The Online Gardener's Handbook 2010
Chapter 7: Lawns
Disease Control on Lawns

Table of Contents

  1. Introduction
  2. Grey Snow Mould
  3. Pink Snow Mould and Fusarium
  4. Leaf Spot and Melting Out
  5. Dollar Spot
  6. Brown Patch
  7. Necrotic Ring Spot
  8. Summer Patch
  9. Pythium Blight
  10. Powdery Mildew
  11. Mushrooms, Fairy Rings
  12. Slime Moulds
  13. Moss
  14. Learn More


In this chapter, a description of various lawn pests will be provided along with suggested management options. These management options will not include the use of pesticides. Some biopesticides and certain reduced risk pesticides are still available to the homeowner for controlling weeds and pests in lawns and gardens. For more information, refer to Chapter 2 of this handbook and the Ministry of the Environment's website. For suggestions on managing specific weeds and pests, consult local horticulturalists, Master Gardeners or your local garden supply centre.

Grey Snow Mould

Snow moulds occur under snow, becoming more severe when snow cover lasts longer than three months. Areas with steady consistent snow cover, therefore, are more likely to be severely attacked by snow mould than are more southerly regions.

Grey snow mould is seen as circular to overlapping patches of dead bleached grass, 10-20 cm in diameter, that appear as the snow melts. The patches may be covered with greyish-white to pink thread-like growths (mycelium), especially on the outer edges, for several days after snow melt. Tiny, black bumps (sclerotia) can also be found on the dead leaf blades.

The fungus remains dormant in the lawn throughout the summer, surviving as sclerotia in the thatch or soil. In the fall, these germinate to release spores that are carried by wind and rain; at other times, germination occurs after snow cover. Infection occurs over the winter, and come spring, the fungus is no longer active.

Management Options

Keep lawn raked to minimize thatch where fungus survives as sclerotia during inactive periods. Avoid rapid succulent growth in fall by continuing to mow until grass stops growing and apply nitrogen no later than six weeks before dormancy. A late fall fertilizer treatment can be applied after grass has stopped growing. Remove piles of snow on grass where disease can occur and break up piles in early spring to speed melting and drying.

Pink Snow Mould and Fusarium

Pink snow mould is similar to grey snow mould, but there are differences. Grey snow mould is a general blight, causing the lawn to appear scalded or bleached and producing sclerotia on the leaf blades. Pink snow mould appears in distinct spots or patches of 10-20 cm in diameter and has an orange to reddish-brown appearance. These can merge to form larger irregular patches. Mycelium can be white or pink and can be seen at the outer margins of the patch up to several days after the snow is gone. As well, pink snow mould attacks all cool season grasses, while Kentucky bluegrass and red fescue are not nearly as susceptible to grey snow mould.

Fusarium patch is caused by the same pathogen as pink snow mould. During times of cool, wet weather in spring and fall, the fungus develops, resulting in small (2-5 cm), blighted patches of lawn with a strong reddish-brown colour. The disease is often more severe in areas of shade, poor air circulation, poor drainage or heavy thatch. When the disease is active, patches may have a brown ring at the outer edge and the diseased grass may be wet and slimy.

Management Options

Rake to minimize thatch where fungus survives during the summer when inactive. Avoid rapid succulent growth in spring and fall by delaying spring fertilizer application, mowing until grass has stopped growing for the season, and applying nitrogen no later than six weeks before dormancy. Prune trees and shrubs surrounding the area to improve air circulation. Do not allow water to sit. Rake leaves in fall.

Leaf Spot and Melting Out

Leaf spot first appears as small, oval, brown to purplish-brown spots on lower blades of grass between April and June. Spots expand to become oval lesions with straw-coloured centres and dark brown margins. When prolonged dull weather is followed by hot weather in early summer, the spots spread to the stems and crowns, creating massive dead or thinning patches known as melting out. Leaf spot occurs over a wide temperature range and is favoured by long periods of leaf wetness, excessive nitrogen fertilization, and low mowing height.

The fungus overwinters as spores on diseased or dead grass and becomes active in spring, producing new spores. All northern turfgrass species, especially Kentucky bluegrass, are attacked.

Management Options

Follow proper fertilizing procedures to avoid rapid succulent growth in the spring and fall. Raise the mowing height to greater than 3.5 cm and rake to minimize thatch build-up. Water early in the morning so that the grass can dry fully. Use a resistant variety of Kentucky bluegrass such as Baron, Sydsport, or Touchdown.

Dollar Spot

This fungus creates circular, bleached, dead spots, 5-7 cm in diameter throughout the months of June to October. When the disease is serious, these spots will overlap, causing large dead areas of lawn. Leaf blades develop hourglass-shaped lesions with bleached centres and reddish brown borders. In September, when warm days are followed by cool nights, cobweb-like fungus growth can often be seen early in the morning before the dew dries. All grasses, especially creeping bentgrass, are susceptible.

Management Options

Follow proper fertilizing procedures. Dollar spot is favoured by low nitrogen levels, and can often be corrected by applying nitrogen fertilizer. Improve air circulation and reduce the level of shade to speed drying.

