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There are several common but generally minor leaf spot and blight
diseases of turfgrasses caused by fungi. The most important are
members of the fungus genera Ascochyta, Septoria, and Leptosphaerulina.
In general, these fungi rarely cause severe damage, but can occasionally
cause concern.
ASCOCHYTA LEAF BLIGHT OR SPOT
Ascochyta leaf blight or spot of turfgrasses is caused by
more than 20 different species of fungi which can cause damage
to Kentucky bluegrass, bentgrasses, Italian and perennial
ryegrasses, fescues (red, meadow, sheep, and tall), redtop,
and many other forage, weed, and wild grasses (Table 1). The
Ascochyta fungi attack grasses during much of the growing
season when humidity and atmospheric moisture are high or
when irrigations and mowing are frequent. The Ascochyta fungi
seldom cause extensive damage.
Symptoms
Small to large turfgrass areas may appear uniformly blighted,
or localized pockets of infection may result in a patchy appearance
with healthy and diseased leaves growing interspersed . Individual
grass blades usually start dying back from the tip, often
progressing downward, sometimes into the leaf sheath (Figure
3). The minute, individual leaf spots are purplish to chocolate
brown. The lesions may later enlarge and merge with their
centers fading to tan and finally straw colored as the girdled
leaf dies. Speck-sized, yellow-brown, rust brown, brick red,
or black fungus fruiting bodies (pycnidia) form in the bleached
areas of dead leaves. The symptoms of Ascochyta leaf blight
may closely resemble that of Septoria leaf spot or tip blight
and leptosphaerulina leaf blight (both following).
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Figure
1.
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Disease Cycle
Figure
2.
. Left, pycnidium of Ascochyta embedded in a grass leaf;
right, spores of six species of Ascochyta as seen under
a high-power microscope (drawing L. Gray).
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| Ascochyta fungi overseason in debris
or in older leaf lesions as mycelia or pycnidia. During wet
weather, 2-celled microscopic spores called conidia (Figure
2), formed within the pycnidia, ooze out in tremendous numbers
in slimy tendrils. Infection occurs after the conidia are transported
to healthy leaves by splashing or flowing water, air currents,
or people (foot traffic and all types of turfgrass equipment).
A wet leaf surface is necessary for infection. Ascochyta fungi
usually invade grass leaves soon after mowing and later grow
downward from the moist, freshly cut end of a leaf blade toward
the leaf base. The pycnidia form after the leaf dies. Ascochyta
leaf blight or spot occurs throughout the growing season. Periods
of damp weather or frequent irrigations favor the disease during
the summer months. Frequent mowing favors the disease by creating
potential infection sites. |
Figure
3.
Ascochyta leaf blight. Note the speck-sized dark pycnidia
of the Ascochyta fungus in blighted portions (courtesy R.W.
Smiley).
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SEPTORIA LEAF SPOT OR TIP BLIGHT
Septoria leaf spot, sometimes called tip blight, is caused by about
15 species of the fungus Septoria. This is a minor disease that
infects many turfgrasses including bentgrasses, bluegrasses, fescues,
ryegrasses, redtop, and bermudagrass (Table 2). Septoria leaf spot
most commonly occurs during cool, wet weather in the autumn, late
winter, or early spring. The disease is rare in warm to hot weather.
Figure
4.
Septoria leaf spot or tip blight on six Kentucky bluegrass
leaves (courtesy R.W. Smiley).
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Symptoms
The overall appearance of diseased turf often resembles damage
caused by a dull mower, scorch, or dollar spot. The grass
blades are often a pale yellow from the tip downward (Figure
4). The small individual lesions, which may be scattered near
the leaf tips, are gray to gray-green or brown before fading
to yellow or light straw color. On perennial ryegrass, however,
the lesions are yellowish green, later turning a dark chocolate
brown. The lesions enlarge, merge, become bleached, and typically
girdle the leaf blades causing the tips to become straw-colored
and mottled. Such leaves commonly defoliate prematurely, thinning
the turf (Figure 5). All Septoria fungi produce speck-sized,
light brown to black fruiting bodies (pycnidia) in the dead
areas of older lesions (Figure 4). They are easily seen with
a magnifying lens and help to distinguish Septoria from dollar
spot, injury from a dull mower, drought, or winter injury.
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Disease Cycle
The Septoria fungi survive unfavorable cold and hot periods
as pycnidia and mycelia in infected grass debris produced
in the previous season. During cool, wet conditions in spring,
autumn, or late winter, large numbers of needlelike, microscopic
spores (conidia) are formed within the pycnidia (Figure 6)
and released. The conidia are splashed and washed or transported
on shoes and all types of turfgrass equipment to healthy leaves.
Here the spores germinate in a film of water and infection
occurs, often in a freshly cut leaf tip. The cycle can be
repeated every 7 to 14 days as long as the weather remains
moist and cool (60 to 75 F or 16 to 24 C).
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Figure
5.
Septoria leaf spot damage on Poa pratensis (courtesy R.S.
Byther).
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LEPTOSPHAERULINA LEAF BLIGHT
Leptosphaerulina leaf blight, caused by Septosphaerulina australis
and possibly one or more other species, is a minor, warm to hot
muggy weather disease in Illinois. Susceptible turfgrasses include
creeping bentgrass (Agrostis palustris), Kentucky bluegrass (Poa
pratensis), and perennial ryegrass (Lolium perenne). The disease
is easily confused with Ascochyta leaf blight and Septoria leaf
spot as well as with dollar spot and Pythium blight. Disease outbreaks
are most common during warm, humid weather when the turfgrasses
are stressed by close mowing, drought, applications of preemergence
annual grass herbicides, and excessive rates of nitrogen fertilizer,
or on newly laid sod that lacks good root contact with the underlying
soil.
