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Summer patch and necrotic ring spot are separate diseases that,
formerly, were collectively known as Fusarium blight. They are serious
and widespread diseases of established Kentucky bluegrass turfs
that are managed intensively and difficult to control. Summer patch,
necrotic ring spot, and perhaps other components of the disease
syndrome produce symptoms that are largely indistinguishable from
each other.
The normally weak pathogenic, soilborne fungus Magnaporthe poae
with a Phialophora state causes summer patch disease when
hot, sunny days follow warm to hot, very wet periods. The soilborne
fungus Leptosphaeria korrae causes necrotic ring spot and
is favored by slightly cool temperatures and dry to wet conditions
from spring to autumn and can persist through winter and early spring.
The disease is most severe in late spring and early autumn. Magnaporthe
and Leptosphaeria, which are ecologically very closely
related fungi, also resemble the take-all fungus, Gaeumannomyces
graminis var. avenae, in their dark brown to black mycelial
growth (runner hyphae) over the roots, crowns, and rhizomes of grass
plants at the edges of the patches and in their disease development.
Also associated with diseased plants are species of Fusarium,
which are widely distributed, but generally considered to be secondary
pathogens or saprophytes. The Fusarium fungi may accelerate
the expression of symptoms but do not appear essential to the final
expression of disease.
Many bacteria, fungal species such as Curvularia, Colletotrichum,
Drechslera, Pythium, Rhizoctonia, Alternaria, Stemphylium, Trichoderma,
and yeasts can also serve as secondary colonizers or saprophytes
of the weakened grass plants, thereby substituting or coexisting
with the Fusarium species.
Grasses susceptible to one or more parts of this disease complex
include Kentucky and annual bluegrasses, Poa trivialis or
rough bluegrass, several fescues (chewings, red, sheep, and tall),
colonial and creeping bentgrasses, Italian and perennial ryegrasses,
bermudagrass, zoysiagrass, centipedegrass, many other forage, wild
and weed grasses, and small grains.
Summer patch and necrotic ring spot can become severe when turfgrass,
especially Kentucky bluegrass, is under stress and entering summer
dormancy, although infection may have occurred several months earlier
during cool weather. Prolonged periods of high humidity and warm
to hot weather, with temperature ranges of 70 to over 100 F (20
to 37 C) during the day and about 70 F (21 C) during night, favor
disease development. Moisture and heat stress, excessive watering,
close mowing, a thatch exceeding 3/4 inch thick, with a soil reaction
(pH) of above 7 or below 5, unbalanced applications of fertilizer—an
excess of nitrogen and low levels of potassium and/or phosphorus—high
populations of parasitic nematodes, such as the stunt nematode Tylenchorhynchus
dubius, compaction, and other factors that predispose turf to
stress conditions usually increase the severity of these diseases.
The abiotic factors that most influence disease development appear
to be related to the soil and thatch environment. In particular,
the factors responsible for the rate of thatch decomposition, including
good aeration under warm, moist conditions and an acidic or alkaline
soil pH, are strongly implicated. These diseases of Kentucky bluegrass
are generally not evident until the second or third year
on newly laid sod and usually not for four years or more after seeding.
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Figure
1.
Summer patch or necrotic ring spot first appears in Kentucky bluegrass
turf in light green patches that soon enlarge and rapidly fade
to a dull reddish brown, then a light tan, and finally to a light
straw color (Courtesy W.C. Steinstra).
Figure
2.
Summer patch, 2nd or 3rd year (R.W. Smiley)
Figure
3.
"Runner hyphae" of the summer patch or necrotic ring
spot fungus growing over two roots (courtesy R.W. Smiley).
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Symptoms
Scattered, circular, slow-growing, often thinned, light green or
grayish green patches, typically 1½ to 6 inches in diameter,
appear first. In warm to hot weather, they soon enlarge and rapidly
fade to a dull reddish brown or bronze, then a light tan, and finally
to a light straw color (Figure 1). The patches may become sunken
elongated streaks, crescents, or roughly circular and 1 to 3 feet
in diameter. The most characteristic symptom on Kentucky bluegrass
in many areas of the United States is a roughly circular, doughnut-shaped
area of dead or stunted grass up to 2 or 3 feet across, with tufts
of apparently healthy grass in the center, giving a "frogeye"
pattern (Figure 2). Plants die as the basal stem, crown, root, rhizome,
and stolon tissues are destroyed by a hard and tough, black, dark
brown, or reddish brown dry rot. Serious turf damage occurs when
the blighted areas are numerous and overlap. The circular patches
tend to increase in size for several years. The dead turf is commonly
invaded by weeds.
Frequently, more subtle symptoms appear. Irregular patches that
resemble drought or chinch bug injury contain living plants scattered
among dead or weak plants. The turf in these areas is stunted and
often pale green to various shades of red, yellow, or tan, eventually
turn a dull tan or brown, but does not develop distinct rings and
spots. Such turf does not readily recover from mowing or adverse
weather conditions.
