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University
of Rhode Island GreenShare Factsheets
Some
Common Fungal Diseases of Turf
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.
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.
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.
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
Pesticides
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
For more
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
University
of Rhode Island Cooperative Extension provides equal program
opportunities.
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