Leaf Spot & Melting Out
Leaf spot and melting out are diseases of stressed turf especially Kentucky bluegrass caused by several species of fungi including Drechslera (Helminthosporium). Leaf spot disease appears during cool, moist weather, spring or fall. The first symptoms of leaf spot are small, purple to black specks on the leaf blades. Some leaf spot can be found on most home lawns in the spring, but it normally does not cause significant damage to the turf. However, newly seeded lawns and certain cultivars of Kentucky bluegrass are particularly susceptible, and a disease outbreak can result in serious injury to the turf. Extended periods of cool, wet spring weather also may trigger leaf spot epidemics on Kentucky bluegrass.
Melting out disease is active during warmer weather. Melting out starts as a black to purple leaf spot, then works its way to the plant base and attacks the roots and crown. Both diseases grow in dry periods alternating with cloudy, wet weather and cool to moderate temperatures. Once the fungus reaches the crown and roots, the entire plant may be killed, causing large areas of grass to thin. Infected areas are irregularly shaped, and range in size from several inches to several feet in diameter. Severe infections may cover the entire lawn
The diseases are enhanced by the use of susceptible cultivars, excessive nitrogen fertilizer, excess water, and a short mowing height.
The most effective means of preventing leaf spot and melting out is to plant resistant cultivars.
Cultural Control: The severity of the disease can be controlled by proper cultural practices that maintain the grass at optimum vigor.
Use resistant varieties when establishing or re-establishing a lawn.
Core aerate the lawn once a year (spring or fall) to help reduce thatch buildup and improve soil condition. Core-Aeration is the best method to reduce thatch. Aeration increases the rate of organic matter decomposition. If the thatch accumulates in excess of 1 centimeter, it should be removed. Thatch provides an ideal medium for the fungi to multiply and also interferes with proper movement of water and nutrients. If clippings are heavy, remove them to reduce thatch accumulation.
Mow grass as necessary to maintain a height of 3 inches. Make sure mower blades are sharp. Never remove more than one-third of the grass blade at a time Mow frequently. Do not mow when grass is wet. During a disease outbreak, the diseased areas should be mowed last to reduce the spread of disease and clipping should be removed.
Water to a depth of 6 to 8 inches as infrequently as possible without creating water stress. Water in the morning or midday so the leaf blades dry as quickly as possible. Avoid frequent light sprinklings.
Avoid excessive applications of fast release nitrogen fertilizer, which induce tender, succulent growth and more susceptible tissue. Apply nitrogen according to soil test results or at the rate of 1 pound per 1,000 square feet 3-4 times a year. Never apply more that 4 pounds of nitrogen per 1,000 square feet in an entire year.
Fungicides are rarely needed to control leaf spot disease. However, if melting out disease has occurred repeatedly in the same areas over a number of years, a fungicide may be warranted. Chemicals are most effective when combined with cultural controls.
Necrotic Ring Spot
Initially, infected lawns show scattered light-green patches, 2-6 inches in diameter in roughly circular or crescent-shaped patterns. As the disease progresses these patches enlarge, the colour changes from a light-green to dull brown then fades to tan; the pattern becomes more distinctly circular or crescent-shaped with tufts of apparently unaffected grass in the center of each patch, producing what is generally referred to as a ‘frog-eye” effect. Individual patches may enlarge up to 3 feet in diameter and these overlap to result in large areas of blighted grass. Plants die as the crowns and roots are destroyed.
Control: Once in your lawn necrotic ring spot is hard to eradicate. The key to control is to keep the grass growing vigorously using sound management practices. Fungicides are generally not very effective against necrotic ring spot. Fungicides are generally not inexpensive and not always effective.
1.Remove thatch when it accumulates in excess of 1 cm. Thatch provides an ideal medium for the fungi to multiply and also interferes with proper movement of water and nutrients. Remove heavy clippings to prevent thatch accumulation. Core-Aeration is the best method to reduce thatch. Aeration increases the rate of organic matter decomposition.
2.Necrotic Ring Spot affects the roots. The roots are not able to function at 100%. The effects of the disease are thus more noticeable in hot, dry weather. During dry periods, water thoroughly, wetting the soil to a depth of at least 6 inches every 7 days or so. Severe damage occurs when weather is hot and humid but dry(no rain). Avoid watering at times when the grass will remain moist all night.
3.Kentucky bluegrasses in general are more susceptible to necrotic ring spot. Overseed with resistant species. We recommend grass seed mixes with at least 20-30% turf-type perennial ryegrass seed. Re-sodding only delays the re-occurrence of this disease.
4.Balanced nutrition is important. Regular feeding is important but not to be overdone. Avoid soluble or fast-releasing nitrogen during the summer.
5.Increase the beneficial microbial population by using organic based fertilizers. The more beneficial microbes in the soil, the less moisture and oxygen available for unwanted organisms. Our regular Turf King fertilizer is high in organic matter.
6.In severe situations, we recommend a regimen of “Sustane” natural fertilizer. Sustane is an organic, composted poultry fertilizer. Increased organic matter increases the population of good microorganisms in the soil. Sustane should be used 3-4 times throughout the season for about 2 seasons.
