Powdery
mildews are common on many kinds of plants. Although they may
look the same, each kind of plant is infected by a different species
of powdery mildew fungus. Powdery mildew fungi on grasses will
not infect lilacs, phlox, roses or other garden plants.
Symptoms:
Powdery mildew may appear quite suddenly, usually in shaded
areas, and most commonly on Kentucky bluegrass. The grass blades
look as if they were dusted with flour or lime. The white to gray
powder is a combination of the mycelium and spores of the powdery
mildew fungus. The mycelium grows over the surface of the leaf,
absorbing nutrients from the plant. Later, the leaf may turn yellowish
and begin to dry up and die, but the leaves often support the
presence of the powdery mildew fungus for some time without significant
injury. Powdery mildew is most common in turf from July to September,
and occasionally in the spring. It is most common during overcast
periods of cool, moist weather.
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| Powdery
mildew on turfgrass |
Close-up
of powdery mildew fungus
on
grass leaves.
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Life Cycle:
Powdery mildew fungi overwinter on infected grass plants and in
survival structures on dead grass. Spores can infect leaves in
less than two hours, and new spores are produced in abundance
in about a week. Air currents carry the spores to new grass plants.
Disease development can be so rapid that powdery mildew may seem
to appear very suddenly.
Cultural
Management: Powdery mildew is usually a disease of shaded
turfgrass. Similar grasses growing nearby in full sun usually
will remain free of powdery mildew. To reduce shade and increase
air circulation, prune shrubs and tree branches. If grass is thin
or weakened in shady areas, re-seed with shade-tolerant
grass species and select cultivars resistant to powdery mildew.
If grass is growing very poorly due to shade, consider its replacement
with shade-loving ground covers such as hosta, pachysandra or
lily-of-the-valley. In moist, shaded areas of lawns, powdery mildew
may be prevented or reduced by careful lawn care. Avoid excess
nitrogen fertilizer, raise the mowing height and water deeply
but infrequently. Avoid frequent, light sprinkling.
Chemical
Management: Fungicides
are not recommended for powdery mildew control on lawns.
Symptoms:
Red thread and pink patch are caused by two different fungi,
but they often appear together and under the same environmental
conditions. Lawns infected by these fungi may have a pink-to-reddish
cast when viewed from a distance. If moist grass is viewed more
closely, pinkish gelati-nous growth of the fungi and tiny cotton-candy
tufts of spores may be seen. In drier conditions, tiny red threads
of the red thread fungus may be observed extending from the leaf
tips. The grass itself may show irregular patches of dead and
dying leaves, giving the lawn a ragged appearance. These diseases
can be found on many turfgrass species but are seen most often
on perennial ryegrasses and fine leaf fescues. The diseases are
restricted to the leaves and are not usually very destructive.
Both fungi occur mostly in the spring and fall in cool (65-70
degree F), moist weather.
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Red
thread on turfgrass. Photo from the Maryland Cooperative Extension.
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Disease
Cycle: The gelatinous masses of both fungi serve as survival
structures in the thatch when dry. The fungi can also survive
as mycelium in infected leaves and thatch. Some spores or pieces
of fungus may be airborne, but the fungi are commonly spread by
water, feet and mowing. Infections are most common on slow-growing
grass stressed by compaction, drought, low temperatures or inadequate
fertility.
Cultural
Management: Although we have no control over periods of prolonged
cool, moist weather, it is possible to prevent or reduce red thread
and pink patch through good lawn care practices. Soil pH should
be maintained at 6.5-7.0 or as appropriate for the turf species.
Watering should be infrequent and deep. Avoid frequent sprinklings
late in the day that extend the time that the grass blades stay
wet. Prune trees and shrubs to increase light penetration and
air circulation. Soil fertility must be balanced. These diseases
are most common in nitrogen-deficient turf, but be cautious about
applying excess nitrogen fertilizer, especially in spring. Other
more serious diseases, such as leafspots
and melting out, can be enhanced by excess nitrogen. If red
thread or pink patch diseases are found in slow growing grass,
it may be helpful to temporarily collect clippings during mowing
to reduce the amount of fungus returned to the lawn. Aeration
of compacted soil will improve turf growth and reduce these diseases.
Chemical
Management: Fungicides are not recommended for red thread
or pink patch on lawns except in extreme cases.
