Question
impatients die with wilting of stem at ground contact. I applied neem based fungicide and changed all soil without any help in next seeding.

Answer
You took the appropriate action by applying the fungicide. However, many soilborne diseases will stay active for up to three years, even after spraying with a fungicide.

At this point, the only alternative is solarization. You need to cover the flower bed with black plastic (3 mil) for 6-8 weeks. This will heat the soil and destroy the spores causing the disease. Once the solarization process is complete, you can replant in the bed next spring.

Good luck, and please write again if I can ever be of assistance.

Regards,

Mike