Q.I Think I Have Stem Rot On My Dahlias. What Should I Do To Fix It? Also, I Heard Something About Not Reusing The Soil Because It C
an spread. Is this true? This is my first time growing dahlias and placed them in a big pot with drainage holes. I water it once a week – typically stop watering once the water drips through the bottom. The dahlias are placed in a sunny location, about 3-4+ hours of direct sunlight. I am not sure about the soil conditions.

Certified GKH Gardening Expert
That soil looks very thick and wet. This will not be good for any plant, and will cause diseases, as you have found out.
You will need to use fresh potting soil, but not topsoil. This article will help:
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/using-topsoil-and-potting-soil.htm
You may want to keep fungicides around:
https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
Watering is best done when needed, rather than on schedule. Use a meter if you are unsure of dryness at the 2 or 3 inch depth mark. This article will help:
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-moisture-in-plants.htm
This article will help you with the care of Dahlias:
https://www.gardeningknowhow.com/ornamental/bulbs/dahlia/tips-dahlia-planting.htm