Fungus Disease of Lawns

The most common fungal diseases of lawns in the Northeast are: powdery mildew, red thread, rust, slime mold, brown patch, dollar spot, leaf spots, necrotic ring spot, summer patch, Pythium blight, and snow molds.

Powdery Mildew

Powdery mildew is most common in turf from July to September, and occasionally in the spring. It is most common during overcast periods of cool, moist weather. While it is common on many kinds of plants, each plant is infected by a different species of powdery mildew fungus; fungi on grasses will not infect other garden plants. Powdery mildew may appear quite suddenly, usually in shaded locations or areas where there is less air movement., and most commonly on Kentucky bluegrass. The grass blades look as if they were dusted with flour or lime. 

To prevent powdery mildew: increase sunlight and air circulation; reseed with a resistant, shade-tolerant cultivar; avoid excess nitrogen fertilizer; raise the mowing height; and water deeply but infrequently. Fungicides are not recommended for powdery mildew control on lawns.

Red Thread

This disease often results in tan to reddish irregular dead patches. It first appears as water-soaked areas, but later as the disease progresses, one can see red threads protruding from the leaf tips and bridging from leaf to leaf. Insufficiently fertilized lawns are susceptible to this disease. It is seen most often on perennial ryegrasses and fine leaf fescues and occurs mostly in cool (65-70 degree F), moist weather. 

To prevent or reduce red thread: soil pH should be maintained at 6.5-7.0; watering should be infrequent and deep; and soil fertility must be balanced. Fungicides are not recommended for red thread. It will fade as the grass dries out.

Rust

There are many different rust fungi that can infect lawn grasses, but they all produce reddish, yellowish or orange spores that give "rusts" their name. The grass itself may not look particularly diseased until after the infection is well developed. In severe cases, spores may leave a reddish dust on mowers, pants, and shoes. Rust is most common  on Kentucky bluegrass and perennial ryegrass and occurs mostly in late summer and early fall.

The most important way to avoid rust infection is to keep the grass growing vigorously so that it will be mowed before spores can be produced. Balanced fertility, aeration to relieve compaction, thatch removal, and adequate water will optimize grass growth. Watering practices that keep foliage dry as much as possible will help reduce infections by rust and other foliar fungi. Fungicides are not recommended for rust diseases on lawns except in extreme cases.

Slime Molds

Slime molds are a superficial fungus-like growth that may appear suddenly on leaf blades of grass in warm, wet weather. Slime molds are initially slimy but mature to a powdery stage when spores are produced. They vary in color from black to powdery gray, pink, yellow, or orange. They may occur in large quantities in rainy weather, enough to be observed even at a distance. Slime molds are commonly observed on tree leaf litter, rotting wood, grass, wood chips and other organic mulches used in landscaped areas. Fungicides are not recommended for slime molds because they do not cause turf diseases and can be easily removed mechanically by raking or hosing.

Brown Patch

Brown patch is a fungal disease of hot, humid weather. It is most common when night temperatures and relative humidity stay high for several days and are accompanied by rain. Disease development can be very rapid under such conditions. The name "brown atch" best describes symptoms of this disease on low-cut grass, such as golf greens, but the same fungus can cause similar, though distinct, symptoms in lawns. Expanding patches of discolored grass up to several feet in diameter mark the presence of a severe outbreak.

There are two important cultural practices that will limit brown patch in lawns: (1) Avoid nitrogen fertilizer applications just before or during hot weather. Several lighter fertilizer applications are less likely to trigger disease than one heavy application. (2) Water eeply and late in the day. When a lawn has had previous brown patch problems, fungicides may be applied when humid weather and hot nights are predicted. Applications should continue according to the fungicide label for as long as the hot, humid weather persists. Fungicides are usually not necessary on Kentucky bluegrass/fine fescue lawns.

Dollar Spot

Dollar spot on lawns usually appears in somewhat irregular circles 3 to 6 inches in diameter. If the spots coalesce, a more general blight may be observed. Dollar spot is most common in early summer, late summer, and early fall in humid weather with warm days and cool nights. Dollar spot rarely causes severe damage on lawn grasses. All grasses are susceptible to dollar spot disease, but some Kentucky bluegrass cultivars are relatively resistant.

