Lawn Fungus Identification Guide | Which Common Fungal Disease Is In Your Grass?


Apr 3, 2020

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After careful manicuring and fertilization of your lawn, the last thing you want to see are defects. When defects, like bare spots or yellowing, crop up, you may immediately think watering and fertilization is all you need. In some cases, though, you’re dealing with a lawn disease caused by a fungal outbreak that requires more than just maintenance.

In our list of common fungi-caused lawn diseases, you’ll learn how the fungi appear and what to do to eliminate and prevent them. You’ll also learn whether the condition is harmful to your turf or not.

Brown Patch

Brown patch, which is caused by Rhizoctonia fungi, is one of the most common lawn diseases you’ll see in cool-season grasses, but it can also affect many warm-season grasses too. The disease becomes most prolific when evening temperatures in the summertime reach 65-70 degrees F. Brown patch starts small and almost unnoticeable but quickly rages out of control, which is why early detection and treatment are essential.

Identifying Brown Patch

Brown patch shows itself as large spots of grass that appear dry or dead. The patches are generally circular or are in an irregular circle that can be up to 3 feet wide. The outside of the patch is often darker than the inside.

Correcting Brown Patch

The best way to prevent brown patch is to avoid overfertilizing or overwatering your lawn, as these can cause the fungus to form and spread more rapidly.

If you’ve already got a brown patch case, you can rectify it with a fungicide designed to wipe out Rhizoctonia. The most effective fungicide is one with fludioxinil as its active ingredient.

Experts recommend only using fungicide as a last resort in high-value turfs, as your lawn will repair itself with proper fertilization and watering.

Pythium Blight

A wide range of Pythium fungi, such as pythium aphanidermaturm, pythium graminicola, pythium ultimum and other, cause an unsightly lawn disease called Pythium Blight. This disease, which also goes by the names cottony blight, spot blight or grease spot, generally impacts cool-season grasses during the hottest and most humid months in the summer.

Identifying Pythium Blight

Pythium blight initially shows itself as 1- to 3-inch orange-colored spots on your lawn. In some cases, the outer edges of these spots have light-gray rings early in the morning. As humidity remains high, the spots will start growing cobweblike fluffy masses of mycelium.

Correcting Pythium Blight

The best way to combat Pythium blight without replacing your lawn is to use a fungicide containing mefenoxam.

If you have a consistent issue with Pythium blight, you can also use mefenoxam as a preventative by applying it in 10-day intervals or as directed by the product’s instruction.

Pink Snow Mold

Despite its name, pink snow mold is only pink for a short time and doesn’t need snow to form. It thrives in areas where it is cool and humid, like in the Pacific Northwest, and can occur year-round. Microdochium nivale is he fungal culprit behind pink snow mold.

Identifying Pink Snow Mold

When your turf gets pink snow mold, it will become matted or turn straw-colored and form a white- or pink-colored cobweblike coating. This generally occurs in the early spring after the snow has melted but can occur anytime the temperature and humidity is right. As the weather warms up, the spots shrink.

Correcting Pink Snow Mold

Generally, pink snow mold fades away in warmer temperatures, and your turf will regreen quickly. To accelerate the recovery, you can gently rake the area to aerate the grass then allow it to dry out. Once the grass is dry and you’ve mowed several times, you can rake out any thatch and continue mowing at a shorter-than-normal length until the mold has subsided.

If pink snow mold is a yearly issue for you, apply a thiophanate-methyl fungicide to your yard just before the first snow to prevent the mold from growing come springtime.

Gray Snow Mold

Gray snow mold, which is caused by various Typhula fungi species, prefers to grow once the snow melts and the ground is moist and cool. That said, it can also arrive without snow if the climate is damp and cool.

Identifying Gray Snow Mold

You can identify gray snow mold by its straw- or gray-colored infection centers that are generally 6-12 inches wide. The telltale sign of gray snow mold versus pink snow mold is the turf is matted down by a grayish mycelium, which is a cobweblike material.

Correcting Gray Snow Mold

Like pink snow mold, your turf will regreen as the temperatures warm and the fungus dries out. If you want to expedite the process, lightly rake the infected area to aerate the grass and mow as normal. After several mowings and the grass has dried out, rake out the thatch and mow at a lower setting than normal until the mold is no longer visible.

Summer Patch

Summer patch is a nasty summertime disease caused by Magnaporthe poae fungi that affects mostly cool-season grasses. The fungus starts its process in the late spring or early summer, but you likely won’t notice the damage until the grass is under its highest stress in late summer. Once the damage starts showing in late summer, there is little you can do, which is why prevention is critical.

Identifying Summer Patch

Summer patch is identifiable by its 2-inch dark-green wilted patches of grass that become yellow or brown as the patches grow. Generally, the grass dies from the tip down. The earlier you can catch the summer patch forming, the more likely you are to save the turf.

Correcting Summer Patch

Once you notice the 2-inch dark-green wilted patches, the fungus has already started its path of destruction, and there are no chemical treatments. The only action you can take is to increase your irrigation to help your lawn recover.

The following late spring, or when soil temperatures reach about 65 degrees F, you can prevent the summer patch from returning by applying a propiconazole fungicide.

