Looks like you are doing good. It is very smart that you are doing your 1st grow more to learn and practice then worrying about too much too soon. The one piece of advice I offer is be sure that if your organic mix needs to be cooked (aged) that you give it the time needed. Most recipes I have seen that need to be cooked go from 1 to 2 months although 1 month seems to be the most common.
The biggest problem most new growers face at the beginning is the patience required and doing the research. Your on the right path. Just for knowing; powdery mildew and many other leaf pathogens are spread by splashing water including rain drops. The water drops hit the infected leaves and some of it bounces onto the next leave and that is how it spreads.
This is why you should not put a healthy plant next to an infected one even if it is a different kind of plant.
I am not so sure that is powdery mildew on the Rhododendron though. The edges on the circular patterns seem too well defined whereas PM has more of a difussed "fuzzy" edge before it eventually blankets the leaf.