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Kentucky Blue Grass Diseases

Kentucky Blue Grass Diseases

Disease resistant cultivars were developed to resist Helminthosporium mold, once a major problem with Kentucky bluegrass. New cultivars are being developed to deal with diseases that took its place. Selecting the right cultivar and proper cultivation remain the best ways to prevent diseases. Because they show similar symptoms, two diseases that frequently strike Kentucky bluegrass, summer patch and necrotic ring spot, were once lumped together as Fusarium blight.

Necrotic Ring Spot

Necrotic ring spot is a patch disease in Kentucky bluegrass and caused by the fungus Leptosphaeria korrae. The disease occurs commonly in the cool, wet weather of April and May. The earliest symptoms start during early summer in the form of 1-1/2 to 3 foot wide patches of blue-green, wilted grass, which rapidly brown, and die. Circles of healthy grass are often in the center of these patches given the appearance of a frogeye. The roots of the affected grass area are decayed under the ground. The best preventative measure is to keep turf healthy so that it is well resistant to disease. Light irrigation in the affected areas helps usually helps to keep turf healthy.

Control Strategies

The primary stresses that influence disease development include excesses of thatch, fertilizer, and turf canopy temperature, as well as incorrect timing of fertilizer applications, low mowing height and low soil pH. Each of these stresses can be reduced through appropriate culture as described below.

Correct excess soil acidity by liming annually to maintain a pH of 6.2 to 6.6. Do not apply even small amounts of fertilizer during the June-August stress period because this will tend to stimulate the disease. Therefore, fertilize only in autumn (September through November) and in late spring (May).

Deep watering is essential for proper root growth. Water the soil under disease-prone areas to a depth of 3 to 5 inches every 7-10 days during dry periods in the summer.  Proneness to disease in turf is increased as the cutting height is decreased. Cut lawns at 2.5 to 3.5 inches height, and do so often enough that less than 1/3 of the leaf blade is removed during each mowing.

Thatch (the layer of organic matter between the mineral soil and the green grass) should be no more than 1/2 inch in thickness. Dethatch using bio-enhanced liquid dethatcher.

Kentucky bluegrass cultivars such as Adelphi, America, Aspen, Columbia, Eclipse, Glade, Midnight, Nassau, Parade, Ram I, Sydsport, Touchdown, Vantage, Windsor, and Victa are less susceptible to Necrotic Ring Spot than others. Blend seed of a resistant cultivar with that of one or more otherwise desirable cultivars. Blending 10-15% (by weight) of perennial ryegrass seed into bluegrass seed will prevent this disease from occurring. Ryegrass can also be seeded into existing lawns.

Summer Patch

Summer patch in Kentucky bluegrass is caused by the fungus Magnaporthe poae. The fungus is active in warm 77 to 86 degree, moist soil. The fungus causes severe root rot and this leads to inadequate water supply to the grass, causing the lawn to wither. These appear in mid- to late summer. These are crescent-shaped or circular. Dying grass on the edges of the patches can have a slightly bronze color. Patches of healthy grass sometimes appear in the center. Symptoms are most severe on knolls, slopes and dry parts of the turf. Cool weather may allow the grass to recover. The fungus festers in the roots for a number of weeks before the symptoms become apparent above ground. Lawns inflicted with summer patch should not be watered frequently since the fungus thrives in wet soil. Let soil dry between irrigation. The symptoms of summer patch are often reduced with the regular use of ammonium sulfate and the ammonium chloride.

Management Strategies

The primary stresses that influence disease development include excesses of thatch, fertilizer, and turf canopy temperature, as well as incorrect timing of fertilizer applications, low mowing height, and pH extremes. Each of these stresses can be reduced through appropriate cultural practices described below.

Disease severity may worsen at a higher pH, so try to maintain the pH of the soil and Rhizosphere at 5.5 to 6.0. Use an acidifying fertilizer where the pH is above 6.0, and try to avoid the use of products that may raise the pH.

For most bluegrass lawns, two to five lbs of nitrogen/1000 sq.ft. is sufficient. Apply this in a fertilizer balanced by phosphorus and potassium. Do not apply even small amounts of fertilizer during the June-August stress period because this will tend to stimulate the disease. Therefore, fertilize only in autumn (September through November) and in late spring (May).

Deep watering is essential for proper root growth. Water the soil under disease-prone areas to a depth of 3 to 5 inches every 7-10 days during the dry periods of the summer. Soaker hoses are very useful for supplementary watering on steeper slopes where other sprinklers are inefficient. The harmful effects of excessive temperature can be reduced by a light sprinkling of the surface at mid-day.

Proneness to disease in turf is increased as the cutting height is decreased. Cut lawns at 2 to 4 inches height, and often enough so that less than 1/3 of the leaf blade is removed during each mowing.

Thatch (the layer of organic matter between the mineral soil and the green grass) should be no more than 1.5 cm (1/2 inches) in thickness.  Dethatch using bio-enhanced liquid dethatcher.

Kentucky bluegrass cultivars such as Adelphi, America, Aspen, Columbia, Eclipse, Glade, Midnight, Nassau, Parade, Ram I, Sydsport, Touchdown, Vantage, Windsor, and Victa are less susceptible to summer patch than others. Blend seed of resistant cultivars with that of one or more otherwise desirable cultivars. Blending 10-15% (by weight) of perennial ryegrass seed into bluegrass seed will prevent this disease from occurring. Ryegrass can also be seeded into existing lawns.