The disease usually occurs on lawns that are nitrogen-deficient. Thick thatch stresses the grass and increases dollar spot. If dollar spot is present in the lawn, it is best to mow only when the grass is dry and to mow high. Water deeply and infrequently. Dollar spot disease responds readily to fungicides, but application of fertilizer and attention to watering practices is usually sufficient to minimize the disease. Dollar spot rarely causes sufficient injury to lawns to justify the use of fungicides.

Leaf Spots

Leaf spots on grass leaf blades begin as small red to purplish ovals that later develop tan centers of dead tissue with darker borders ("eye spots.") The leaf spots may coalesce and cause extensive blighting. The fungus may even invade the crowns and roots, leaving the plants weakened and rotted. In this severe stage, called "melting-out," large areas of dead or badly weakened grass may result. Leaf spot fungi may be active in all seasons at a variety of temperatures. All are associated with frequent moisture on the leaf blades and high relative humidity. Mowing increases infection by leaf spot fungi. 

Fungicides should not be applied routinely for leaf spot fungi. If cultural management has not prevented severe infection or if prolonged wet weather occurs, several contact fungicides are available. In damaged areas, re-seeding with resistant cultivars is a better long term solution to leaf spot problems. Grass suffering from severe "melting-out" and crown and root rot will not be improved by fungicide applications.

Necrotic Ring Spot and Summer Patch

Symptoms of necrotic ring spot and summer patch include rings and circular patches of dead or dying grass. Sometimes there is a center of relatively healthy grass in the "doughnut" or "frog-eye" patch. Both diseases are most severe on Kentucky bluegrass lawns that were seeded or sodded two to four years previously. Fine-leaf fescues are also susceptible. Necrotic ring spot infects grass in spring and fall in wet weather, but symptoms become severe when environmental stress conditions impact the diseased plants from early summer on. Summer patch symptoms are most  severe in mid-summer, especially following a heavy rainfall. The times of occurrence of these diseases overlap, so it is not always possible to separate the diseases strictly by season or field symptoms. Severe damage is most common on sodded Kentucky bluegrass lawns that have been planted on poorly prepared sites with poor drainage and compacted soil.

Systemic fungicides can help prevent these diseases but are not generally cost-effective for use on lawns. Because of the expense and difficulty in fungicide applications to grass roots, cultural controls are strongly recommended for lawns.

Pythium Blight

Pythium fungi commonly cause seed rot and seedling death, resulting in the poor establishment of newly seeded lawns: infected seeds fail to germinate; infected seedlings are discolored and many topple over at the soil line and die. Pythium blight begins in mature lawns as small slimy or greasy looking spots which may assume a reddish color if weather conditions become cooler and drier. When the grass is wet or where air circulation is poor, the cottony, white, weblike mycelium of the fungus may be observed. This disease can develop rapidly and extensively in poorly drained areas.

For established lawns, water deeply, to a depth of 6 inches, only when necessary. Water early in the day so leaf blades will dry before dark. Avoid excessive nitrogen fertilizer, especially when hot, humid weather is expected. When the mycelium is active, it is easily spread by feet and mowing. Mow grass when dry to avoid spreading the fungus. Mow low or poorly drained areas last. Pythium blight is not a common lawn disease in New England. Therefore, correction of drainage problems and attention to the other cultural practices described above are usually more effective and appropriate than fungicides. 

Snow Molds

Damage from snow mold fungi usually becomes apparent as the snow melts and exposes the grass in late winter. Snow mold symptoms consist of roughly circular patches (at least 3 to 12 inches) of dead and matted grass blades. In severe cases, these patches coalesce and may not be recognizable as individual circles. Two common types of snow mold are found in New England: The web-like mycelium of pink snow mold may initially look white and mature to a faint pink to salmon color. Gray snow mold is white to gray. The mycelium  of both types of fungi will disappear quickly as the grass dries. It is useful to determine whether the disease is pink or gray snow mold because gray snow mold rarely damages more than the blades of the grass. In contrast, pink snow mold may invade the crowns and roots causing more serious injury. It is not unusual for both types of snow mold to be found in the same area. All common lawn grasses may be infected, but Kentucky bluegrass fescue lawns are the least susceptible to severe damage.The most important means of preventing or reducing snow mold problems in lawns is the care of the grass at the end of the summer season. As long as the grass continues to grow, it should be mowed. Fall fertility programs should be timed so that they do not influence the ability of the grass to become dormant for the winter season. Fall fertilizers should be applied more than six weeks before dormancy. Fungicide applications for snow mold are not recommended for home lawns except in extreme circumstances.

Adapted from the UMass Cooperative Extension, 2000