TURF GRASS DISEASE MODELS
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The disease Phytium blight, also known as Cotton Blight, Grease Spot is caused by several different Pytium species. Cool-season turfgrasses are susceptible to Pythium. Under favourable conditions, the whole turfgrass could be destroyed within a few days.
Pythium symptoms will first appear as small, irregularly shaped spots, which will join together to form large patches that will often be long streaks. Pythium often develops into these streaks because it is the direction of water movement (drainage) most often ruts from golf carts and mowing equipment. The turfgrass leaves will at first look and feel water-soaked, greasy, or slimy. Once dew or moisture dries up the blade will shrivel up and collapse, often causing a matted brown turf. The turfgrass will begin to develop patches that fade to a light brown or gray color. With high humidity in early morning or throughout the day, diseased leaves may be covered with the white, cobwebby, mold-like growth of mycelium.
Pythium may survive in the soil for extended periods of time, often coming from debris from past infected plants or spores living in the soil.Pythium spreads by the movement and growth of mycelium and spores from plant to plant. Pythium thrives in hot and humid weather typically day temperatures of 27°C to 35°C in areas that have little air movement but high moisture content. In lower temperatures of 13°C to 18°C and extended periods of wet weather Pythium is still prevalent.
Pythium will most commonly appear during the «150 rule», which is when day + night temperatures are over 66°C. Pythium is most common when dew remains on the grass blade for 14 hours or more. Turf stands that have excess nitrogen and lush growth are very vulnerable to Pythiuum and will spread rapidly due to high nitrogen levels. Alkaline soils (above a pH of 7) and calcium deficient soils also tend to favor Pythium.3 Pythium survives overwinter as oospores found in the soil. The pathogen, therefore, is easily spread with the movement of diseased plants, soil movement, surface water, or even from shoes. Pythium also causes «Damping off», «seed decay», or «seedling blight» of turfgrasses. This is most common in Perennial ryegrass (Lolium perenne) and happens in areas that are high above the recommended seeding rates. Not only does Pythium devastate the Turf canopy but the oomycete can also attack the roots and crowns, which will reduce growth, become off-colored, and cause thinning of turf.
- Infection and disease development is associated with daytime air temperatures above 30°C
- night temperatures are above 20°C
- relative humidity for 15 or more hours above 90%.
- high nitrogen nutrition appear to be more susceptible to the disease as are young/germinating seedlings, so care must be taken during overseeding or establishment in hot weather
The model shows a riks period on the 27th of July because on that day more than 15hours of the relative humidity of above 90% was determined and the temperature was between 20 and 35°C. The wet period afer that date does not show risk, because the temperature was too low.
Kentucky bluegrass and Fescue are less susceptible to Pythium blight than Perennial ryegrass and Bentgrass. Creating an environment that includes adequate drainage, good air movement, and balanced fertility will help prevent the disease. Removing dew during hot and humid weather will also help prevent Pythium. Dew can be removed from the grass from mowing, using a backpack blower, or dragging a hose across the grass. Install an internal drainage system if you have severe drainage problems. Improve air circulation (Installing fans, removing trees or shrubs) and avoid irrigation practices that will leave moisture on the grass blades for extended periods of time. An adequate aeration program will relieve compaction and improve drainage. Aeration should annually disrupt between 15-20% of the total surface area. Be sure that you are not applying excess nitrogen to your soil. Avoid using quick release fertilizers, try using slow release ammonium sources. If your soil pH is above 6.5 then use ammonium sulfate, which will acidify your soil. Most balanced fertility programs for Kentucky Bluegrass lawns will consist of applying two to five lbs of nitrogen/1000 sq.ft. a year.
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The Dollar Spot disease is caused by the pathogenic fungal species Sclerotinia homoeocarpa and infects the foliar of warm and cool season turfgrasses.
