Could be that plants are`nt getting enough or to much water.Or they might not be getting enough nutrients either.You can areate the soil around the plants then mix some iron chelate into the soil.If the pores on the leaves are clogged this will produce yellow leaves to.You can clean the leaves by spraying them with some spolied milk .Let the milk stay on the leaves for a fwe minutes then wash the milk off.This will clean the pores on the leaves then allow the leaves to breath.Avid gardner for 25 years here