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Corn leaf spots and blights are common foliar pathogens
throughout Illinois. Although the intensity can vary due to weather,
tillage system, and hybrid resistance, these diseases are among
the most common plant disease problems in the Corn Belt.
One of the best known of the leaf blights is southern
corn leaf blight (SCLB), which caused extensive and widespread
damage to the corn crop in 1970. A new race of this fungus, designated
Race T, attacked both inbreds and hybrids with the Texas male-sterile
(Tms) cytoplasm. An estimated 80-85% of the dent corn grown in
1970 had Tms cytoplasm. Race T not only attacked leaves, but also
leaf sheaths, ears, and stalk tissues.
Race T unexpectedly multiplied and spread rapidly
during 1970. The disease began in the southern states and by early
to mid-August, it was established as far north as Minnesota, Michigan,
and Maine. An estimated 250 million bushels of corn was lost to
SCLB in Illinois alone.
In 1971, losses to Race T virtually disappeared.
The production of normal cytoplasm (N) seed was greatly increased,
weather conditions were not as favorable for SCLB infections, infected
residues were buried by farmers, non-host crops were planted in
affected fields, and earlier planting was used. Early planted corn
generally escaped severe damage in 1970. Since this time, the use
of normal cytoplasm plus other management factors has controlled
Race T as well as reducing losses to the more common leaf-infecting
Race O.
A major change has occurred with respect to the renaming
of some of the more common members of the leaf-blighting group.
Fungal taxonomists have removed northern and southern corn leaf
blight from the Helminthosporium group and placed
them in other genera. However, there is not yet full agreement
or acceptance of the new names. The list below gives the "old" names
as well as the most commonly used "new" name and some
other synonyms. To avoid listing all the names, the term "Helminthosporium leaf
blights" will be used in this text when referring to the group
as a whole:
SOUTHERN CORN LEAF BLIGHT (Helminthosporium
maydis): Bipolaris maydis, synonym: Drechslera
maydis, Cochliobolus haterostrophus.
NORTHERN CORN LEAF BLIGHT (Helminthosporium
turcicum): Exserohilum turcicum, synonyms: Bipolaris
turcicum, Drechslera turcicum, setosphaeria turcica.
NORTHERN CORN LEAF SPOT (Helminthosporium carbonum): Bipolaris
zeicola, synonym: Drechslera zeicola, cochliobolus
carbonum.
Other common leaf blights include Gray Leaf Spot
(Cercospora zeae-maydis), Eyespot (Kabatiella
zeae), Anthracnose Leaf Blight (Colletotrichum
graminicola) and Yellow Leaf Blight (Phyllosticta
maydis).
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Southern Corn Leaf
Blight
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Southern corn leaf blight (SCLB) is favored
by warm temperatures (68-90 F) and high humidities. Thus,
it tends to be more of a problem in the southern half of
Illinois, although it can be found farther north if weather
conditions are favorable. Frequent rainy periods enhance
disease development.
The symptoms of SCLB are leaf lesions ranging
from minute specks to spots of one-half inch wide and one
and one-half inches in length. They are oblong, parallel-sided,
and tan to grayish in color. A purplish to brown border
may appear on the lesions depending on the genetic background
of the plant. Early and severe infections in susceptible
plants predisposes them to stalk rots.
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Figure
1. Southern corn leaf blight.
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The fungus overwinters in corn debris as spores or
mycelium. Spores are spread by wind or splashing water to growing
plants. After infection and colonization, sporulation from these
primary lesions serves as the source for secondary spread and infections
as long as weather conditions are favorable for disease development
and living tissues are present. The disease cycle may repeat every
few days under ideal conditions. Germination of spores and penetration
into the plant can occur within six hours when free water is present
on the leaf surface and temperatures are between 60 and 80 F. Control
of SCLB is easily accomplished with resistant hybrids. Although
some slight flecking may be found in some hybrids, this is simply
a part of the resistant reaction and does not lead to any economic
loss.
Burial of crop residue is helpful where erosion is
not a problem. Crop rotation is especially suggested where no-till
is used or where heavy crop residues are found. Since this fungus
overwinters on debris, the planting of corn into such residues
may result in earlier infection and poor seedling performance.
Foliar fungicides are useful in seed production fields. For optimal
control, it is important to control foliar disease during the period
from 14 days before tasseling to 21 days after tasseling. Research
has shown that this four-week period is the most critical for leaf
blight damage and that yields and quality are most affected if
susceptible inbreds are not protected at this time.
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Northern Corn Leaf
Blight
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Northern corn leaf blight (NCLB) is favored
by moderate temperatures (65 -85 F) high humidity and heavy
dews during the growing season. Dry conditions greatly reduce
disease incidence. Early infections before silking can cause
yield reductions of 50% or more on susceptible inbreds and
hybrids. If disease onset is delayed until six weeks after
silking, losses are minimal. In addition to grain losses,
forage value is reduced and plants are predisposed to stalk
rots.
NCLB is recognized by long, elliptical lesions
that are typically cigar-shaped. Lesions may be as large
as 3/4 inch in width and 2 inches in length. Lesions first
appear on the lower leaves. The disease progresses upward
until, in severe cases, nearly all of the leaves are infected.
However, this is not common since dent corn hybrids planted
in Illinois have genetic resistance to this pathogen. Damage
can be extensive in susceptible inbreds if lesions occur
at or above the ear leaf.
