It's the drought.
cool nights, warm days, and drought contribute to powdery mildew on many plants.

Here is a link that might be useful: WSU powdery mildew

There are literally thousands of different species of powdery mildew, most of which are limited to infecting very specific plant species. IOW, the species of PM that affects azaleas, Erysiphe azaleae, does not affect any other plants besides azaleas and their rhody cousins and is not spread by the lamium, which, if infected with PM, is of another species entirely. PM is generally considered to be a cultural and climate-related disease, typically occurring late season when soils are dry but air is mild and moist and is encouraged by poor air circulation/overcrowding and overhead watering.

Deciduous azaleas are frequently very prone to PM as are certain species/cultivars of rhododendron.

Powdery mildew on lamium is not overly common in our climate but it can happen. Typically plants that are happy in dry shade can develop PM if excessively watered (high humidity) or if overcrowded. Cutting back can clean that up handily.