Affected grasses show white to straw-colored lesions that progress downward from the leaf tip or laterally across leaf blades. A brown border usually surrounds each lesion. Older lesions on higher mowed grass frequently appear hourglass-shaped, being narrower in the middle than at the top or bottom. Individual leaf blades may contain many small lesions or one large lesion or the entire leaf blade can become blighted. Infected leaves become blighted, turning white to straw-colored as lesions expand and coalesce. Blighted leaves are formed in aggregates that appear as circular, sunken patches, measuring from < 1 to > 10 cm (< 0.5 to > 4 inches) in diameter. On golf putting greens and other closely mown areas, the patches appear as white to straw-colored spots that are similar in diameter to a silver dollar, hence the name dollar spot.
Dollar spots may coalesce into large straw-colored areas of blighted turf measuring 15 cm – 3 meters (6 inches to roughly 10 feet) in diameter. Dollar spot-affected turfgrass areas often become thinned of foliage and invaded by weed species.
Be aware that symptoms of dollar spot, Pythium blight, and the brown patch may be similar at certain stages of disease development. Usually, the dollar spot is not associated with a rapid kill of turfgrass plants as are Pythium blight or Rhizoctonia brown patch. The fungi that cause dollar spot and brown patch often produce distinct lesions on infected leaves, but Pythium blight does not. Even though dollar spot symptoms are confined to aerial parts of turfgrass plants, Sclerotinia homoeocarpa produces a metabolite that is toxic to bentgrass roots. The toxin causes roots to thicken, cease to elongate, and become devoid of root hairs.
The fungal pathogen penetrates leaves directly by forming an appressoirum or by entering through stomata or cut leaf tips. The hypae grow and colonize the epidermal and mesophyll cells. The fungal pathogen also secrets enzymes and toxins, which result in necrostic tissues. The fungus survives as mycelium or stromata in infected plants, while spores are not produced.
- optimal temperatures between 15°C and 32°C
- long periods of leaf wetness from dew, rain, irrigation
- most prevalent during springtime and fall.
- low soil moisture (which stress the plant and makes it more susceptibility to the pathogen)
- turfgrass under low nitrogen fertility show mowre dollar spot than onces with optimum fertility.
Allen, T.W., A. Martinez, and L.L. Burpee. 2005. Dollar spot of turfgrass. The Plant Health Instructor. DOI:10.1094/PHI-I-2005-0217-02)
Couch, H.B. 1995. Diseases of Turfgrasses. Krieger Publishing Company, Malabar, FL.
Detweiler, A.R., J.M. Vargas, Jr., and T.K. Danneberger. 1983. Resistance of Sclerotinia homoeocarpa to iprodione and benomyl. Plant Dis. 67: 627-630.
Endo, R.M. 1966. Control of dollar spot of turfgrass by nitrogen and its probable basis. Phytopathology 56: 877
Golembiewski, R.C., J.M. Vargas, Jr., A.L. Jones, and A.R. Detweiler. 1995. Detection of demethylation inhibitor (DMI) resistance in Sclerotinia homoeocarpa populations. Plant Dis. 79: 491-493.
Goodman, D.M. and L.L. Burpee. 1991. Biological control of dollar spot disease of creeping bentgrass. Phytopathology 81: 1438-1446.
Nelson, E.B. and C.M. Craft 1991. Introduction and establishment of strains of Enterobacter cloacae in golf course turf for the biological control of dollar spot. Plant Dis. 75: 510-514.
Nelson, E.B. and C.M. Craft. 1992. Suppression of dollar spot on creeping bentgrass and annual bluegrass turf with compost-amended topdressings. Plant Dis. 76: 954-958.
Smiley, R.W., P.H. Dernoeden, and B.B. Clarke. 1992. Compendium of Turfgrass Diseases 2nd ed. American Phytopathological Society, St. Paul, MN.
Smith, J.D., N. Jackson, and A.R. Woolhouse. 1989. Fungal Diseases of Amenity Turf Grasses. E. & F.N. Spon, Ltd. New York, NY.