NCLB overwinters in corn debris. Conidia (spores)
can be windblown over long distances. Water splashing can
also cause lower leaf infections and result in seedling blighting
where continuous corn is planted. Infection is initiated
when free water is present on the leaf surface for 6 to 18
hours and the temperature is between 65 and 80 F.
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Figure
4. Northern corn leaf blight.
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Control of NCLB is based upon selection of resistant
hybrids. Many hybrids with resistance to NCLB also carry resistance
to SLCB. At least two types of resistance to NCLB are known: small
lesion size and few lesions (controlled by multiple genes) and
chlorotic lesions with little or no sporulation and a yellowish
halo (controlled by a single gene). Thus, even where resistant
hybrids are planted, leaves may show some flecking or small lesions,
but no economic damage occurs.
Residue burial and crop rotation also will reduce
NCLB levels. Although the spores are easily disseminated by winds,
rotating to soybeans or another non-host crop helps reduce disease
levels. Foliar fungicides are also helpful in seed production fields
where susceptible inbreds are planted. Applications should be made
as for SCLB during the pollination period. Maintaining high balanced
fertility based upon a soil test is also helpful. Do not apply
excessive nitrogen since this may increase infection levels.
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Northern Corn Leaf
Spot
Northern corn leaf spot (NCLS) is primarily a concern
in seed production fields where susceptible inbreds are planted.
Hybrids may show some minor flecking or small lesions, but most
hybrids carry adequate resistance to prevent economic losses from
occurring. NCLS is favored by many of the same conditions as for
NCLB and SCLB. It is primarily limited to northern Illinois, but
may be found in seed fields in north central Illinois when moderate
temperatures and high moisture levels occur. Spores are produced
abundantly in damp weather.
Lesions of NCLS can vary depending on the race present.
Race 1 lesions are tan, oval to circular with concentric zones
and are commonly 1/2 inch in width and 1 inch in length. Race 2
infections are rare. Lesions are oblong, dark brown to blackish
in color and 1/8 inch in width and 1 inch in length. Race 3 lesions
are most common in the Corn Belt. These lesions are narrow and
linear in shape, with lengths less than 1 inch and widths less
than 1/8 inch. Lesion shape and size may vary with the genotype
of the plant. Lesions are grayish-tan and surrounded by a pigmented
border. Control measures are not usually necessary for commercial
hybrids. Seed production fields can benefit from fungicide applications,
especially for highly susceptible inbreds.
There are at least three known races of NCLS. Race
1 is highly pathogenic on some inbred lines; Race 2 is much less
pathogenic; Race 3 is primarily a problem in seed production fields.
It can also produce lesions on commercial hybrids but does no economic
damage. There is also evidence that a fourth race may occur or
it may be a biotype of one of the other races.
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Gray Leaf Spot
Figure
2. Gray leaf spot.
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Due in part to the adoption of reduced and
no-till practices, gray leaf spot is increasing in severity
across the Midwest. This pathogen survives readily in corn
debris and sporulates profusely in the early spring if weather
conditions are favorable. The disease is most severe in continuous
no-till corn and can cause extensive damage in reduced tillage
fields if crop rotation is not practiced.
Lesions are identified by their rectangular
or blocky appearance on susceptible corn plants. Lesions
are pale brown or gray to tan in color and are 1/4 inch to
two inches in length. They are restricted by the veins and
usually have blunt or squared-off ends. In susceptible hybrids,
lesions may coalesce causing extensive tissue necrosis.
GLS typically appears on the lower leaves because
of the spores being either windblown or rain-splashed from
previous crop debris. Extensive blighting may result followed
by death of the plant and stalk breakage or lodging. If favorable
environmental conditions occur, GLS may kill entire fields
prior to maturity.
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GLS is favored by warm, humid conditions and frequent
rainfall. Fungal spores can survive at humidities as low as 60%,
but infection and colonization of the host does not occur unless
relative humidities reach above 85%. This pathogen has a long latent
period when no symptoms are visible. This may last from 2-4 weeks
in length. Thus, once initial symptoms are evident, the disease
severity may already have reached the epidemic point.
Control of gray leaf spot should begin with identification
of potential problem fields and the selection of resistant or tolerant
hybrids for these fields. Since GLS is favored by high humidities,
only tolerant or resistant hybrids should be planted in these fields.
River bottom fields, for example, are typically humid and offer
the most favorable environment for GLS infection. Crop rotation
is also important for GLS control as this pathogen cannot survive
for extended periods without a host plant. Thus, rotating soybeans
or another non-host crop helps to reduce the inoculum level.
Plowing heavily infected fields will also reduce
carry-over inoculum levels. Once buried, the fungus cannot produce
spores and infect the corn crop. However, care should be taken
with plowing, especially with regard to slope of fields and erosion
considerations. Separation of fields can be a minor, although important,
method of reducing infections by GLS. Since this is primarily a
wind dispersed pathogen, corn crops should not be planted adjacent
to fields with high corn residues, if GLS was a problem in that
field the past season. If winds blow across the residues, spores
may be transported to the new corn crop and early infections begun.
Fungicides are not commonly recommended for commercial corn fields.
Several products are available for seed production fields and these
should be used with proper scouting to detect the disease in the
early stages.
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