too much nutes or too little nutes causes the stem to go purple also very low temps aswell i cant see the white splotches on the pics..but i think its powdery mildew and i got that on me plants its fungus systemic i used 9 part of water to 1 part of skimmed milk and soft sponge to wipe the leave then sprayed them with a 10% sodium bicarbonate water this will prevent the PM of growing on leaf again buy making the ph on surface of leaf high