old leaves that have no nutrients left to give up is whats yellowing. yup for fertilizing and keep it out of sight for a while would go a long way
when a leaf starts to show the fat layer deteriorate its a sure sign of nutrient deficiency. the skin on top of the leaf is just like your skin. its a layer of fat and protection. it should be thick and waxy on a healthy leaf. the leaf is thin and weak when its not. when you see it start to shred the plant is vulnerable. a good fungicide wont hurt either. i use daconil rotated with physan 20. daconoil leaves a white residue but that wont hurt nothing but your eyes