I had limited success growing hops in OH (not sure where you are) but one of my hills was Magnum. Looks to me like stress from heat/water -- either too much or too little of either. Some vatieties (e.g. Sterling) show this appearance all summer once the heat is on, but not Magnum. Whatever extreme your weather has been supplying, (attempt to) counter it. I don't think it's a mildew.