You probably can pinch the lower branches off, and actually help the growth of the plant. Your plant can grow pretty big (4 feet) so to encourage tall growth (and keep the leaves away from the dirt, which probably caused the yellowing) pull them out.
If it goes on, pull the plant out (don't compost it) and be thankful it is early in the year.
It's probably not the water, unless you're watering directly on top of the plant and causing them to get muddy. Try watering at the base and making a little dirt moat around the plant so that the water goes into the ground and the roots have to grow down to get it.