Why do leaves on japonica turn yellow?

Overwatering, pathogens, pests, soil chemistry and sun can account for japonica leaves turning yellow. Japanese quince (Chaenomeles japonica) and glossy-leaved paper plant (Fatsia japonica) have yellow leaves when the soil pH (power of hydrogen) impedes intakes of present macro- and micro-nutrients. They also show chlorotic shoots from fungal or viral diseases, heavy droughts or floods, improper light and shade and incorrect irrigation schedules.