You can consider yourself very lucky not have encountered this problem before. PM is typically a regular late season resident in PNW gardens - our climate, mild and very dry in summer, is just very conducive to it. Dry soils and and a high dewpoint/cool evenings are a very fertile breeding ground for the spores. To prevent or retard development in the future, make sure your plants get a periodic deep, slow watering through the driest periods of our summers and mulch to help retain soil moisture.

For now, carefully wipe all signs of the disease off the foliage. You can spray with Neem oil or any other remedy you favor for PM (milk, baking soda, etc.) And keep the soil around the plant evenly moist until fall rains appear.

FWIW, PM seems to be more prevelent in heavily shaded conditions. You might consider relocating your hellebore into more sun, which it will probably appreciate anyway. And if it is a form of x hybridus, you can safely remove badly affected foliage without worry. It will replace the foliage. We regularly cut back diseased plants at the nursery without problems.

Wow--I had no idea that dry conditions would lead to this. For some reason, I was convinced that dry conditions would have actually prevented mildew. In fact, once I noticed the mildew, I started watering the hellebore even less in the hopes that that would help! Poor thing. I've probably accidentally been underwatering it for weeks now.

As for the sun, luckily when I bought it from the hellebore grower, he told me that this particular hellebore loves sun, so it's always been in a sunny-ish place.

Thanks so much for your help! I appreciate it. I'll try cleaning the PM off. By the way, if you have time, could you tell me how I would know if it is x hybridus? I have no tags or info for the hellebore, so it's likely I won't be able to determine anything.