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