If you can, smell the bottom of the pot where the excess water comes out. The water that runs out the bottom should generally have a neutral smell; a 'stinky' smell is an indicator that your plant may be suffering from 'root rot' or other problems.
Hydrogen Peroxide in 3% - 35% strength.
Mix 1 cup of 3% Hydrogen Peroxide with a gallon of water. For 35% strength Hydrogen Peroxide, mix one tablespoon to a gallon of water.
Water your plants with the mixture to kill any bacteria living in your root area. Unfortunately, hydrogen peroxide also kills good bacteria, which can protect your plant from future infections.
1cup=0.236lite
1gallon=3.785lite