i've noticed a flat, bright-red blotchy "rash" or discoloration on the underneath of several leaves of my silver maple. what is this and how do you treat it if necessary?
I don't know. People will ban me but, I'd simply spray with both an insecticide and a fungicide as directed on the label. Personally, I'd choose liquid copper for my fungicide and either a horticultural oil or pyrethren derivative, or an inorganic insecticide. In any event, all listed above would do the job. Continue as needed. Follow the label for application intervals.
Easy solution. Firewood.