The Online Gardener's Handbook
Chapter 7: Lawns
Disease Control on Lawns
Table of Contents
- Grey Snow Mould
- Pink Snow Mould and Fusarium
- Leaf Spot and Melting Out
- Dollar Spot
- Brown Patch
- Necrotic Ring Spot
- Summer Patch
- Pythium Blight
- Powdery Mildew
- Mushrooms, Fairy Rings
- Slime Moulds
- 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and come spring, the fungus is no longer active.
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.
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
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
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.
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.
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.
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.
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,
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.
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.
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,
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.
Maintain balanced fertility, avoid drought stress and excess thatch.
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.
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 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.
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.
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 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.
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
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.