Brown "streaks" on my tomato plant stem?

Question:I am growing a tomato plant in one of those upside-down grow bags, and I've noticed that part of the stem near the base has some brown "streaks". It looks exactly like streaks of dirty water that have dried on the stem - so much so that I even tried rinsing it off (with no luck of course). The plant seems otherwise fine, with no mottling or spots on any of the leaves. What could this be?

Answers:
Could be bacteria from the water. Try using boiled or distilled water with miracle grow