So many times I have heard "I have brown spots in my lawn, it must be a fungus". No it's not fungus, it probably something else. Why? Fungi are such pathetic weak pathogens, what little damage they do is cosmetic, sub-lethal and temporary. Fungi are not invisible ninjas, moving silently in the shadows killing your lawn. Fungi are as delicate as a snowflake, so fragile, they can be stopped by a puff of air or a ray of sunlight.
If Fungi are Harmless, Why All the Fungicides?
Golf courses and marketing drive the promotion of fungicides. A fungus on a golf green will cause an uneven putting surface and an ugly blotch on the green. Fungicides keep the green smooth and appealing. That same fungus on a St Augustine lawn mowed at 4" is barely even noticed, much less a reason for concern. But fungicide makers have learned that the homeowner is willing to buy fungicides to rid their lawn of the boogey man - fungus.
Fungi and Your Lawn
Fungi must have perfect conditions to damage your lawn. Whatever those perfect conditions are, as soon as the conditions change - i.e. the sun rises, the wind blows or leaves dry - the fungus is dead. It is no longer viable. It cannot damage your lawn any longer. Most fungi can do little more than damage a few leaves which are replaced by the grass plants within days. The average homeowner is more likely to mistake water stress for a fungus problem and think a fungus problem is normal.
Fungi are Beneficial
Fungi are part of the soil microbial community and necessary for the enzymatic digestion of organic materials - composting. Their job as microbes in the soil is to digest dead dying tissue and recycle it into plant food. Under certain conditions - usually very temporary conditions - fungi can attack living tissue. But only while conditions remain perfect. St Augustine turf has few problems with fungus and the few that can cause problems are weak, sub-lethal and temporary. There is no need to constantly treat for fungus in a St Augustine lawn - it does more harm to the ecosystem of the soil than good for the grass.
Some Fungi are Worse than Others
In Pinellas, brown patch is the worse. Brown patch can cause "crop circles" in a St Augustine lawn that are quite ugly. It only occurs during specific weather conditions - the moist cool nights of fall and spring. Prevention is nearly impossible, curative treatments help to reduce damage and the lawn will recover and look great in a week. But even as ugly as brown patch is, it is not a lethal threat that will kill your lawn. It is merely ugly. A temporary ugliness.
You Got a Fungus - We got a Cure, for a Price
Many of the larger lawn spraying companies, like Trugreen, sell fungicide as a add on to their regular service. They have learned what the fungicide makers already knew - homeowners will buy fungicides to get rid of the boogey man in their lawn. It's magical - you had a fungus, we sprayed it and it went away. But in practice applying fungicides to a St Augustine lawn is very profitable to cure a minor problem that would have gone away with the change in the weather - anyway.
My Lawn is Sick, I'll Give it some Medicine
Many homeowners want to cure what ails the lawn by giving it a shot of medicine. The fact is disease caused by fungi are minor, insignificant and temporary problems in a St Augustine lawn. Save yourself some time and money and avoid fungicides as a cure for what ails your lawn. If you want to spend time and money to cure what ails your lawn, upgrade the irrigation system and buy a mower that will mow a lawn at 4". A better irrigation system and mowing at 4" will have a greater positive impact on your lawn than the most expensive fungicide.