Blight is the first thing that springs to mind but a lack of water or to much heat can also cause leaves to turn yellow. It is a fine line between to much or not enough of both heat and water and it can get quite frustrating! Feel the soil around the plant and for a few cm below the surface and if it is dry then up your watering. If you have them in a greenhouse then make sure there is plenty of ventilation. I leave my door open during the day and close it up at night.