Powdery Mildew is most often found in shaded, moist areas that suffer from poor air circulation. Most often it is found near the shade of the house or a tree during August –October. Grasses under stress are particularly susceptible to this disease. Powdery mildew first appears as a powdery white growth confined to the upper surface of the grass blade. The fungus grows rapidly and soon covers the leaves of the entire plant. If the disease progresses, its suffocating activity continues to point where the blades yellow and plants die.
In general, powdery mildew is not considered to be a serious problem and is usually more of a nuisance. Plants are seldom killed, but they become more likely to suffer from other diseases and stresses. Kentucky blue grasses are the most susceptible.
Control: Improve air circulation and reduce shade by pruning nearby trees and shrubs. Prune during the growing season to be able to tell which branch to remove for best light advantage. Regular fertilization is important but over-fertilization leads to too much succulence and increased susceptibility.
In addition, our recommendations would include aerating to keep the lawn healthy. Winter fertilization is one way to round out the lawn’s nutritional needs.
Rusts are often first noticed when you walk through the lawn, especially with white running shoes. You find that your shoes have turned orange. This orange-red dust is composed of the many spores that produce this disease. At this time, you may then notice the rusty appearance of the lawn. The spores are carried by the wind to other lawns. If they land on grass blades with sufficient moisture, a new infection is started.
Rusts can affect all turf grasses but perennial ryegrass and Kentucky bluegrass are the most susceptible. Infection occurs with 4-8 hours of 22-25oC temperatures, cloudy weather and high humidity. Rust is most common during July and August but can occur from July to October.
There will be small chlorotic flecks on either the upper or lower leaf surface, followed by reddish brown pustules containing spores. The spores are easily rubbed off. Severe cases may result in yellowing and wilt. Where the turf is killed, the pustules become brown-black on the dead tissue.
Control: Rusts are a minor disease in Ontario. Mowing and managing fertility will usually control the disease and prevent damage. Provide a balanced Nitrogen feeding to the lawn. This will avoid nutrient stress and improve leaf growth. Either too much or too little Nitrogen can favour rust development. Reduce shade and improve air circulation. Use resistant varieties if possible. Water infrequently but thoroughly early in the day.
When the disease is present, increase the mowing height and frequency. These 2 practices will encourage faster growth. This will reduce the disease incidence and severity by allowing the grass to grow faster than the disease. Our recommendations also include an aeration to improve the general health of the lawn. Consider adding a Winter fertilizer to your program to improve the lawn’s nutrition.
The term “Snow Mold” refers to lawn diseases that occur primarily from late fall to mid-spring. Snow Mold or Snow Mould appears as a fluffy white, pink or gray residue that seems to follow the retreating snow line. Cool, wet conditions with or without snow cover cause this disease to flourish.
A properly fertilized, well-maintained lawn is most resistant to damage from snow mold. A vigorous raking in early spring through the snow mold residue will help reduce this disease and quicken the turf grass healing process.
The two most important diseases that occur at low temperatures are Pink and Gray Snow Mold.
PINK SNOW MOLD
Symptoms begin as small, circular spots which can grow to rusty-brown patches up to 12 inches in diameter. The diseased areas are initially pale yellow and later turn tan or whitish-gray as the leaves die. Infected leaves have a bleached appearance and feel slimy when wet. The dead grass blades have a matted appearance. The disease spread rapidly when under snow cover, or during periods of cool, wet weather. The grass at the edges of the spots may be covered with a white cottony growth called mycelium. This mycelium will often develop a salmon-pink or rusty brown colour when exposed to light.
GRAY SNOW MOLD
This disease often occurs when there is a early, deep snowfall coving the lawn before the ground has frozen in the fall. The symptoms are visible when the snow melts from the lawn. The matted patches are light yellow to bleach brown in colour and up to 24 inches in diameter. Around the margin of the infected area, a grayish-white, aerial mycelium can be observed during damp weather. Leaves of infected plants are often matted together and break free easily. Injury to the turf is aggravated when the snow is compacted by walking, skiing, snowmobiling, or sledding.
Control options are the same for both diseases.
1. Avoid thatch build-up in lawns, as this serves as an ideal environment for survival and growth of these fungi. Thatch (the layer of decaying grass residues and roots between the leaves and the soil) can be reduced by core aeration and proper lawn care practices.
2. Avoid applying fertilizers with excessive quick release nitrogen to the lawn during late summer and fall before the grass goes dormant. Too much nitrogen promotes lush growth, which will not harden off properly, and is therefore very susceptible to snow mold and other types of winter injury.
3. Cut grass to recommended heights until growth stops in fall. Excessive foliage under snow cover provides ideal conditions for snow mold fungi.
4. Make sure leaves are removed from the lawn in the fall. Avoid piling snow that you have shoveled off walkways onto your lawn.
5. In areas with a history of snow mold problems, apply a recommended fungicide in late fall just before snow cover.
6. No matter how tempted you are to start fussing with your lawn fungus, wait until the grass has had a chance to dry out before attempting any first aid
7. Rake the area affected with snow mold and fertilize lightly to encourage new growth. Re-seed areas that have heavy damage.