What Causes Fungus in Grass?

There are several causes of fungus in grass, most of which are just the result of bad lawn care practices.

Quick Answer

In my experience, the top causes of turfgrass fungus, starting with the leading issues, are:

  • Excessive Heat and Humidity 
  • Undernourishment 
  • High Night-Time Temperatures 
  • Poor Air Flow 
  • Compacted Soil 

I provide more details on each cause below, with my experience in treating the fungus for each scenario.

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1. Excessive Heat and Humidity 

To reiterate, fungus occurs when your lawn is under stress, such as if you overwater including excessive rain, or underwater your lawn, and/or during times of drought.

Heavy watering and long periods of rain leave water on the leaf surface of the grass. The mycelia (root-like structure of a fungus) use this accumulated water as a path to travel from leaf to leaf, spreading the fungus along the way.

Sprinkler in lawn affected by fungus

Using your lawnmower or even walking on your lawn when it’s battling fungus can help spread the fungus across the grass.

2. Undernourishment 

As a homeowner, you probably already know about proper fertilization practices, but many of us still make the big mistake of under-fertilization or over-fertilization, resulting in brown borders, and/or fairy rings disease.

A thumb rule is to apply nutrient-rich fertilizer 4 times a year to your lawn. However, it’s important to perform a soil test to determine the right amounts of nitrogen fertilizer or other types of lawn food. But make sure you perform a soil test first before applying any fertilizer to your lawn.

And remember, fertilizer isn’t the only source of nutrients for your lawn. There’s also water from rain or sprinklers that helps the grass absorb essential nutrients like nitrogen that’s already present in the soil.

The faster the nitrogen is used up when it rains or when you water, it will need to be replenished or else fungus will develop on your lawn.

3. High Night-Time Temperatures 

If the night-time temperatures are just as high as daytime temperatures in your area, it could encourage lawn fungi to develop and thrive.

Even though there’s not much you can do in this situation, and I can’t advise you to water your lawn at night as this may provide the right conditions for fungus to grow, you should be aware that this could be a possible reason for lawn fungi growth.

4. Poor Air Flow 

Lawns with poor or restricted airflow have a greater chance of developing lawn fungus. Good airflow decreases moisture and humidity levels much faster than stagnant air.

Poor airflow can be caused by nearby privacy fences, buildings, and dense trees and can increase fungal activity in your lawn, surrounding garden sheds, above-ground pools, and outdoor furniture.

5. Compacted Soil 

Compacted soil affects the flow of water to penetrate the grassroots, which can cause root diseases including fungal growth.

Apart from an infected turf, soil compaction may also cause weak, thin grass, that’s prone to weeds and pests.

There are plenty of issues that can cause soil compaction including poor soil structure. Sometimes soils may get compacted due to an issue you’ve inherited and will need to be addressed a bit differently.

How Does Turfgrass Fungus Spread? 

Grass fungus spreads easily and fast via several different ways including rain, wind, grass clippings, and even your lawnmower.

This is perhaps one of the biggest reasons why lawn care professionals avoid mowing lawns that are infected with fungi without being treated, as the fungi can spread to neighboring lawns.

To prevent the spread of turfgrass fungi, many homeowners wait it out in the hopes of getting rid of it on its own. But not taking action against grass fungus such as fungicide applications and/or improving nitrogen levels can be detrimental to your lawn’s short and long-term health.

Before You Go…

I’ve written a detailed article on the most common types of lawn fungus and their remedies and the list includes fairy ring, dollar spot, powdery mildew, snow mold, and red thread.

Also, check out my article on the best fungus treatment for grass. I provide more details on how to get rid of fungus with herbicides or using natural methods.

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