Vargas, J.M., Jr. 1994. Management of turfgrass diseases. 2nd ed. CRC Press, Boca Raton, FL.
Vargas, J.M., Jr., R. Golembiewski, and A.R. Detweiler. 1992. Dollar spot resistance to DMI fungicides. Golf Course Management 60: 50-54.
Williams, D.W., A.J. Powell, P. Vincelli, and C.T. Dougherty. 1996. Dollar spot on bentgrass influenced by displacement of leaf surface moisture, nitrogen, and clipping removal. Crop Sci. 36: 1304-1309.
Zhou, T. and G.J. Boland. 1998. Suppression of dollar spot by hypovirulent isolates of Sclerotinia homoeocarpa. Phytopathology 88: 788-794.
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Sensors: Soil temperature, Precipitation and Air temperature needed
The disease Brown Patch is caused by the fungal pathogen Rhizoctonia solani and occurs during summertime in golf courses and lawns. The most susceptible grass species are perennial ryegrass, bentgrasses and Festuca arundinacea(tall fescue or Rohrschwingel).
In early morning on dew-covered turf, white mycelium of the causal fungus can often be seen on and between grass leaves and stems in the patch. Sometimes, all the grass within the patch is killed, creating a sunken or “pocket” effect. More often, the turf in these patches is thinned rather than completely killed. Occasionally, no circular pattern can be seen, and the disease appears as a diffuse blight.
On tall fescue, symptoms of brown patch can be observed on individual leaves and not necessarily in patches. Symptoms on leaves appear as irregular tan or light brown lesions surrounded by dark brown borders. In severe cases, the entire stand may look discolored and thinned.
Symptoms of brown patch on creeping bentgrass putting green; note dark rings around periphery of patch (smoke rings). Picture source: Plant Science
A distinguishing feature of brown patch on golf course is the presence of dark purplish rings around the periphery of the patches. These are called smoke rings. Smoke rings are more pronounced in the early morning hours, usually fading by midday.
Symptoms of brown patch on tall fescue leaves; note irregular, light brown lesions with dark brown borders (photo courtesy of Dr. Noel Jackson. Picture source: Plant science
R. solanii overwinters in the form of resting bodies called sclerotia, either within infected grass tissue or in the soil. The fungus is capable of surviving in soil for years in the absence of a susceptible grass. Disease activity is prevalent when surface moisture and humidity are high, night temperatures are above 20°C and daytime temperatures average 27°C or above. Rainy weather and a saturated atmosphere (100% relative humidity) greatly speed disease development. Disease severity is greater on lush, succulent turfgrass maintained with high nitrogen levels than on grass maintained with moderate levels.
Applying nitrogen fertilizers on the turf with a known history of the brown patch during hot and humid weather may create the need for fungicide applications to control the disease. Removal of dew or guttation water that collects on the grass leaves each morning has proven effective as an aid in reducing brown patch. This removal can be achieved by mowing or by dragging a water hose across the area. Necessary watering should be done in time for the grass to dry before nightfall.
Fungicide treatment should only be needed on high-value ryegrass or bentgrass turfs. Fungicide treatment usually is made on a curative basis; the first spray should be applied immediately after the onset of symptoms, especially if prolonged hot, humid weather is expected. In areas where brown patch causes severe thinning on putting greens, preventative fungicide applications may be justified.
Conditions for the diseases are temperatures around 20°C during night and day temperatures around 25°C and high relative humidity (rain, dew).
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The fungal disease Snow mold occurs in early spring after snow melting. The two types of snow molds (gray and pink) become active under the snow cover and are caused by the fungus Typhula spp.(gray snow mold) and Microdochium nivalis (pink snow mold).
Gray snow mold survives hot summer temperatures in the soil or in infected plant debris as sclerotia, resistant fungal structures, while pink snow mold survives as mycelium or spores in infected plant debris.
