A bargain autumn rhododendron with grey lower leaves: a real problem or mostly cosmetic?
You describe a grey, milky coating low down on the leaves. In the photograph, it is on the older, lower leaves near the ground. That tells us quite a lot about the severity and what is worth doing now.
The rhododendron you have brought home appears to have a fairly mild case of powdery mildew. The coating is visible on the lowest, older leaves close to the ground, consistent with a superficial fungus in an area where air is still and leaves stay damp for longer.
As you are planting in early September in Sweden, I would not concentrate on fungicide. Instead, remove the most obviously grey-white leaves, clear away fallen material and avoid wetting the foliage when watering. Your east-facing position with partial sun is unlikely to put the plant under extreme sun stress, but make sure it is not crowded so that the lower growth can dry after dew and rain.
Otherwise, the plant looks capable of establishing, so the more important autumn concern is rooting before winter rather than the mildew itself. Give it acidic, airy soil, avoid planting too deeply and keep the soil evenly, lightly moist until the cold arrives. If new leaves higher up develop the same white coating after you have removed the lower ones, it is worth following up the problem next season.
Sources and images 1
- Original reader contribution ↗Reader question