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Your Guide to Identifying 15 Common Lawn Diseases

Jul 16, 2024

(And When You Need A Professional)

Is your lawn looking patchy, discolored, or off in some way? When lawn disease strikes, your lawn can undergo extensive damage in a short period of time, so It’s important to act quickly to identify the disease and control it before it spreads across your property. 


In this guide, you’ll find all the basics about 15 common lawn diseases in North America, including symptoms and underlying causes.


Brown Patch 

Brown Patch DiseaseDescription: Brown Patch is a leaf disease that can appear on your lawn during periods of hot and humid weather. Brown Patch will often appear first as rough, circular patches varying in size, from a few inches to several feet across. In the early morning dew, you may notice something that looks like purple or grayish-brown cobwebs; this is actually fungal growth. While the disease rarely causes damage to Kentucky Bluegrass, it can cause significant damage to Tall Fescue, Perennial Ryegrass, and Creeping Bentgrass.


Causes: Brown patch is caused by the Rhizoctonia fungi, which infects the grass foliage and crowns. This fungus colonizes the organic matter in the thatch layer, and, during times of stressful conditions, can cause disease in the grass plant. Brown patch is likely to develop in cool season grasses when temperatures are above 85°F with high relative humidity. 



Dollar Spot

Dollar Spot Disease


Description: Dollar Spot is a fungal disease that can affect residential lawns, particularly those with cool-season grasses like Kentucky Bluegrass, Perennial Ryegrass, and Fine Fescue. It’s called Dollar Spot because the affected areas typically show up as straw or tan colored spots that are small, round, and the size of a silver dollar. The spots may also have a reddish-brown border. In severe cases, the spots may merge and form larger patches of dead or dying grass.

Causes: Dollar Spot is caused by the fungus Sclerotinia homoeocarpa, which thrives in warm, humid conditions and spreads through the grass blades by contact or through the air. It can also be spread by mowing equipment, shoes, and other lawn tools.


Fairy Ring

Fairy Ring Disease

Description: Fairy Ring disease is recognized in early summer by the appearance of dark green, circular bands in the lawn. Most of the time, Fairy Ring forms in a circular pattern, but it’s also common to see an unformed circle. Bands can vary in width from 4-12 inches, and rings can vary in diameter up to 50 yards. During periods of wet weather, mushrooms and toadstools may rise along the circumference of the rings.


Causes: Fairy Ring is caused by a fungus called Marasmius oreades, also known as the Fairy Ring Mushroom. The fungus is contained within the soil, as deep as 12 inches below the surface, and creates mushrooms that can range in size from a few inches to several feet in diameter. It attacks at a central point and grows outward along the inner margin of the rings, where the grass becomes very weak and often dies out temporarily. In small rings, the weakened grass may occupy the entire central area. The fungus feeds on dead organic matter in the soil and can also affect the growth of the grass, causing it to be thicker or thinner than the surrounding grass. High soil moisture, poor drainage, and the presence of dead organic matter, such as leaves or tree stumps, can contribute to the development of Fairy Ring.


Leaf Blight

Leaf Blight Disease

Description: Leaf Blight disease typically affects cool-season turfgrasses, such as Kentucky Bluegrass, Tall Fescue, and Perennial Ryegrass. The symptoms of Leaf Blight include elongated, water-soaked lesions on the leaves, which eventually turn yellow or brown and can even cause the leaves to die. In a heavily infested lawn, the damage appears in circular patches that often form large, bleached areas. Closer inspection of the lawn reveals that individual blades of grass are yellowing from the tips down.


Causes: Leaf Blight usually enters the grass plants after the lawn is mowed in the evening,  followed by excessive watering at night. The disease is caused by fungus and is most common in warm, humid weather conditions. It can be spread by mowing or watering equipment, as well as by foot traffic. Overall, Leaf Blight disease can be a frustrating problem for homeowners, but with proper care and attention, it can be managed effectively.



Gray Leaf Spot

Gray Leaf Spot Disease


Description: Gray Leaf Spot is a fungal disease that tends to affect warm-season grasses, such as Bermuda, Zoysia, St. Augustine, and Tall Fescue, in the warm and humid regions of the United States. The disease appears on the leaf blades of the turf as circular or oval-shaped, tan-colored lesions with brown or purplish borders. In severe cases, lesions can develop on leaf

sheaths and stems and the leaves will wither and die. As the disease progresses, the affected area will grow larger and the grass will begin to thin out, turn brown, and die, sometimes killing large areas of the lawn. Unless the disease is controlled, it can cause devastating damage.


Causes: Gray Leaf Spot is caused by the fungus Pyricularia grisea, which thrives in warm and rainy climates.



