Where is that discoloration on the plant? Top, bottom, or middle? It doesn't really look like much to worry about. With how dark the leaves are elsewhere I would imagine its not a nitrogen deficiency. It could just be a weird defect that happened to crop up. The first thing I would do would be to check the runoff ph or make a soil slurry and check its ph to make sure it is not some sort of ph issue. If your ph is good I would probably just let it ride for now and see if it gets any worse. It is just at such an early progression that it is really hard to identify.