Your plant is doing super and nothing wrong at this time. Don't put any fertilizer on it or you will hurt it because she is growing just the way she supposed to do. The older leaves are always going to be fading all the way up until harvest. By the time your spaghetti squash finishes, it will have lost a lot, if not all of the oldest leaves. The leaves store food for the plant and when the plant calls for food the oldest leaves are the first to go. You are doing a fantastic job going your spaghetti squash.