I have a large corn plant (Dracaena) that began having yellow spots on the leaves a few months ago. The yellow spreads, eventually turn brown and seem burned. I have repotted thinking it was root rot (no rot). I moved it closer to a window because it has been quite dark during Seattle winters; how it continues to have yellow spots and appear limp. What could be causing the problem? Should I keep it away from my other plants? Any advice would be appreciated!