Reports on Plant Diseases |
| RPD No. 313 -
Anthracnose of Forage Grasses and Cereals |
November
1984 |
[ Symptoms
] [ Disease Cycle ] [ Control
]
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Anthracnose, caused by a widely distributed, soil-inhabiting fungus
Collecotrichum graminicola (C. graminicolum), attacks practically
all grasses and cereals grown in Illinois. Anthracnose is usually
of little importance except in coarse, sandy soils where fertility
is low or unbalanced and where a more or less continuous grass-cereal
culture is followed. Early attacks of the disease cause a general
reduction in vigor, premature ripening or dying, and shriveling
of the seed. Defoliaton of sorghum and Sudangrass reduces the value
of plants for forage and may lower the sugar content of the stalks
in very susceptible varieties. Losses vary greatly from year to
year because the Colletotrichum fungus is greatly affected by variations
in weather. Periods of humid, hot weather (about 82°F or 28°C) are
optimum for disease development.
In severe attacks, plants may die prematurely. Heads may become
bleached and sterile. Winter varieties of oats suffer more severe
attacks than spring varieties. G.H. Boewe, former plant pathologist
with the Illinois Natural History Survey, has reported almost 100
percent of the oat plants in fall-sown field to be infected in certain
years. Many of these plants are weakened and killed prematurely.
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Symptoms
Anthracnose is most noticeable on many grasses and cereals as plants
approach maturity. Sudangrass, sorghums, broomcorn, and Johnsongrass–all
species of Sorghum–however, suffer most during hot, damp weather
in midsummer at the height of their vegetative growth.
If conditions are favorable (warm and moist), the disease causes
stunting and wilting and sometimes kills grass and cereal seedlings.
It occurs commonly over a temperature range of about 60° to 90°F
(15.5° to 32°C). Affected plants often appear to recover as the season
progresses. In older plants, the leaves, leaf sheaths, and stems
(culms) are affected. Severely infected plants are stunted and have
few tillers.
Anthracnose often develops on cereals and grasses as a bleaching
of the stem bases, followed by a brown rotting of the crowns and
roots. If the disease is severe, the collectotrichum fungus may
also spread into the crowns and roots of perennial grasses, causing
lodging. Affected stands may die in the second or third year, especially
in sandy, dry, unfertile soils.
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Figure
1. Anthracnose on Orchardgrass (courtesy University of Wisconsin).
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Figure
2. Anthracnose on oat leaves. Note the black fruiting bodies
of the Colletrichum fungus in the affected area. Infection frequently
occurs at the base of a leaf.
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On sorghums a red or purplish rotting of the basal stalk often
occurs (called red rot). When stalks are split lengthwise, the pith
is discolored in areas and interspersed with white, giving a marbled
appearance to the decayed area. Depending on the cultivar, the discolored
areas range from tan to purplish red. A similar symptom occurs when
the peduncle or upper stem below the head becomes infected. Rotted
stalks frequently break near the middle of the stalk or just below
the seed head. Diseased plants may not lodge but produce small,
poorly filled heads. (For a more complete account of the root rot-crown
rot disease complex of cereals and grasses, read Report on Plant
Diseases No. 113, "Root and Crown Rots of Small Grains").
Small, well defined; round to elliptical, elongate, or irregular;
often zonate leaf spots or lesions are commonly produced on the
lower leaves of susceptible plants. The lesions usually have light
tan or straw-colored centers (reddish brown on oat leaves) with
dark red to brown borders (Figures 1 and 2). As the disease progresses,
it gradually affects all leaves on highly susceptible species like
Sudangrass and sorghums. Individual lesions may merge, causing entire
leaves to wither, die, and drop prematurely. On broom corn, production
of heads may be poor.
The fruiting bodies of the Colletotrichum fungus are elongated,
slightly raised, dark brown to black specks called acervuli. They
may form in the bleached centers of older lesions on both living
and dead leaves (Figure 1 and 2), at or near the joints (nodes)
of stems, and on diseased heads when moisture is abundant. Clusters
of small black "spines" or setae (visible with a hand
lens or strong reading glass) protrude from the acervuli and help
to distinguish Collectrichum from other leaf-blighting fungi. (See
Report on Plant Disease No. 116, "Scald of Cereals and Forage
Grasses"; No. 309, "Helminthosporium Leaf Spots and Blotches
of Forage Grasses"; No. 310, "Brown Stripe or Leaf Streak
of Forage Grasses"; No. 311 "Selenophoma Leaf Spot or
Speckle of Forage Grasses"; and No. 312, "Stagonospora
Leaf Spot or Blotch of Forage Grasses"). Back to Top
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Disease Cycle
The tiny black acervuli, found on above-ground parts of diseased plants,
produce large numbers of spores (conidia) in pinkish masses during warm
to hot, damp weather (optimum temperature about 77°F or 25°C). Wind and
splashing or blowing rain disseminate these spores to nearby plants, where
they germinate, penetrate directly through the epidermis or stomata, and
start new infections. The Colletotrichum fungus survives the winter on
both living and dead tissues–seed, leaves, and stubble–of grains,
forage, and weed grasses, Quackgrass, Johnsongrass, cheat, wild barley,
foxtails, hairy crabgrass, barnyard grass, bermudagrass, orchardgrass,
redtop, red fescue, switchgrass, and other grass weeds are susceptible
and are common sources of conidia to initiate new infections. Primary
infection is from conidia and fungus threads (mycelium) on crop refuse.
Infected seed that is not properly treated with a thiram- or captan-containing
fungicide is a possible source of seedling root and crown infection. As
plants mature, secondary spread of anthracnose is general from conidia
as well as from mycelium on crop residues.
Isolates or pathogenic races of Colletotrichum graminicola that attack
corn do not infect Sorghum species and small grains, nor do isolates from
Sorghum spp. and cereals infect corn.
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1. Maintain adequate to high, balanced soil fertility–especially
of potassium and phosphorus–based on a soil test. Remember that anthracnose
is most severe in old sod or continuous grain fields where fertility is
low or unbalanced.
2. Where possible, sow only thoroughly cleaned, certified, plump, disease-free
seed of improved, well-adapted small grain and grass varieties recommended
by University of Illinois agronomists and your county Extension adviser.
The seed should be properly cleaned to remove the light, infected kernels.
3. Treat the seed of small grains, sorghums, Sudangrass, and other forage
grasses with a protectant fungicide. (Read Report on Plant Disease No.
1001, "Seed Treatments for Field Crops," for details). Seed
treatment helps to prevent the introduction of the Colletotrichum fungus
carried on the seed, to new fields.
4. Where feasible, rotate forage grasses and cereals with soybeans, forage
legumes, or corn for one year or more. Rotation helps to prevent disease
buildups.
5. Avoid the following:
a. A sequence of closely related grasses and cereals.
b. Pure dense stands of a single grass variety. Where practical, seed
a mixture of forages.
c. Leaving a heavy mat of hay on the grass during warm, damp weather.
6. Plant at the recommended rate in a fertile, well prepared seedbed.
7. Keep down weed grasses by cultural or chemical means.
8. Where feasible, plow under crop and weed debris cleanly.
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For further information concerning diseases of crucifers
and other vegetables, contact Mohammad Babadoost, Extension Specialist in Fruit
and Vegetable Pathology, Department of Crop Sciences, University of Illinois
at Urbana-Champaign.
University of Illinois Extension provides equal
opportunities in programs and employment.
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