I just bought this plant about three to four weeks ago. It seemed healthy at the time of purchase and it is in a size appropriate pot.

I went out of town for three days and came back to several dark brown spots on the edges of the leaves that spread quickly and vastly within a week (photo below). a few of the leaves have now fallen off and I’m afraid that all leaves will soon be affected. the worse leaves are at the bottom and the higher up on the tree, the fewer the spots. The top portion of the soil seemed dry when I checked but not sure what we should do at this point. Any help would be much appreciated. thank you!!