Thank you for writing.
I need to see a picture of the plant and the underside of the planter.  This time of year, the most common cause is overwatering, soggy roots and root rot. Root rot is almost impossible to reverse and is partly related to the water applied and poor drainage from the pot. Anyway, send me the initial photos and I may ask you to dig out a plant and show me its roots. 

Thank you for the pictures.  This is pretty typical for edema necrosis (water death of leaves.  The veins are engorged, the cells have exploded.  My guess is that the plant has soggy roots that may rotted and the pot has a poor or no drainage system.  Lift the plant up and show me the roots and the bottom of the hole.  How does the pot drain?