When I use fungicide should I treat the soil too?

If the fungus is on the soil, you could treat it. But, if the fungus is on the plant you should treat the plant.

Related Videos:
Spraying Fungicide
Mixing and Applying Fungicide
Understanding Common Fungicides: An Essential Guide

Related Resources:
Fungicides and Bactericides for Home Fruits, Vegetables, and Ornamentals (UT Redbook)
Using Organic Fungicides
Proper Fungicide Use