you agree?
Next steps: Do I need to remove the entire (pictured) plant, or just the infected parts, if possible? And remove the few affected spots on the younger Silver Swan plant? I don't see any orange on my other plants. My rose bush just has some black bugs crawling around the flowers right now! Do I need to worry about this spreading?
Many thanks,
Gwen
King County Washington
Expert Response
Hi Gwen,
Thank you for contacting us!
We can't be sure of a diagnosis without examining a sample, but the symptoms in your photos do appear consistent with a rust-type pathogen. In researching this, I discovered that rust on spurge is one of the few types that appears on both sides of the leaves. Most are only visible on the underside.
Treatments for this are preventative rather than reactive or curative. This unfortunately means that it's hard to stop them once they have started. I've copied a link to more information about rust diseases on ornamentals here: http://hortsense.cahnrs.wsu.edu/Search/MainMenuWithFactSheet.aspx?CategoryId=12&ProblemId=4009
Most rust diseases are host specific and won't spread to other species, however they do spread among plants of the same species. The situation you describe seems like it should favor healthy growth. Most spurges do well in a slightly challenging environment, and the wind would normally do a good job of blowing spores away from plants.
Depending on your feelings about the plants, you might remove the worst affected one and cut off infected parts on the other; remove both plants and hope it doesn't appear on the third. The most recently planted spurge likely wouldn't tolerate being pruned to the ground, but the other might. (It would be a risk, as it might not recover.)
It might help to talk to a Master Gardener in person, or bring a sample to one of their clinics. I've attached a link to a map with more information about dates and times of these clinics.
I hope this helps, but let us know if you have additional questions.
- I don't see a link to the Master Gardener map and clinic info-- could you please resend?
Hi Gwen,
It sounds like you are on the right track. You are right that pruning back the older plant may not be enough, but there is no harm in trying.
Sorry about the link to the Master Gardener plant clinics, I don't know what happened, but I'm trying again here: https://www.mgfkc.org/resources/map-of-mg-clinics-and-gardens
You can also email them directly at this address: <personal data hidden>. It might be worth getting a second opinion. They may have locally relevant knowledge that I don't.
Thanks again for your question!
