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Diseases of Turfgrass: Identification and Management

 

Turfgrass diseases are diagnosed by identifying symptoms and signs of infection. Symptoms are the response of the plant that results from an infection by a plant disease causing organism or stress. These include leaf spots, yellowing (chlorosis), stunting, dead or rotted plant parts, and wilting. Signs are the visible evidence of the pathogen--white cottony growth of a fungus or water mold, fruiting bodies of a fungus (mushrooms or rust pustules), or the resting bodies of a fungus (sclerotia). Most residential lawn diseases can be managed without using fungicides, but when chemical control is warranted, fungicides should be applied by a licensed lawn care professional with appropriate spray equipment. If a disease is present at a noticeable level, it is usually too late for a fungicide to be effective. To be effective, most fungicides need to be applied preventively (before disease becomes established and produces spores). Newer fungicide chemistries are targeted and specific in their mode of action. If disease occurs yearly in a landscape or occurs in a high value turf site, fungicide application may be recommended. Some diseases such as brown patch cause browning and dieback on the above ground portion of the plant, but do not kill the crown and roots. When the weather becomes more favorable for turf and less favorable for disease, turf will recover without a fungicide. Knowledge of diseases that occur in the region and knowledge of the turf species being grown can help to determine what management strategies are needed.

Some regionally significant diseases are described below in the general calendar order that they normally occur, under Delaware conditions.

Pink snow mold. This disease usually occurs in very early spring or late winter, but can occur in late fall, and is also called Microdochium patch. Causal agent Microdochium nivale. Symptoms: Patches of matted leaves that have a pinkish or red-brown color. Patches may be several inches in diameter and center of may be bleached white. Occurs under snow cover, but can also occur during cool periods with high moisture. All turfgrass species are hosts, but are not generally killed. Management: Avoid high nitrogen and remove thatch. Mow late into the fall.

Red thread. This foliage disease occurs primarily in the spring and fall, but during rainy periods in summer. Causal agent Laetisaria fuciformis. Symptoms: Irregular blighted patches sometimes with a pink cast. Blades are water-soaked and covered with the diagnostic pink gelatinous fungal growth. Dead blades in affected areas are straw brown, tan or slightly pinkish. Dark pink/coral antler-like strands of mycelium or pink cottony tufts can be seen extending from the tips of dried infected grass blades. All turfgrass species are hosts especially perennial ryegrass and fine fescues. Affects only the blades does not kill turf. Management: Maintain adequate nitrogen, remove excess thatch, and reduce stress.

Leaf spot and melting-out. This group of leaf spot diseases occur in spring, summer, and fall. Melting-out occurs primarily in late spring, in mowed and managed turf. Causal agent Drechslera spp. Symptoms: Leaf spots can be brown to purple brown, oval-shaped or elongate, with tan centers. Melting out symptoms are more severe and result in dieback of roots and crown, and thin turf. Found primarily on Kentucky bluegrass, but all turfgrass species can be infected. Bluegrass can be killed by the root and crown phase (melting-out). Management: Over-seed bluegrass lawns with a resistant cultivar, and avoid high nitrogen.

Necrotic ringspot. This root rot/patch disease occurs in cool weather in spring and fall. Causal agent Ophiosphaerella korrae. Symptoms: Rings of dead turf several inches to over a foot in diameter with green turf or weeds in the center. Blades appear yellow to brown, roots and crowns are decayed. Bluegrass and fine fescues are affected. Plants are killed. Management: Avoid high nitrogen and drought stress. Over-seed or replace with tall fescue or ryegrass that is resistant.

Dollar spot. This disease produces spots two to six inches diameter in affected turf, in late spring through fall. Causal agent Sclerotinia homoeocarpa. Symptoms: Leaf spots are hour-glass-shaped, extend across the blade with bleached centers and brown to purple borders. In the morning cottony fungal growth can be seen on infected blades. Bluegrass, ryegrass, and fine fescues are hosts, especially in low fertility turf. Foliage is affected but plants are rarely killed. Management: Adequate nitrogen, mow high, remove thatch, and reduce stress.

