Tomato: Leaf malady

I'm not sure whether to water the tomato or if I'm watering it too much. The leaves are all droopy and it is looking listless. The outside temperature is 25°C and humidity is through the roof. Tomatoes like warm, dry temperatures, so this humidity may be pretty uncomfortable for it.

But the problem I'm most worried about is a white powder on the leaves. It looks like a fungus, but I can't find anything relevant online about it.

If anyone has any idea what the problem is and what the solution should be, please leave a comment.