Rhizoctonia Yellow Patch

Rhizoctonia yellow patch is a serious fungal disease of Kentucky bluegrass caused by Rhizoctonia cerealis. The disease occurs frequently in wet, cool weather and is common in the new sod which is grown on heavy soils. The disease is characterized by light yellow-green 2 to 3 inch patches which rapidly increase in size and start turning brown. The edges of the leaf blades are red or red-purple. Best preventative measures include reducing irrigation and making sure that soil is well-drained and aerated.

Management

Using a slow release fertilizer to ensure moderate amounts of Nitrogen will help in reducing the chance of Yellow Patch. Reducing the leaf wetness period will also help prevent Yellow Patch. Night time watering can increase the leaf wetness period and lead to disease.  Allow for better air movement in order to help dry the lawn when moist. Remove excess thatch and aerate. Ensure that when mowing the lawn is mowed at a high level when dry and in the morning allowing for the grass blades time to heal. Early evening mowing will increase chance of Yellow Patch as well as a dull mower blade.

Powdery Mildew

Powdery mildew, caused by Erysiphe graminis and other fungi, appears as a gray to dusty white powdery coating over the leaf surface. As the spots grow larger they come together to form a mat of mildew that looks like dirt or dust. Powdery mildew is a problem with baron, flyking and merion cultivars of Kentucky bluegrass if they grow in the shade. Warren and nugget are varieties adapted to the shade. Treat powdery mildew with fungicides containing triadimefon, fenarimol or propiconazole, among others.

Treatments

There are several organic treatments that fight powdery mildew. Milk contains salts and amino acids beneficial to plants and that combat the disease. To use, mix 1 cup of skim milk with 9 cups of water and spray on infected areas every two to three days.   Make a spray of 1 tbsp. of baking soda in 1 gallon of water and spray on infected areas every three to five days.

Organic Neem oil is a broad-spectrum fungicide. Mix 2 tbsp. and 1 1/2 tsp. dish soap per gallon of water. Spray all plant surfaces, including the undersides of leaves, until wet.

Cleanup is key. Gather all fallen leaves and flowers that are affected and place in the trash. Do not compost them.

Striped Smut

Striped smut, caused by a variety of fungi, causes yellow-green streaks that turn gray. The gray streaks then rupture, revealing masses of black spores. The disease shreds the grass leaves, which turn brown and die. Striped smut usually strikes in cooler spring weather and the grass dies in the hottest parts of summer. Merion is a highly susceptible Kentucky bluegrass cultivar. Numerous varieties have been developed that will resist striped smut to some degree.

Management Strategies

Some varieties of bluegrass survive the effects of the disease better than others. Merion Kentucky bluegrass, for instance, is very susceptible to stripe smut, while Park and Newport are fairly resistant. Other more resistant varieties are A-20, A-34, Aquila, Baron, Birka, Bonnieblue, Fylking, Geary, Glade, Nugget, Pennstar, South Dakota Certified, Sydsport, Vantage, and Victa.

Optimum fertilization with a complete (not nitrogen only) fertilizer and conscientious watering helps maintain the vigor of the stand and to increase the survival of infected plants.

Gray Snow Mold

Gray snow mold (Typhula incarnate) appears in circular patches up to 2 feet wide under snow. When the snow melts, the patches of infections are usually matted and surrounded by a halo of gray to white, fluffy fungal growth. This disease, which rarely kills the grass, is found more often on bentgrass than bluegrass. Fungicides containing the active ingredients azoxystrobin, thiophanate-methyl, chlorothalonil and propiconazole, among others, can be used to treat snow mold.

Disease Cycle:

Snow mold fungi remain inactive during the warm months when other disease fungi are most active. They survive in thatch and on plants as sclerotia (gray snow mold) or as mycelial threads (pink snow mold). In the fall, Typhula species sometimes produce small, white or pinkish, club-shaped spore-producing structures that may be seen among grass blades in lawns that have not been recently mowed. They grow from sclerotia that were produced in the previous winter for survival over the summer.

As cool, wet weather develops, the fungi begin to grow and infect grass plants. Like all living organisms, these fungi require moisture to survive. The cold, dry air of winter prevents active growth. The shelter of leaves, snow or any other cover on the grass maintains the necessary moisture for growth. Optimal conditions for snow mold activity occur when snow falls suddenly and remains on ground that has not yet frozen. In such cases, grass is often still lush, providing an excellent food base for the fungi.

Cultural Management:

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, or they should be applied after leaf blade growth has stopped but while the grass is still green. Addition of nitrogen fertilizer just before the grass becomes dormant will stimulate a late burst of succulent green growth, making the grass prone to winter injury caused by frost, ice or exposure and also providing the snow mold fungi with vulnerable host plants. This condition is particularly dangerous when an early snowfall occurs.

Because snow mold activity is greatest beneath covers that maintain moist conditions, all leaves or other materials should be removed from the lawn. In addition, it is best to avoid piling snow deeply along sidewalks and driveways where it will form a long-lasting snow bank. In large lawn areas, the strategic placement of snow fences and landscape plants may prevent deep drifting of snow. In the spring, rake away dead and matted foliage from damaged areas to allow the new growth to begin.

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About the Author

Age: 62, There's a lot of experience in these gray hairs!

Education: Master of Science in Quality Management, Quality Control

Status: Married 38 years, 4 children, 10 grandkids

Company: The Organic lawn Care Store - http://www.theorganiclawncarestore.com/

 

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