Whenever it is needed.  

At different times during the growing season lawns may be damaged by fungal diseases, also known as lawn fungus. In most cases the fungal diseases will only cause a slight discoloration of your lawn, but in some rare severe cases it can cause large areas of your lawn to die completely.

Fungal diseases are caused by many things.  Heat, humidity, fertilization levels, excessive rain or drought, and too thick of a thatch layer, are all contributing factors that can cause lawn fungus.

My lawn program is designed to keep fungal diseases to a minimum, but if they do occur I can treat them with different types of controls.  I will keep an eye on your lawn to try to spot any disease problems before they get out of hand and work with you to fix any these problems.