Answer: You are describing classic symptoms of root rot (and stalk collapse) due to overly moist potting soil. I don't know that the plant can be saved or if it would be better to toss it out and start all over again. Overwatering is a common problem with houseplants. When you keep the soil too moist you drive out the oxygen; roots need both moisture and air to maintain good health. Dieffenbachia grows best in fast-draining potting soil that is allowed to dry slightly between waterings. Water the plant thoroughly and then allow the top inch of soil to dry out before watering again. If the leaves begin to droop, that is also an indication it needs water. With your new plant, try to adjust your watering to the plant needs rather than to days of the week (many people water their plants on specific days of the week i.e. Tuesdays and Saturdays, without ever check the soil to see if plants actually need water). Soggy soil will lead to root rot.
Hope you have success with your houseplants!
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