Brown Patch

Roughly circular or crescent shaped brown patches, often with a central green tuft of grass, is an indication of brown patch infestation. It usually develops during the hot, humid weather of mid to late summer when temperatures are between 25-30°C and when night temperatures are higher than 20°C. On higher-cut turf this may give rise to a frog-eye appearance. On lower cut turf, diseased patches are initially purplish green and die out to a light brown. Patches may appear blighted or may just display a circular pattern of thinned-out turf of 1 m in diameter or more. All northern turfgrasses, particularly bentgrass, annual bluegrass and perennial ryegrass, are susceptible.

Highly maintained lawns, those with dense, highly fertilized stands, those that are watered frequently, and those with poorly drained thatch, are more prone to this disease. Shaded areas and areas with poor air circulation are also more susceptible. Moisture must be present on the grass for the fungus to attack healthy lawns. The fungus overwinters in the lawn.

Management Options

Prune trees and shrubs to improve air movement in affected areas. Avoid applying high-nitrogen fertilizers during hot weather. Minimize thatch buildup and avoid night watering.

Necrotic Ring Spot

This disease commonly occurs on sodded lawns three to four years after establishment. Symptoms include grass that is easily pulled out and root surfaces that are black. Outer blades of grass plant turn yellow while younger blades turn purple, then brown. Severely infected grass has few roots left because the fungus invades roots, rhizomes and crowns. It overwinters on roots and lower crowns and spreads in cool, moist fall conditions, showing symptoms from September to October. Kentucky bluegrass and fine fescues are particularly susceptible in late spring and early autumn, in full sun or shade.

Management Options

Ensure that adequate phosphorus and potassium are present and apply nitrogen lightly several times during the growing season. Rake to promote vigourous growth. Overseed affected areas with resistant cultivars such as Kentucky bluegrass cultivars Miracle, Unique, Bristol, Adelphi, Miranda, joy, Mystic, Alpine, Washington, Princeton, NE80-88.

Summer Patch

Summer patch often attacks Kentucky bluegrass and annual bluegrass during July and August, when lawn is under stress and soil temperatures are greater than 25°C. Distinct circular patches appear, with a ring pattern up to 10 cm in diameter. Infected roots are dark brown and the grass quickly withers, turns brown and dies.

Management Options

Maintain balanced fertility, avoid drought stress and excess thatch.

Pythium Blight

All turfgrass species, especially creeping bentgrass, annual bluegrass and perennial ryegrass are susceptible to Pythium blight. Though rarely found on mature home lawns, it can be a problem on newly seeded lawns when hot, humid conditions occur. It is more severe in areas of poor air circulation and drainage, and where the lawn has high levels of nitrogen.

Symptoms are initially similar to dollar spot, but under wet conditions the Pythium patches will be darker grey, greasy or have a water-soaked appearance, particularly in the morning. During hot, humid weather when day temperatures reach 30°C and night temperatures are above 20°C, patches grow quickly, often stretching in the direction of mowing or surface drainage.

Management Options

Do not apply excessive nitrogen that will result in lush growth during the summer. Do not water at night. Improve drainage and air circulation and reduce shade by pruning adjacent trees and shrubs. No species is resistant.

Powdery Mildew

Powdery mildew is most common on Kentucky bluegrass. Greyish-white, powdery masses on leaves and stems occur in shaded areas in cooler weather, especially in fall. If severe, leaves turn yellow and die.

Management Options

Use resistant varieties. Improve air circulation and reduce shade by pruning adjacent trees and shrubs. Keep lawn vigourous with balanced nutrition, and avoid producing too lush a lawn, as this increases susceptibility to the disease.

Mushrooms, Fairy Rings

Mushrooms may be a nuisance, but they do not damage grass. They grow on organic matter in the ground, usually decaying roots, scrap lumber or thatch.

Fairy ring, which is caused by several fungi in the soil, weakens and sometimes kills the grass in circular patterns. It is more likely to develop where there is a lot of organic matter or a high thatch build up, and is more common during long dry periods, particularly during summers which follow a wet spring. Symptoms appear as a ring of abnormal grass growth or an arc of mushrooms. Rings usually grow a few millimetres a year.

Management Options

Remove rotting rubbish to reduce growth of fungi. Frequent heavy watering, fertilizing, heavy raking or aerating help to stimulate regrowth of grass to hide damage.

Slime Moulds

Slime moulds are fungi that produce masses of unsightly yellow, blue-grey or black slimy growths on grass. This eventually changes to dry, crust-like globules that produce spores. With dry weather, sunshine, or vigourous raking, the mould disappears. Although unsightly, slime moulds are not harmful and do not require control.

Management Options

None required.


Moss is most often a problem in lawns where growing conditions are not suitable for grass. Heavy shade, low fertility, or damp, poorly drained and compacted soils encourage moss growth while retarding grass growth.

Management Options

The causes of the poor growth conditions for grass must be removed before moss growth can be controlled. If trees or shrubs are causing the shading, you may wish to prune them back or remove several of them to allow more light through the canopy. Where shade cannot be corrected, consider using ground covers such as vinca, pachysandra or English ivy, or more shade- tolerant grass such as fine fescues.

Drainage can be improved either by rerouting run-off water, installing tile drainage beneath the lawn or raising the lawn surface by adding soil. Low soil fertility can also be a problem because the roots of trees and shrubs are competing for moisture and nutrients. Have soil analyzed at soil testing laboratory before changing fertilizer program.

Learn More

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