Necrotic Ring Spot

While necrotic ring spot, which is caused by the Ophiosphaerella korrae fungus, is a severe condition to any lawn, it is particularly harmful to Kentucky bluegrass. The fungus feeds on the soil, thatch and dead leaves in your yard and kills your lawn’s root system in the process.

Necrotic ring spot generally shows up in the cooler, wet months of spring or fall.

Identifying Necrotic Ring Spot

Necrotic ring spot, like many lawn diseases, starts with the grass fading to a yellow to light green color before eventually thinning out. The telltale sign of this severe lawn disease is its pattern, as it often created 3- to 15-inch frog-eye-like rings in your yard. In some cases, these patches can be up to 3 feet wide. After this pattern emerges, the grass eventually dies.

Correcting Necrotic Ring Spot

The preferred method for controlling necrotic ring spot is with maintenance. This includes frequent aeration to ensure deep root growth, proper irrigation and a balanced nitrogen fertilization program.

If you must take the chemical route to control necrotic ring spot, it’s all about timing and temperature. You must apply a fungicide when the fungus is most active, which will be when the 3-inch soil temperature is 60-70 degrees F. This fungus attacks at the root level, so the fungicide will be most effective when applied right before or after irrigation.

There is a wide range of fungicides that correct necrotic ring spot, including those containing thiophanate-methyl, propiconazole or fenarimol.

Rust Disease

Rust disease is a common lawn issue caused by various fungi. It’s generally most prolific between spring and fall when temperatures are 68-86 degrees F. It is a fan of wet weather but can also crop up in sunny conditions after a spell of wet, humid weather.

While rust disease can hit virtually any grass type, it’s most fond of ryegrass, tall fescue, Kentucky bluegrass and zoysiagrass.

Identifying Rust Disease

Irregular yellow patches of grass in your yard are telltale signs of rust disease. You can also check the individual blades of grass and find small yellow flecks. As the disease takes hold, these fleck rupture and cover the grass is a light-yellow powder that rubs off on your shoes and pants as you walk through your yard.

Correcting Rust Disease

The initial correction process for rust disease is through maintenance. Fertilize and water your lawn as recommended. This should be all you need, but if things don’t improve in about three weeks, you may have to resort to a fungicide containing propiconazole.

Red Thread

Red thread, which is caused by the fungus Laetisaria fuciformis, thrives in climates that offer warm days and cool, moist evenings, making yards in northern states frequent victims of this unsightly disease. While red thread generally doesn’t harm your lawn directly, it does make it susceptible to other diseases and pests, so ridding your turf of it will help keep things green.

Identifying Red Thread

Red thread is relatively simple to spot. From a distance, you will notice patches of grass with a reddish hue. This is the red strands of the fungus showing through the grass. When you approach the patch, you’ll notice the grass blades are bleached or yellowing. Patches are generally no more than 2 feet in diameter.

Red thread generally thrives in low-nitrogen soils and in climates where the temperatures is 68-75 degrees F and humid.

Correcting Red Thread

Experts don’t recommend treating red thread chemically, as it’s simple to control by strengthening your existing lawn. Fertilize as recommended with nitrogen and keep your soil aerated. Over the course of about two years, the red thread will thin out and disappear.

For extreme cases, you can opt for a fungicide containing strobilurins, but this is generally limited to professional use.

Powdery Mildew

Powdery mildew, which is caused by Blumeria graminis fungi, doesn’t discriminate. It will impact nearly any type of plant, even your grass. It thrives in warm, wet weather, making the southeastern U.S. a hotbed for this disease. It doesn’t cause any lasting damage, but it can be rather unsightly.

Identifying Powdery Mildew

Powdery mildew is easy to spot and almost impossible to mistake for any other lawn disease. This disease appears as white and grey powdery spots on your grass.

Correcting Powdery Mildew

Though it won’t impact your turf’s health or transmit to other plants, powdery mildew is not attractive, so obliterating it may be in your best interest. There are many fungicides designed to treat powdery mildew, including those containing metconazole, myclobutanil, propiconazole, tebuconazole, triadimefon or triticonazole.

Fairy Ring

More than 40 species of fungi cause fairy rings, so it’s no shock there’s not a single grass species immune to it. It thrives from spring through fall and can create havoc in your yard if left untreated.

Identifying Fairy Rings

There are three types of fairy rings, but only one type has a lasting negative impact on your yard. These harmful fairy rings are unmistakable. The harmful ones are large, irregular rings strewn throughout your yard. They can range from dark green to a tan color but the real telltale signs are puffballs or mushrooms appearing along the ring.

Correcting Fairy Rings

Correcting these fairy rings can be a chore, as they impact the grass and the soil. Start with thorough aeration of the impacted area and about 3 feet beyond the edge of the ring. Add a wetting agent to the area to help break down the hydrophobic qualities the fungus creates in the soil. Water your lawn for 5-10 days straight. Once the lawn responds, add fertilizer to spur growth, reseed or install new sod in the area.