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Figure
4.
Dark pseudothecia of Leptosphaeria korrae the necrotic
ring spot fungus, growing out of Kentucky bluegrass root tissues
(courtesy R.W. Smiley).
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Figure
5.
Magnaporthe poae with candidiophores and conidia (drawign
by Lenore Gray).
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Disease Cycle
The causal fungi grow outward radially at the rate of two to three
inches per year for several years. Any one patch may "die out"
or disappear after reaching a certain size. Leptosphaeria and
Magnaporthe, which are difficult to identify and to isolate,
are believed to survive seasons as mycelium and sclerotia in colonized
grass debris in the thatch or soil and possibly on perennial parts
of living grass plants. Healthy crowns, stems, and underground parts
of turfgrass plants are invaded by the mycelium of the two fungi
in cool to hot, moist weather. The fungi spread from plant to plant
by strands of dark brown to black mycelium (runner hyphae) that
grow over roots (Figure 3), crowns, rhizomes, or stolons and by
infected plant debris adhering to mowers, aerification dethatching,
and other turf equipment. Associated with the mycelium, and developing
on crown, leaf sheath, and root tissues, are dark brown, flattened
sclerotia and black, flask-shaped and beaked fruiting bodies called
pseudothecia of Leptosphaeria korrae (Figure 4) that, when
mature, contain large numbers of microscopic, needlelike ascospores.
Magnaporthe poae (Figure 5) produces large numbers of microscopic
conidia or phialospores. Symptom-free but infected sod, sprigs,
and plugs also may serve to spread these diseases.
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Control
Follow the suggested cultural measures outlined below. Avoid as many
environmental stresses as possible and keep the turfgrass growing steadily
throughout the growing season.
1. The initial step in managing turf for control of summer patch/necrotic
ring spot is to properly prepare the planting bed. These diseases are
often most devastating in new housing areas where much of the topsoil
is removed and sod is laid on a fine-textured, infertile soil that has
been heavily compacted by grading and construction equipment and vehicles.
Summer patch/necrotic ring spot can also occur when seed is planted into
poorly prepared soils. Soil preparation is extremely important for the
overall, long term health and quality of a turf.
Remove stumps, large roots, construction lumber, bricks, concrete, plaster,
mortar, and other debris. Provide five to seven inches of good topsoil.
Any soil amendments, such as peat moss, calcined clay, sand, or compost
should be mixed uniformly into the upper six inches of soil. When needed,
provide subsurface drainage with coarse textured amendments or underground
drain tile. Provide good surface drainage by grading for a slope of two
to four percent and filling in depressions to achieve a smooth, uniform
surface. Test soil and follow the soil test suggestions to adjust soil
reaction (pH) to between 6 and 7.
Grow locally adapted, disease-resistant grasses in blends or mixtures.
Avoid pure stands of very susceptible grass cultivars. Table 1
lists a number of modern Kentucky bluegrasses resistant to one or more
diseases. Other Kentucky bluegrass cultivars generally considered resistant
to summer patch and/or necrotic ring spot include Admiral, Banff, Bristol,
Challenger, Columbia, Georgetown, Glade, Majestic, Mystic, Nassau, Rugby,
and Trenton. Susceptible Kentucky bluegrasses include Argyle, Delft,
Kenblue, Merion, Newport, Nugget, Park, Plush, and Ram I.
Overseeding diseased turf with a blend of perennial ryegrasses or resistant
cultivars of Kentucky bluegrass provides control. Cultivars of perennial
ryegrass and tall fescue differ in their resistance to the summer patch
fungus. Red fescues are generally considered resistant to these diseases.
4. Maintain adequate to high balanced fertility, based on soil tests
and the recommended fertilization program for your area and the grass
or grasses being grown. Avoid overstimulation with fertilizer, especially
with a water-soluble nitrogen material, just before or during summer stress
periods. Necrotic ring-spot and summer patch occur much less frequently
when fertilizers that contain slow release forms of nitrogen. are applied.
5. Mow frequently at the suggested maximum height (2 to 3 inches) for
upright, lawn-type grasses; one-half inch or less for bentgrasses and
bermudagrass. Remove no more than one-fourth to one-third of the leaf
height at one cutting. Maintain a mowing height on Kentucky bluegrass
turfs of 2 to 2½ inches in the spring and fall; 2½ to 3
inches in the summer.
6. Where feasible, collect the clippings. This is especially important
under conditions of very high nitrogen fertilization during hot, very
humid weather.
