Wilt and wither are synonyms of each other. They both indicate that a plant leaf, for example, is drooping from exposure to heat, lack of water, disease, etc. There is a subtle different, in my opinion, but most people would consider these two words to mean the same thing. To me, wilt makes me think of a more droopy, aged plant. Wither, on the other hand, makes me think of dried up plants, which isn't necessarily the same thing.