Lawn Fungus, Causes, Identification, and Treatment

Like bacteria in the soil, fungi are not inherently good or bad. Yet there are countless species that can cause unsightly damage or even death to turf grass if left untreated. 

Because the signs and symptoms of the fungal disease can be quite subtle, many cases go unnoticed by homeowners until serious damage is done. Once you know what to look out for, however, identifying and treating fungal infections becomes a routine part of maintaining a healthy lawn.

Treating lawn fungus requires understanding the most likely causes of such infections as well as how to identify common varieties that affect turf grass. I’ve covered all of this and more below.

Common Causes of Lawn Fungus

No lawn is 100% invulnerable to disease. However, we do know of several contributing factors that make certain lawns more likely to suffer from fungal infections than others. The presence of one or more of these issues can cause what would have been a mild fungal disease to turn catastrophic.

If you’re wondering why your lawn fell victim to a fungus while your neighbor’s lawn remains unscathed, one of the factors below could be to blame:

1. High Night-time Temperatures

Fungi require very specific conditions to thrive and reproduce. Many common lawn diseases are kept at bay by cool evening temperatures. Unfortunately, the opposite becomes true when night-time temperatures are nearly as high as daytime ones.

In terms of disease prevention, there’s not much you can do about this contributing factor. Just be aware of any changes to your lawn’s appearance or health when night-time temperatures are elevated.

2. Humidity

Nearly all fungal diseases that affect turf grass prefer moist conditions. When air humidity is high, the grass stays damp longer after rain, morning dew, or irrigation. High humidity can encourage fungal growth even when moisture is otherwise kept to a minimum. All of these scenarios create the perfect breeding ground for fungi in and around your lawn.

3. Overwatering

In my experience, overwatering is one of the biggest contributors to fungal disease in residential lawns. Watering too much or too often allows no time for the topsoil to dry out. Since fungi thrive in consistently moist areas, the consequences of overwatering should be obvious.

4. Poor Air Flow

Fungus is a common problem in lawns with obstructed airflow caused by nearby buildings, privacy fences, and dense trees or shrubs. The most likely reason is that moving air decreases moisture and humidity levels significantly faster than stagnant air. 

Even if your lawn has relatively good air circulation overall, you may notice increased fungal activity in the areas surrounding outdoor furniture, garden sheds, decks, above-ground pools, children’s play gyms, and similar structures.

5. Compacted Soil, No Aeration

You might not realize it but air circulation is just as important below the soil’s surface as it is above ground. Proper aeration allows oxygen molecules to penetrate the soil and reduces fungal growth. The most common cause of poor soil aeration is compaction. Compacted soil is a problem for plant health because it contains few if any, natural air pockets.

A depressed footprint in damp soil is an example of mild compaction with many lawns slowly becoming compacted over several years simply due to everyday foot traffic.

However, severe compaction can occur when vehicles or heavy garden equipment are driven or parked on turf grass. 

6. Excessive Fertilizer

Applying more fertilizer at one time than your lawn can use may encourage fungal growth. 

Like plants, fungi love nitrogen. So the fungi living in your lawn will happily soak up any excess nitrogen left behind by overfertilizing. Too much fertilizer can also stress turf grass and leave it more vulnerable to fungal infections. 

7. Mowing Too Short

While mowing your lawn extremely short may seem like an easy way to keep your landscape looking clean and tidy, it can also lay the groundwork for fungal infection. 

Each variety of turf grass has an ideal mowing height that will keep it strong and healthy. Mowing below this recommended height will stress your lawn and create opportunities for fungi to take hold.

How to Identify Lawn Fungus

You can’t effectively treat an illness without first knowing the cause. This is as true for plants as it is for any other living thing.

Fortunately, lawn fungus identification doesn’t require being a mycologist. Since a surprisingly small number of fungus species are responsible for the majority of diseased lawns, you should be able to narrow down the culprit fairly easily.

To get you started, here are seven types of fungi likely to be found in your backyard:

Red Thread

red thread.

If your lawn appears to have pink or red hairs growing from its blades, the most likely culprit is a fungal disease caused by Laetisaria fuciformis. The pinkish-red areas usually start quite small but will quickly enlarge if not promptly treated. While ailing grass will stay green at first, it will eventually dry out and turn brown.

The red thread is most commonly seen during rainy, cool seasons such as spring and early summer. According to Penn State University, Kentucky bluegrass, fine fescue, and ryegrass are typically affected by this disease.

Fusarium Disease

Fusarium Disease

Fusarium blight is a common fungal disease caused by both Fusarium culmorum and Fusarium tricinctum. The most common symptom of this condition is gray or yellow patches of dry grass that range in size. Sometimes, affected patches will form a ring with green grass in the center.

This fungus affects all cool-season turf grasses but Kentucky bluegrass is the most vulnerable. In contrast to many other fungal diseases, Fusarium blight is most likely to occur with heat and drought.

Dollar Spot Disease

Dollar Spot Disease.

Dollar spot is the common name for a fungal disease caused by Clarireedia jacksonii. According to the University of Massachusetts, lawns growing in dry and low-nitrogen soils are most susceptible to this fungus.

From a distance, this disease presents as small patches of yellowed grass blades. You’ll often see what looks like fine spiderwebs when moisture is present. A closer look at individual blades affected by this fungus often reveals visible lesions resembling bleached horizontal stripes with brown margins.

Fairy Ring Disease

Fairy Ring Disease.