Slime molds. Surface molds that produce a gray, purple, or black material on blades of turf. Causal agent several species. Symptoms: Mold often appears after a rainy period in circular patterns in lawns. Slime molds are usually found from late spring to fall on the leaf blade surface and do not harm the plants. Management: The molds can be removed by mowing, raking, or washing with a hose. Control thatch.

Anthracnose. This disease occurs in spring and summer and can occur on the foliage and spread to the crown and roots. Causal agent Colletotrichum graminicola. Symptoms: Yellow to brown patches may turn bronze colored and crowns turn dark brown. The causal fungus survives in plant debris, spreads by splashing water and is common in humid weather. Management: Proper fertility, drainage, and aeration will reduce stress and improve root health, reducing incidence of anthracnose.

Pythium blight. This leaf and root disease occurs during the spring and summer, and is favored by wet, humid periods and poorly drained soils. Causal agent Pythium spp. Symptoms: Small, spreading, circular areas are matted, tan/gray, or water-soaked. Affected areas follow surface water flow patterns or occur in areas with poor drainage. Infected blades appear greasy, wilt and die rapidly. Tufts of mycelium are found on tops of plants in the morning. Dead plants are often red/brown in color and matted together. Bluegrass, fescues, and perennial ryegrass are killed. Management: Apply nitrogen only in the fall and remove thatch. Improve drainage.

Brown patch. This disease occurs during the summer when night temperature are above 70 F, and humidity is high. Causal agent Rhizoctonia solani. Symptoms: Leaf blight in patches five inches to two feet in diameter. Leaf spots irregular with a narrow red-brown border. Small patches can coalesce to blight larger areas quickly under favorable weather conditions. All turfgrass types are affected, especially tall fescue and ryegrass. Crowns and roots are not affected and mature turf is rarely killed. Management: Apply nitrogen fertilizer only in the fall, water early in the day so leaves will dry, and remove thatch.

Summer patch. This root and crown rot occurs during periods of high temperature and drought. Causal agent Magnaporthe poae. Symptoms: affected areas are light green then fade to a tan/straw color. Often circular or irregular patches have living grass or weeds in the center. Patches can merge and blight large sunken areas of turf with brown borders. Bluegrass and fine fescues older than two years may be killed. Management: Reseed with tall fescue or resistant cultivars of bluegrass. Apply nitrogen fertilizer only in the fall, water early in the day so leaves will dry, and remediate compaction.

Powdery mildew. This minor disease occurs primarily in the fall, especially in shady areas. Causal agent Blumeria graminis. Symptoms: Leaves of infected plants have a white to gray powdery growth on the surface. Leaf blades only are affected, which may turn yellow and die. All grass species are susceptible, especially in shady conditions. Management: Apply nitrogen only in the fall, mow high, increase light levels or plant shade tolerant grasses.

Rust. This foliar disease is usually seen in the fall. Causal agent Puccinia spp. Symptoms: Superficial yellow-orange flecks on individual grass blades that develop into rust/red pustules. Spores rub off easily on shoes or clothing, and affected areas can appear a red/brown color. Perennial ryegrass and bluegrass are the most susceptible and leaf blades may die with a heavy infection. Turf can be thinned but rarely is killed. Management: Maintain fertility, good air circulation and plant resistant cultivars.

Fairy ring. Disease can be seen any time of the year but mushrooms are most often seen in the fall. There are three types of fair ring diseases on turf, each with differing symptoms. Causal agent various fungi. Symptoms: Circular areas of dead grass bordered by zones of dark green grass, or rings of very green grass without a dead zone. Affected areas from a few feet in diameter up to 20 feet across. Mushrooms may or not be present in rings. All turfgrass can be affected, especially nutrient deficient or drought stressed turf. Management: Fertilize and rake mushrooms to remove fruiting bodies. Remove thatch, reduce stress, core aerate and drench with a wetting agent if soil is hydrophobic.

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