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Anthracnose is a widely distributed but generally minor disease
of turfgrasses caused by two fungi: Colletotrichum graminicola
(synonym C. cereale; sexual state Glomerella
graminicola) and Microdochium bolleyi (synonym
Gloeosporium bolleyi). The Glomerella sexual state
of Colletotrichum is rare in nature.
In the Midwest the disease is most commonly found on closely cut
annual bluegrass (Poa annua). Bentgrasses, fine-leaved
fescues, perennial ryegrass, and Kentucky bluegrass are also commonly
found infected. Probably all turfgrasses are susceptible during
prolonged, warm to hot 80 to 90 F (27 to 32 C), dry or very moist
weather when the grass is under severe stress. If such conditions
persist, this damage can become quite extensive (Figure 1). Anthracnose
usually occurs where the turfgrass is weakened by such causes as
"Helminthosporium" leaf spot and melting-out diseases,
insect damage, soil compacted from heavy machinery or foot traffic,
low or unbalanced fertility, poor soil drainage, a thick thatch,
or drought stress.
Symptoms
During cool, wet periods – or during hot weather on closely
cut Poa annua greens and fairways – water-soaked
lesions occur on the stems. These lesions later become bleached
and girdle the tiller, and scattered individual or small patches
of plants turn yellow and die. The crowns and roots can also be
infected. During warm to hot weather, especially when the soil is
dry and the turf and atmosphere are wet or very humid, the anthracnose
fungi readily colonize older leaves apparently hastening leaf and
tiller senescence and death. Occasionally, lesions are found on
individual grass blades which are round to oblong, or elongated,
reddish brown to brown spots or blotches surrounded by a yellow
halo (Figure 2). These lesions may enlarge and merge to blight and
kill the entire leaf blade. The anthracnose fungi commonly infect
grass blades from the tip down, especially grass that has been freshly
mowed.
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Figure
1.
Anthracnose with early yellowing followed by bronzing symptoms
on annual bluegrass.
Figure
2.
Anthracnose lesions on five Kentucky bluegrass leaves. Healthy
leaf to the right (courtesy Dr. R.W. Smiley).
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Infected leaves turn yellow, then light tan to brown as they die.
The chief diagnostic feature of the more common Colletotrichum
anthracnose fungus is the appearance of numerous minute, raised,
black fruiting bodies, called acervuli, which are widely distributed
on senescing and dead leaves. The acervuli, easily seen with a magnifying
lens, are covered with long black bristles called setae which easily
distinguishes Colletotrichum from other fungi which infect or grow
on turfgrasses. The Microdochium anthracnose fungus does not produce
setae. Acervuli are also evident on young leaves when either fungus
is active.
Shoot infections result in stem girdling and can develop into yellow-bronze
patches of turf varying in size from 2 inches followed by a slow
deterioration of large, irregular areas of turf 10 to 20 feet or
more in diameter (Figure 1). Anthracnose-blighted plants are reddish
brown at first fading to yellow, then tan or brown. The stem bases,
crowns, and root systems of severely infected plants may be brown
or black and shallow.
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Disease Cycle
The anthracnose fungi produce large numbers of microscopic, one-celled,
bean- or crescent-shaped spores (conidia) in the elongated acervuli in
leaf and stem lesions. The conidia of Colletotrichum are
larger (4 to 25 microns long) than those of Microdochium
(5 to 9 microns). The conidia are distributed by splashing or flowing
water, air currents, grass clippings, shoes, and on all types of turfgrass
equipment. Both fungi overseason as dormant mycelium in colonized grass
debris in the thatch layer. When atmospheric humidity is high and leaves
are wet, both germinating conidia and fungal hyphae penetrate foliar,
stem, or root tissues under stress from temperature extremes, drought,
soil compaction, unbalanced fertility, or another cause. Acervuli form
on dead tissues and conidia are disseminated to healthy plants repeating
the disease cycle.
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Control
1. Keep the turfgrass growing vigorously throughout the growing season
by carrying out the following cultural practices:
a. 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.
b. Grow locally adapted, disease-resistant grasses in blends or mixtures.
Check with University of Illinois Extension Turfgrass Specialists and
your local county extension adviser for suggested grass species and cultivars
to grow and for available disease resistance in turfgrass cultivars.
c. Purchase only top quality, certified sod, sprigs, plus, or pathogen-free
seed from a reputable dealer. Plant at suggested rates in a fertile, well-prepared
seedbed.
d. 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.
e. 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.
f. 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.
g. 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.
h. Remove excess thatch in early spring or fall or when it accumulates
to 1/2 inch for higher-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.
i. Core aerify 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.
j. 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.
k. Where feasible, golf greens and other high-value turf should be cooled
down once or twice on very hot days by syringing for a few minutes during
the hottest part of the day.
l. Control other turfgrass diseases such as those caused by "Helminthosporium".
These diseases have been shown to promote the outbreaks of anthracnose.
m. Strains of creeping bentgrass and annual bluegrass are tolerant to
most pathogenic races of the anthracnose fungi. Resistant hard fescues
include ‘Aurora', ‘Reliant', ‘Spartan', and ‘Waldina'.
2. The cultural practices outlined previously should keep anthracnose
well under control. If not, there are several protective fungicides which
have proven effective. Suggested fungicides, when and how they should
be applied, are given in Illinois Extension Urban Pest Management Guide,
which is revised annually. It should be available in all Extension offices.
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