Powdery mildew, a form of mold, can occur at any phase of the growth cycle poplars. Black spot, rust, pythium, the list goes on and on. Especially the quite humid area of the country he lives in. But I wouldnt worry about an afternoon t-shower, especially in veg, even early flower. If its going to get severe though, just bring it in and put it next to a south facing window. A few days of in and out is not going to hurt it a bit. Just try not to temp shock it. You know, ac cooled house at 72, outdoors 85. Get what I mean?