Answer: What you describe sounds like symptoms of Fusarium stem and root rot. Initial symptoms include yellowing of the lower leaves and slight wilting. Examination of the root systems will reveal blackened water-soaked roots. This disease spreads through irrigation water so minimizing water applications may control the spread. If the plants continue to decline, I'd remove and destroy them. If you decide to plant new cordylines, sanitize the planter with a bleach solution and use fresh potting soil. To avoid this problem in the future, water only once a week, making sure that the soil drains quickly after watering.
Best wishes with your cordyline.
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