7. Water established turf thoroughly during drought periods. Moisten
the soil to a depth of the root system at each irrigation. Repeat as needed,
usually every 7 to 10 days if the weather remains dry. Maintaining adequate
moisture on south- and west-facing slopes and next to sidewalks, driveways,
and parking lots is important because these sites dry faster and have
higher soil temperatures than other areas such as east- or north-facing
slopes. Avoid overwatering and frequent sprinklings in late afternoon
or evening. Daily watering may be needed in certain areas to prevent wilt
and symptoms of summer patch and necrotic ring spot. Midday heat stress
can be reduced by sprinkling (syringing) lightly one or more times between
about 11:00 a.m. and mid-afternoon.
8. Reduce soil compaction in turf by directing traffic to paved areas
through the use of fences, patios, ornamental plantings, or other means.
Core aerify to reduce compaction one or more times per year when turf
is actively growing (spring or fall for cool season turfs). Repeat operation
until there are 18 to 36 holes per square foot, 1½ to 2½
inches deep.
9. Remove thatch in spring or fall when turf is actively growing and
thatch has accumulated to one-half inch or more on higher cut turfs or
one-eighth inch or less for fine turf. Use a vertical mower, power rake
or similar dethatching equipment, often available from garden supply or
tool rental stores. Take care using these power tools when working in
areas with thick thatch; turfs may be rooted into the thatch and much
live turf may be removed by these activities.
FUNGICIDE MANAGEMENT
Where the cultural practices outlined above (1-9) are not providing the
expected control, preventive fungicide drenches may be applied. First,
consider the total cost of the disease management program and potential
loss from disease. Apply the first fungicide drench before disease
is evident or when soil temperatures at the 2-inch (5 cm) level are about
70 F (or 21 C). Two or three applications are needed at 21- to 28-day
intervals.
Carefully follow the manufacturer's directions on the container label.
Use one of the fungicides listed in the current edition of Illinois Commercial
Landscape Turfgrass Pest Management Handbook. The only two fungicides
that control both summer patch and necrotic ring spot are Banner
and Rubigan.
(Mention of a trade name or proprietary product does not constitute warranty
of the product and does not imply approval of this material to the exclusion
of comparable products that may be equally suitable).
Dollar spot of turfgrasses is believed to be caused by species of the
fungi Lanzia and Moellerodiscus (formerly known as Sclerotinia
homoeocarpa). It has been reported in most areas of the United States.
During warm moist weather in the northern part of this country dollar
spot is a serious disease of creeping bentgrass, Kentucky bluegrass, annual
bluegrass, and fine-leaf fescues. All lawn and fine turfgrasses grown
in the Midwest may be attacked. Perennial ryegrass and tall fescue are
potential hosts with Bermudagrass and zoysiagrasses the most severely
affected southern turfgrasses. The disease occurs during warm (60 to 85
F or 15 to 39 C), humid weather, particularly in turf deficient in nitrogen.
Drought stress increases the severity of disease, although leaf wetness
within the canopy is also required.
Table 1. Modern Kentucky Bluegrass Cultivars Adapted
to Illinois and Reported to be Moderately to Highly Resistant (R)a to
one or More Diseases
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Kentucky Bluegrass Cultivars
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"Helminthosporium"
diseases
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Leaf Smuts
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Leaf and stem rust
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Summer patch & necrotic
ringspot
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Sclerotinia dollar spot
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Typhula blight
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Septoria leaf spot
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Red thread
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A-20
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R
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R
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R
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R
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R
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(b)
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R
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A-34 (Bensun)
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R
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R
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R
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R
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Adelphi
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R
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R
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R
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R
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R
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R
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R
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R
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Baron
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R
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R
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R
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R
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R
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R
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R
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Bonnieblue
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R
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R
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R
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R
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R
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R
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R
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Brunswick
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R
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R
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R
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R
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R
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Cheri
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R
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R
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R
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R
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R
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R
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Enmundi
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R
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R
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R
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R
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R
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Enoble
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R
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Fylking
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R
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R
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R
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R
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Geronimo
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R
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R
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R
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Glade
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R
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R
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R
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R
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Majestic
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R
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R
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R
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R
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R
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R
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R
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Monopoly
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R
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R
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R
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R
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R
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Nugget
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R
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R
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R
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R
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R
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R
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Parade
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R
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R
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R
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R
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R
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R
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Plush
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R
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R
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R
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R
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R
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Rugby
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R
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R
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R
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R
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R
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Sydsport
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R
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R
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R
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R
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R
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Touchdown
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R
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R
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R
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R
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R
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Vantage
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R
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R
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R
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Victa
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R
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R
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R
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R
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R
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a A resistant (R) rating does not mean that a particular cultivar will
be resistant in all locations every year. Due to the presence of physiological
races or strains of the various fungi that cause these diseases, a cultivar
may be susceptible in one locality and highly resistant in another. This
is especially true of powdery mildew and is the reason we omitted this
disease from our ratings.
b A blank under a given disease does not necessarily indicate susceptibility.
In some cases it means that no data are available on which to evaluate
the relative susceptibility or resistance to a particular disease.
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