Fairy rings aren’t just something found in storybooks — they’re a very real phenomenon. Unfortunately, they’re the result of fungal disease rather than any magical creature.

Fairy ring disease presents as dry, discolored grass blades that form a full or partial circle with healthy grass on either side. According to the University of Wisconsin, these fungal rings can reach up to 15 feet in diameter. Some fairy rings also produce mushrooms.

Lawn Rust

Unlike the real deal, lawn rust is not caused by oxidizing metal. However, it’s easy to see how the fungal disease caused by Puccinia spp. got its name. Lawn rust typically appears as an orangish powder on grass blades. The powder (which consists of fungal spores) is easily spread from blade to blade.

According to Iowa State University, you’ll often see signs of rust in the middle of summer when there is a lot of moisture and temperatures between 65 and 85. Evening rain or watering creates the perfect conditions for lawn rust to form.

Mushrooms

lawn mushrooms

A common misconception is that mushrooms are fungi in their entirety. However, it’s more accurate to think of mushrooms as the metaphorical flowers of fungi living under the soil’s surface.

Many fungi spread via mushrooms that send out spores. You may want to remove mushrooms from your lawn — even if there are no other symptoms of the fungal disease — to prevent further spread. (Before you dispose of those mushrooms, however, consider using them to help you identify the type of fungus living in your yard!)

Not all lawn mushrooms are harmful. Mushrooms normally appear when there is a lot of organic material in the ground that needs decomposition. Since they favor moist conditions, a flush of mushrooms may also indicate poor drainage.

Powdery Mildew

Powdery mildew is easily one of the most prevalent fungal diseases in the horticultural world. While there are many fungus species that cause this disease, Erysiphe graminis is the one responsible for powdery mildew symptoms in turf grass.

Patches of what looks like fine spiderwebs are the first sign of powdery mildew in the lawn. The next sign — from which the disease gets its name — is a layer of gray-white powder on small sections of grass. As the disease progresses, the grass itself may turn white and, eventually, yellow.

Treating Lawn Fungus

Like the common cold in humans, some plant diseases are inevitable. For that reason, I don’t believe you should beat yourself up if your lawn succumbs to a common fungal illness. 

However, there are steps you can take to prevent or minimize cold symptoms — e.g., wash your hands, take vitamin C, or wear a face mask. The same is true of plants and their diseases.

Below you’ll find what I consider to be “good hygiene” for turf grass prone to or actively suffering from the fungal disease:

Dethatch Your Grass

Thatch is a natural layer of mostly dead grass tissue that forms on the soil’s surface. An excessively thick layer of thatch may be visible, especially where your lawn meets surfaces like pavement or gravel. However, most thatch layers are hidden beneath the healthy grass blades.

While thatch is perfectly normal, it can pose a few problems if left to grow too thick. One such problem is that it provides a protective home for fungus spores to live and multiply.

Most grass varieties should be dethatched every few years as needed. Keep in mind that a thin layer of thatch is often beneficial to grass health — you should always leave some thatch intact.

Aerate Your Turf

Aeration is by far the best way to fight compaction and improve airflow within the soil. You’ll get the best results from core aeration, which removes small plugs instead of simply piercing the topsoil.

I typically recommend aerating once per year unless your lawn’s health requires otherwise. Cool-season grasses should be aerated in spring or fall. Warm-season grasses should ideally be aerated in early summer.

Amend Soil pH to Suit Your Turfgrass Type

Healthy turf grass will always stand up better to fungal diseases. One way to improve your lawn’s health is by ensuring the soil pH is at an appropriate level. Each variety of grass prefers a certain pH level, so be sure to reference those specific guidelines before moving forward.

Some research shows that fungi prefer acidic soil conditions. You may see an improvement in fungal activity by raising your soil’s pH slightly (while remaining in the suggested range for your grass type).

Only Water Your Grass Early Morning

Watering your lawn in the morning allows plenty of time for the grass to dry before nightfall. Watering after mid-day means that the lawn will stay damp for much longer, creating an ideal environment for fungi to reproduce and spread.

Apply a Fungicide Treatment

Serious fungal infections may require chemical treatment. Products that kill fungi are called fungicides. 

Not all fungicides are effective against all diseases, so be sure to identify your lawn’s assailant before proceeding with this strategy. I recommend reaching out to a local university extension office or similar resources for sample testing. This is the best way to get a positive ID on a fungal disease and determine the best chemical treatment.

Despite the efficacy of many fungicides, I still advise treating mild fungal infections without them. Some fungi will become resistant to common fungicides if exposed to them year after year. I suggest reserving the use of fungicides for particularly serious infections or lawns that are too weak to fight off disease on their own.

Apply a High Nitrogen Fertilizer

There are several fungi that appear when there is too little nitrogen in the soil — dollar spot disease is just one example. Underfertilized lawns will respond well to an application of nitrogen at least once per year.

As always, be aware that you can apply too much of a good thing. Applying excess nitrogen will do more harm than good in the fight against fungal disease. I recommend conducting a basic soil test to determine how much if any, nitrogen your lawn needs to thrive.

Verdict: Fixing Lawn Fungus

Whether healthy or not, fungi can be found in all residential lawns. Often, they’re invisible to the naked eye. It’s only when fungi start overtaking turf grass and causing visible damage that they become a problem.

Unsurprisingly, prevention is the best strategy against fungi. Maintaining a healthy lawn with plenty of airflows will stop most infections in their tracks. If the fungal disease does take hold in your lawn, following the cultural practices laid out above and — only if necessary — applying fungicide should return your yard to its former glory.