what is causing the bottom of my evergreens to die and what can I do to stop this?
This is usually an infection in the soil. This can be corrected, simply, by dusting wettable sulfur over the soil around the tree according to the instructions on the package. This will usually stop the spread, but they may never have growth at that spot again. They will have to be pruned and shaped to deal with the injury. Over time, with this treatment, it will recover on its own, and the spread will stop completely.