Planting vinca after pansies has been a long time problem. The main disease that affects pansies and vinca is Phytophthora parasitica. The fungus enters the plant at the soil line mostly in warm wet conditions.
This can be passed on to vinca thru the soil. The crown turns soft, watery, and greenish-brown killing the whole plant. There is another soilborne fungus Thielaviopsis basicola called Black Root Rot. The fungus is found across the southern part of the United States in a wide variety of temperatures.
Proper application of fungicides may control Pythium or Phytophthora root rot. Root diseases can be minimized by not planting vinca directly after the pansies are finished. Root fungus thrives when the plants have poor drainage, are over watered, and over fertilized. It is better to wait a year and use another plant along with a fungicide to control
An application of fludioxonil, iprodione, PCNB, thiophanate methyl or triflumizole in a drench will help to reduce spread of the disease from pansies to vinca. Always fungicides apply according to the directions
on the label.