The stem is just sunburnt. This isn't a problem. The yellow leaves suggest you need more nitrogen. Find a fertilizer with a high nitrogen ratio. The 3 numbers on the bottle represent N-P-K. You want a fertilizer with an N number higher or close to the other two. So 3-1-2 would be a good choice but 5-10-21 would be a bad choice because the other two nutrients are so much higher.