Same word as disimprove:

abate, backslide, cheapen,decay,decrease,degenerate,depreciate,deteriorate,diminish,disimprove,disintegrate,droop,drop,dwindle,ebb,fade,fail,fall, fall off,flag, go downhill, go to pot, go to the dogs, hit the skids,languish,lapse, lose value,lower,pine,r