Fungal growth begins in the winter, beneath a cover of snow on unfrozen ground. Growth can take place at temperatures slightly below freezing and may continue after snow melt, as long as the grass remains cool and wet. Gray snow mold activity stops when the temperature exceeds 7°C or the surface dries. Pink snow mold activity may continue during wet weather in the fall and spring, as long as the temperature is between 0°C and 15°C.
Symptoms are firsty seen in the lawn as circular, straw colored patches when the snow melts in the spring. These patches continue to enlarge as long as the grass remains cold and wet. Grass within the patch often has a matted appearance and colored fungal growth. The fungal growth may cover the entire patch or develop along the margins, with gray snow mold being white to gray in color and pink snow mold being white to pink in color. Occasionally, fungal fruiting bodies (mushrooms) may be seen emerging from infected turf. Hard structures, called sclerotia, may also develop on the leaves and crowns of plants infected by gray snow mold, not pink. The sclerotia are spherical in shape and roughly the size of a pinhead. Their presence helps to distinguish gray snow mold from pink snow mold.
Snow molds do not occur in the home lawn every year, but are most common during years when an early, deep snow cover prevents the ground from freezing. A cold, open winter will not promote snow mold, but may cause winter injury. The damage caused by snow molds is seldom serious. Generally, infected areas are just a little slower to green up. Gently rake affected areas of the lawn to promote drying and prevent further fungal growth. Fungicides are not usually recommended, but in severe cases a preventative spray may be helpful.
The following steps can be taken to minimize damage in future years:
- Avoid excessive applications of nitrogen fertilizer in the fall.
- Continue to mow the lawn at the recommended height until it is no longer actively growing. The taller the grass, the more likely it will mat down and encourage snow mold development.
- Rake up leaves in the fall.
- Manage the thatch layer to avoid accumulations of more than ½ inch.
- Spread out large snow piles to encourage rapid melting. Use snow fencing to minimize snow accumulation in problem spots.
Lamey, H. Arthur, Cynthia L. Ash, and Ward C. Stienstra. 1988. Lawn Diseases, PP-950(ND) or AG- FO-3386(MN). NDSU Extension Service, North Dakota State University. MN Extension Service, University of Minnesota
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For successful overseeding, you need to:
- Choose the proper seed.
- Properly prepare for and time the overseeding.
- Carefully maintain the overseeded grass.
- And attentively manage the spring transition back to the warm-season grass.
You also need to maintain a healthy warm-season turf all year. It’s particularly important to keep the soil fertile, relieve soil compaction and prevent excessive thatch.
The proper seed is the grass with the characteristics best suited to your particular needs. Annual ryegrass has fast been replaced by perennial ryegrasses because of their improved quality, stress and pest tolerance and manageability. Use seed treated with fungicides, too, such as Apron or Koban. This is particularly true for early fall, since seedling blight diseases can be a problem at this tim
The right seeding rate depends on how you want it to look and how much traffic the turf will bear. Higher-traffic areas need higher seeding rates. However, higher seeding rates may also mean a more difficult spring transition.
- Soil temperatures at a 4-inch depth approaching 24°C.
- Night temperatures consistently around 10°C.
- Average midday temperature below 21°C, or two to four weeks before the average annual first killing frost.
The best way to make the actual overseeding successful is to get good soil-to-seed contact. Seedbed preparation generally consists of close mowing or scalping, with some light vertical mowing, and sweeping or vacuuming up the loose plant debris.
In Fieldclimate we show optimum dates for overseeding with either WARM (GPW) or COLD season gras species (GPC).
Example 1: Optimal time span to overseed with cold season gras species during late summer and autumn.
Example 2: Optimal time span for warm season gras species.
Gil Landry, PhD., Coordinator — The Center for Urban Agriculture, The University of Georgia.
Fall is overseeding time (from Mid of August till Mid of October).