Dollar Spot

Dollar spot is a widespread lawn disease caused by the fungus Sclerotinia homoeocarpa. Dollar spot prefers perennial ryegrass, fine fescue and centipede grass, but tall fescue, Kentucky bluegrass, zoysiagrass and Bermudagrass are far from immune to it.

Dollar spot is most prolific during the summer and can cause lasting damage.

Identifying Dollar Spot

Dollar spot is relatively simple to spot with its roughly 1-inch-diameter circles of tan-colored grass. You’ll often observe these spots in small clusters. In the early morning, you may notice cottonlike mycelia in the tan spots.

Correcting Dollar Spot

In residential yards, a fungicide is rarely a necessity, as you can control dollar spot through a fall-applied nitrogen fertilizer and another around mid-summer. You’ll want to apply the fertilizer at a rate of 0.2 pounds per 1,000 square feet.

Some fungicides can kill the fungus and accelerate the recovery process, but you’ll want to enlist the help of a professional to get the job done.

Gray Leaf Spot

Gray leaf spot is a common disease found in St. Augustinegrass caused by the fungus Pyricularia grisea, but it has also made its way to annual ryegrass, perennial ryegrass and tall fescue recently. This unsightly disease can ruin a great-looking yard in no time, so quick detection and elimination is critical.

Identifying Gray Leaf Spot

In St. Augustinegrass, gray leaf spot starts with tan to gray spots in the grass blades that are depressed in the center and have purple or brown borders.

Gray leaf spot is harder to detect in cool-season grasses, as they generally don’t show spots. Instead, the disease shows as small pinprick lesions. If left untreated, these lesions quickly turn water-soaked and progress to twisted necrotic leaf tips.

Correcting Gray Leaf Spot

The first step in controlling gray leaf spot is getting back to normal maintenance. Fertilize and water your yard as recommended and frequently scout your yard to watch for improvement. Fungicides are not recommended except in circumstances where natural treatments are ineffective.

If you feel fungicides are the only way, azoxystrobin, pyraclostrobin or fluoxastrobin can control gray leaf spot for up to 28 days. Because fungicides are temporary solutions and require careful use, consult a lawn care professional to get the job done.

Dog Spot

Dog spot isn’t a fungus, nor is it a disease. It is, however, an unsightly and often misunderstood lawn issue. It’s no mystery that dog urine kills grass, but what is a mystery is why it kills the grass. Some claim it’s the pH that kills it, but this is untrue. What kills the grass is the high nitrogen content in the urine. Much like overfertilizing, this high nitrogen content burns the grass.

Identifying Dog Spot

Dog spots are relatively simple to identify, as they are random, irregular patches of brown grass. They have no other identifying characteristics of a fungus, including the cottonlike material found in some fungus-caused diseases.

Correcting Dog Spot

There’s not much you can do to a dog to change the nitrogen concentration in its urine. Instead, train your dog to urinate in a mulched or rocky area to prevent this issue.

If you have an existing dog spot, fixing it is simple. If the spot is smaller than your fist, just let it be. The surrounding grass will eventually take over. For larger spots, rake out some of the dead grass – no need to rake it all out – and reseed the area.

Leaf Spot

Leaf spot refers to a wide range of plant diseases caused by bipolaris, drechslera and exserohilum fungi and can affect a wide range of grasses, though they prefer bluegrass and Bermudagrass. Leaf spot causes thinning and an unsightly appearance in its early phases, but it eventually causes rot, which can start killing your turf.

Identifying Leaf Spot

Leaf spot creates random brown spots on the grass blades, which is easy to mistake for insect damage or drought issues. If left untreated, it can reach the melting out stage. At this point, the grass will turn a reddish-brown and eventually yellow, wilt and die due to rot.

Correcting Leaf Spot

Preventing leaf spot is as easy as mowing at the correct height and proper fertilization. This creates a thick, lush lawn that’s resilient if leaf spot appears.

If you choose to go the fungicide route, QoI fungicides (stobilurins) are your best bet to get things under control. Fungicides containing iprodione have proven effective too. Because many of these fungicides are heavily regulated, you may have to call a lawn care professional.

Slime Mold

Slime mold, which is generally from fungi in the genera Mucilaga or Physarum, may not be overly harmful to your lawn, but it creates odd patches of discoloration that stand out in your otherwise-green lawn. Ridding your lawn of this disease will get it back to its lush, even appearance.

Identifying Slime Mold

Slime mold is fairly obvious. One type of slime mold coats your grass in a greyish material that may be slimy when wet but easily wipes off when dry. From a distance, this slime mold will look almost like someone dumped ashes on your yard.

The second type of slime mold looks like a yellowish blob and almost resembles dog vomit. As it ages, this yellow blob will turn grey and powdery.

Correcting Slime Mold

Slime mold won’t harm your grass, but it’s still an eyesore. You can rectify it by letting nature run its course. If you have a significant infestation that you want to get rid of, you can rake it out or spray it off with moderate water pressure.

Time to Identify and Cure Your Lawn Disease

With a firm grasp of the various grass fungi and the diseases they cause, you can now scout your yard effectively and sniff out these diseases. In most cases, quick action is essential, so use this new understanding to get a quick jump on these fungi and diseases before they ruin your gorgeous turf.