Some time prior to 2246 a disaster of some sort had taken place on Epsilon Sorona, requiring Starfleet to evacuate the approximately 6,000 colonists. These refugees were subsequently resettled on Tarsus IV.
Unfortunately, seeds brought from the planet reacted poorly to the soil on Tarsus IV, causing a fungal infection to arise which attacked the food stores on Tarsus IV. Using the crisis as an excuse, Adrian Kodos briefly seized control of the colony and had 4,000 colonists put to death Many of the refugees from Epsilon Sorona died in the resulting massacre. (DSC novel: Drastic Measures)