Leaf Spot

Leaf Spot Disease


Description: Leaf Spot is often grouped with Melting Out disease, due to their similar symptoms and effect on turfgrass. Leaf Spot will appear as small, circular or elongated spots on the leaves of the grass plant. These spots are typically brown or tan in color, with reddish or purplish borders. As the disease progresses, the spots may enlarge and merge together, eventually causing the grass blades to turn yellow or brown and die. 


Causes: Leaf Spot is often caused by a pathogen that does the most damage during the heat of the summer months.



Melting Out

Melting Out Lawn Disease

Description: Melting Out disease has similar symptoms and effects on turfgrass as Leaf Spot disease. Melting Out becomes apparent by the discolored spots on the grass blades, but as the disease progresses, the fungus enters the base of the plant and attacks the crown and roots. Affected turf appears yellowish, thin, and shabby, with irregular patches of dead grass. When these affected areas are raked, dead grass plants are easily removed. Although it can affect most cool season grasses, it also tends to cause most damage in warm season grasses such as bermudagrass and zoysiagrass.


Causes: The Melting Out pathogen is active during cool, wet weather.





Necrotic Ring Spot


Necrotic Ring Spot

Description: Necrotic Ring Spot is a fungal disease in the soil that affects cool-season turfgrass, such as Kentucky Bluegrass, Perennial Ryegrass, and Fine Fescue. It results in circular patches of dead grass that have a distinct brownish-yellow ring surrounding a healthy-looking center. The rings can vary in size, but are generally about a foot in diameter. It’s sometimes called “Frog Eye” because of the shape of the rings.


Causes: Although the symptoms of Necrotic Ring Spot are most apparent during the cool conditions of the spring, when the fungus is active, symptoms can appear throughout the growing season. The disease is often intensified by stress from heat and drought conditions, so

symptoms generally reappear during the hot, dry summer months.





Powdery Mildew

Powdery Mildew


Descriptions: Powdery Mildew is a fungal disease that can infect any turfgrass, but is most common on Annual Bluegrass, Bentgrass, and Fescues. The fungus causes a white or grayish powdery coating to appear on the leaves and stems, feeding on the plant tissue and causing damage. The affected leaves may also turn yellow or brown, and the grass may become stunted and weakened. 


Causes: Powdery Mildew occurs in cool, humid conditions, most commonly in the spring and fall when temperatures are between 55°F and 70°F. It can spread rapidly in heavily shaded areas, periods of low light intensity, and periods of poor air circulation.





Pythium Lawn Disease

Descriptions: Pythium Blight is a fungal disease that affects all types of turfgrass, including lawns, golf courses, and sports fields. If the conditions are ideal for the fungus, Pythium Blight causes the grass to turn brown and die, potentially within a few hours. The grass will appear greasy with sudden wilting or yellowing of the leaves and a foul odor from the affected area. Water-soaked leaves will collapse and become matted together by a cottony, purplish-gray or white mass called mycelium. Damage will often appear in the lawn as long streaks, because the spores and mycelium are spread by mowing equipment or drainage patterns.


Causes: Pythium Blight is primarily found on poorly drained soils with a pH greater than 7.0 and is exacerbated by high nitrogen fertility from quick release nitrogen. The disease thrives in warm, wet weather and can spread rapidly, making it difficult to control. During cooler parts of the season, the fungus grows more slowly and generally does not infect foliage, though in some cases it will cause the roots to rot. 



Red Thread

Red Thread Lawn Disease


Descriptions: Red Thread disease, also known as “Pink Patch,” is a common fungal disease that affects cool season turfgrasses, such as Perennial Ryegrass, Kentucky Bluegrass, and Fine Fescues. Red Thread can appear overnight if the conditions are ideal, making it very difficult to control or prevent. The fungus produces thread-like strands or web-like masses, which range from coral pink to deep red in color, on the tips of the brown grass blades. The strands can protrude up to 1⁄2 inch above the blade and are easily seen, hence the name “Red Thread.” Patches of the disease can grow up to 6 inches in diameter and may become circular or irregular in shape. As the disease progresses, the patches can enlarge, merge, and become more pronounced, leading to a thin, weak, and unattractive turf. 


Causes: Red Thread disease is most prevalent during periods of warm, humid weather, especially in poorly drained or nutrient-deficient soils. Red Thread disease occurs during humid periods of the spring and fall when temperatures are between 60°F and 75°F.




Rust Lawn Disease


Description: Rust is a fungal disease that affects many types of turfgrasses. It is appropriately named for the orange or brown rust-colored spots it produces on the grass blades. The disease can cause the leaves of grass plants to turn yellow, wilt, and die, leading to thin and patchy lawns. Rust favors warm and wet conditions, occurring most prominently in late summer and fall, when there are conditions of low light intensity and temperatures between 70°F and 80°F. 