Symptoms:
There are many different rust fungi that can infect lawn grasses,
but they all have in common the production of reddish, yellowish
or orange spores that give "rusts" their name. If only a small
amount of infection has occurred, the rust spores will only be
seen by close observation of the tiny, powdery pustules on the
leaf blades. In severe cases, there may be enough spores present
to leave a reddish dust on mowers, pants and shoes. The turfgrass
itself may not look particularly diseased until after the infection
is well developed. In fact, because spore production does not
occur until one to two weeks after infection, rust-infected turfgrass
leaf blades are usually mowed away before rust spores can be produced.
In rare severe infections, the lawn may look thin and be weakened
and more susceptible to other stresses such as drought or winterkill.
Rust is most common on Kentucky bluegrass and perennial ryegrass,
and occurs mostly in late summer and early fall.
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| Rust
fungus on grass blades |
Disease
Cycle: Most rust fungi have complex life cycles that involve
two unrelated host plants and many spore stages. The disease cycle
on lawns is usually limited to the rust-colored spore stage which
rarely survives New England winters. As a result, most rust infections
are initiated by spores that are blown up from warmer southern
areas. Rust fungi cannot grow without a living host plant. Infection
only occurs when water is on the leaf surface and can occur at
most normal summer temperatures. There is usually not enough time
for spores to be produced on lawns that are mowed regularly, so
epidemics are not usually severe.
Cultural
Management: The most important way to avoid rust infection
on lawns is to keep the turfgrass growing vigorously so that it
will be mowed before spores can be produced. Balanced fertility
based on soil tests, aeration to relieve
compaction, thatch removal and adequate
water supply are among the important factors that will optimize
turfgrass growth. Watering practices that keep foliage dry as
much as possible will help reduce infec-tions by rust and other
foliar fungi.
Chemical
Management: Fungicides are not recommended for rust diseases
on lawns except in extreme cases.
Symptoms:
A superficial fungus-like growth may appear suddenly on leaf blades
of turfgrass in warm, wet weather. Slime molds are initially slimy
but mature to a powdery stage when spores are produced. They vary
in color from black to powdery gray, pink, yellow or orange. They
may occur in large quantities in rainy weather, enough to be observed
even at a distance.
Disease
Cycle: Slime molds are saprophytes, obtaining their nutrients
from dead organic matter in thatch and soil. They are unable to
infect turf or other plants as parasites and do not cause any
injury beyond that caused by coating the leaf blades. Slime molds
are initiated from spores and develop into a slimy film. As they
mature, the slime is converted into a mass of powdery spores which
may be airborne to new sites. Slime molds are commonly observed
on tree leaf litter, rotting wood, turfgrass, wood chips and other
organic mulches used in landscaped areas.
Cultural
Management: Because slime molds do not infect turfgrass and
simply cover the surface of the leaves, they can be removed by
raking or hosing the superficial substance away.
Chemical
Management: Fungicides are not recommended for slime molds
because they do not cause turf diseases and can be easily removed
mechanically.
Symptoms:
Leaf smuts are named for the powdery black spore masses produced
by the smut fungi that infect Kentucky bluegrass and, occasionally,
perennial ryegrass. In the spring and fall, at temperatures of
50-65 degrees F, plants may look stunted and yellow, turning grayish
just before the streaks of black spores break through the leaf
surface. Later, leaves appear twisted and frayed. During hot summer
weather, smut symptoms often disappear, but the plants are still
infected. They are very susceptible to various stresses, such
as heat and drought, and many will die. Smut symptoms may return
in the fall, and the fungus will continue to live inside the plant
until the plant dies. Smut is usually not reported until lawns
are three to four years old, when yellowing and thinning is first
noticed and black streaks of spores are discovered.
Disease
Cycle: The black dusty smut spores survive in soil, thatch,
and on seed. They are easily transported via air, soil, water,
feet, seed and equipment. Turfgrass plants are infected by smut
only at growing points, and they will remain infected until they
die.
Cultural
Management: Only smut-free seed and sod should be used to
establish new lawns. A few infected plants are easy to overlook,
and smut is usually not discovered until infections are more extensive
in older lawns. In established lawns, smut will be worse with
excess thatch, soil pH below 6.0, frequent watering and excess
nitrogen fertilizers. These same factors encourage other diseases
as well and should be avoided. After infected plants die out during
summer stress, reseed in the fall with a blend of resistant cultivars.
Chemical
Management: Only penetrant/systemic fungicides are useful
against smut fungi. They are expensive and usually must be applied
more than once. Fungicide applications are not economical or practical
for most lawn situations compared to reseeding damaged areas.
Adapted
from the UMass Cooperative Extension, 2000.
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