My four o'clocks are very large and seem to be healthy. They flower and part way into the season the leaves wilt down, become limp, and there is a dark ring at the base of the stalks. The plant will eventually die. I have tried using insecticide granules for cut worms or grubs. I sprayed a fungicide in case it might be that. The ground is damp but not soggy wet. They grow in an area in my yard where they are sheltered from the direct mid day sun.