Causes: Rust is caused by a group of fungi commonly found in soil and plants. It survives on living and dead leaf tissue in the thatch layer of the lawn. When conditions become suitable, the disease will occur quite rapidly. A variety of factors can contribute to the growth of Rust, including improper watering, high humidity levels, and fluctuating weather patterns. Reddish orange spores develop in large numbers on the leaves of the grass plants and are easily rubbed off on shoes, clothing, animals, mowing equipment, or other objects that pass through the infected areas. However, as conditions improve and cultural practices are altered, the grass is normally able to recover.


Slime Mold

Purple Slime MouldYellow Slime MouldBlack Slime Mould


Description: Slime Mold will sometimes appear as a thin layer of slimy, paste-like material that covers the leaf blades. It can appear in different colors and will later dry to form powdery growths on the leaves. It is not harmful to humans or animals, and it does not damage lawns directly. However, it can be unsightly and may cause concern for homeowners who are not familiar with the organism. 


There are various species of Slime Molds throughout North America. Each type differs in appearance and behavior, though they all share the characteristic of being slimy and gelatinous. They are not specific to any certain type of grass and can affect both warm- and cool-season grasses. Some of the more common Slime Mold varieties are: Purple Slime Mold, Yellow Slime Mold, Dog Vomit Slime Mold, and Black Slime Mold.

Causes: Slime Mold is a fungus-like organism that can appear in lawns, particularly during damp or humid conditions, and can spread quickly to cover large areas of the grass. Slime Mold feeds on bacteria and other microorganisms in the soil, so it might indicate an excess of organic matter in the soil, such as dead leaves or other debris.


Snow Mold

Snow Mold Disease

Description: Snow Mold is a disease that commonly occurs in residential lawns in regions with cold winters and snow cover. There are two types of Snow Mold: Gray Snow Mold and Pink Snow Mold. Gray Snow Mold is the more common of the two and appears as circular patches of grayish-white or light brown grass in the lawn. Pink Snow Mold is less common but more severe, and appears as circular patches of pinkish, slimy grass.


Causes: Snow Mold is caused by the growth of fungal spores that become active under the snow during the winter months. When snow accumulates on the lawn for an extended period of time, it provides a moist, humid environment that is ideal for the growth of fungal spores. Areas of the lawn that are shaded or have poor drainage are more prone to Snow Mold.




Summer Patch Disease

Description: Summer Patch is a disease that affects cool-season grasses, such as Kentucky Bluegrass, Tall Fescue, and Fine Fescue. As the name suggests, it typically occurs during the hot and humid summer months, particularly in areas with poor soil drainage or heavy thatch buildup. Symptoms include yellowing and browning grass with patches of dead or dying turf. These patches may be circular or irregular in shape and can range in size from a few inches to several feet in diameter.


Causes: Summer Patch is caused by a fungus called Magnaporthe poae, which attacks the roots and crown of the turfgrass plant. Damage from this disease will often occur where the grass receives direct sun and is located on south-facing slopes or near sidewalks, driveways, buildings, or otherwise stressed areas. In the cool weather of autumn, the grass may begin to grow into these dead areas again. 



Yellow Patch Disease


Description: Symptoms of Yellow Patch will appear as yellow, brown, or tan patches on the lawn, ranging in size from a few inches to several feet in diameter. The patches may have a distinct yellowish border. The grass in the affected areas may be stunted and have a thin or sparse appearance. In severe cases, the patches may merge together and form large, irregularly shaped areas of dead or dying grass. The roots and crowns of the grass plants may also be affected, leading to an overall decline in the health of the lawn. The damage is usually superficial, but with prolonged periods of wet weather in the late winter early spring, thinning can occur.

Causes: Yellow Patch is a fungal disease that can occur in both warm- and cool-season grasses. It is most prevalent from late fall to early spring when temperatures are cooler and rainfall has been above average.


Controlling Lawn Diseases

A thick, healthy, well-maintained lawn is always the best defense against lawn disease. In most cases, the spread of the disease will be reduced by changes in temperature and weather, but you can also adjust some cultural practices to improve the health of the lawn and limit the spread. 

  • Avoid watering when lawn disease is active.

  • Avoid mowing when lawn disease is active. If you do have to mow, make sure the blades are sharp to minimize the spread of disease.

  • Fertilize regularly. 

  • Aerate to improve soil drainage, reduce soil compaction, and allow water and nutrients to penetrate the soil, reducing the likelihood of lawn disease.

Depending on the size of the infestation and the scope of the damage, other forms of control may be necessary and you may need to repair heavily damaged areas with sod or the appropriate grass seed. 

If you suspect that any of the diseases listed above are affecting your lawn, contact your local Weed Man for a diagnosis and customized treatment recommendations.

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