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Symptoms
Large turf areas may become uniformly blighted or appear patchy
with individual leaves dying back from the tip (Figure 7). Uniform
yellow or brown lesions may extend down to the leaf sheath. Water-soaked
lesions, which quickly fade to a bleached white, may also occur
on the leaf blades. Such lesions closely resemble the damage due
to high temperatures, frost, or a dull mower. Tiny, pale brown fungus
fruiting bodies (perithecia), which closely resemble the pycnidia
of Ascochyta and Septoria, form in the dead tissues of older lesions.
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Figure
6.
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Figure
7.
Perithecia of leptosphaerulina australis in blighted leaves of
Poa pratensis (courtesy R.W. Smiley).
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Disease Cycle
The weakly pathogenic Leptosphaerulina fungus (or fungi) overseason
as perithecia and mycelia in dead grass tissue. The perithecia produce
club-shaped asci, each of which contains 8 muriform ascospores which
have both cross and longitudinal walls (Figure 8). The microscopic
ascospores are produced and released in warm, wet weather and are
blown, splashed, and transported on shoes and turfgrass equipment
to healthy leaves. The spores germinate and penetrate the leaf blades
in a film of moisture.
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Control
These three diseases are usually easily controlled by adhering
to the following cultural practices. No fungicide applications are
normally needed.
1. Provide good surface and subsurface drainage when establishing
a new turf area. Test the soil reaction (pH) and follow the recommendations
in the soil report. A pH between 6 and 7 is best for all turfgrasses
grown in the Midwest.
2. Grow locally adapted, disease-resistant grasses in blends or
mixtures. Check with University of Illinois Extension Turfgrass
specialists or Extension adviser for suggested grass species and
cultivars to grow and for available disease resistance in turfgrass
cultivars.
3. Purchase only top-quality, certified sod, sprigs, plugs, or pathogen-free
seed from a reputable dealer. Plant at suggested rates in a fertile,
well-prepared seedbed.
4. Fertilize according to local recommendations and soil tests.
Recommendations will vary with the grasses grown and their use.
Adequate to high levels of phosphorus and potassium (potash) may
help to suppress disease development. Do not apply fertilizer during
periods of drought or high temperature. Turfgrass fertilizer recommendations
can be obtained from University of Illinois turfgrass specialists
or your nearest Extension adviser.
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Figure
8. Spores
of leptosphaerulina australis as seen under a high-power microscope
(courtesy R.W. Smiley)
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5. Mow frequently at the height recommended for the area, season, and
grasses grown (1 1/2 to 2 1/2 inches for lawn-type grasses and 1/4 inch
or slightly less for fine bentgrass turf). Remove no more than one-third
of the leaf height at one cutting. Keep the mower blades sharp. Do not
mow when the grass is wet.
6. Water established turf thoroughly during droughts. Moisten the soil
to a depth of at least 6 inches at each irrigation. Repeat every 7 to
10 days if the weather remains dry. Water as infrequently as possible
to allow gaseous exchange between soil and atmospheric air. Avoid light
sprinklings, especially in late afternoon or evening.
7. Increase light penetration and air movement to the turfgrass area
and speed drying of the grass surface by selectively pruning or removing
dense trees, shrubs, and hedges bordering the turf area.
8. Remove excess thatch in early spring or fall when it accumulates to
1/2 inch for high-cut grasses and 1/8 to 1/4 inch for fine turf. Use a
vertical mower, power rake, or similar dethatching equipment. These machines
may be rented at most large garden supply or tool rental stores.
9. Core heavily compacted areas one or more times each year, using a
hand aerifier or power machine. Eliminate foot and vehicular traffic by
putting in walks, fences, shrubbery, patios, parking areas, and so on.
10. Follow suggested insect and weed control programs for the area and
grasses being grown. Follow recommendations of University of Illinois
Extension Entomologists and Turfgrass specialists. Insects commonly transmit
disease-causing fungi or provide entry wounds for them, while weeds may
harbor pathogenic fungi.
11. Kentucky bluegrass cultivars reported as having good resistance to
one or more species of Septoria include ‘Adelphi', ‘Cheri',
‘Fylking', ‘Geronimo', ‘Majestic', ‘Nugget', ‘Parade',
"Sydsport', and ‘Touchdown'; ‘Newport', ‘Park', and
‘Trenton' are reported as having intermediate resistance; ‘Common'
and ‘Delta' are rated as susceptible; and ‘Merion' as very susceptible.
No information about cultivar resistance is available for Ascochyta and
Leptosphaerulina leaf blights.
12. Avoid applying herbicides or installing new sod before or during
hot, humid weather.
The cultural practices outlined above (1-12) should keep these three
diseases well under control. If not, the fungicides suggested for controlling
"Helminthosporium" diseases and dollar spot should be at least
partially effective when properly applied just before infections take
place. Suggested fungicides for turfgrasses are given in Homeowner's Guide
to Pest Management, and Illinois Commercial Landscape and Turfgrass Pest
Management Handbook. These publications are available at your nearest
Extension office or the Office of Information Technology and Communication
Services, Ag Services, P345, 1917 South Wright St., Champaign, IL 61820
(1-800-345-6087).
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