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Disease and causal agent
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Plant parts affected
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Conditions favorable for the disease
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Appearance and
effect
of the disease
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Survival
and
spread
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Control
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Bacterial wilt (Stewart’s wilt) & leaf blight
Erwinia stewartii
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Foliage, stem pith, roots, tassels, cobs, and kernels
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High temp., high levels of ammonium N & P, low levels
of Ca and K increase plant susceptibility; mild winters favor survival
of flea beetles, hence increases chance of transmission. Abundance of
bacterial wilt is limited by the availability of the corn flea beetle.
An index of potential problems is calculated from the sum of the monthly
mean temperature for the previous winter (December, January, and February).
When the index exceeds 100, bacterial wilt is highly probable.
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Linear pale green to yellow streaks with irregular or
wavy margins that may extend the length of the leaf. These streaks soon
dry and become brown. Infected plants may produce premature bleached and
dead tassels. Internal stem discoloration seen in cross section of stem.
Reduced yield due to reduced photosynthetic areas.
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Bacterial over-winter in corn flea beetles; sometimes
in kernels; dispersed by insects and, rarely, by seed.
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Use resistant or tolerant hybrids and varieties. Early
applications of insecticide to control vector on susceptible varieties.
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Southern leaf blight
Cochliobolus heterostrophus
(syn. Helminthosporium)
maydis (anamorph Bipolaris maydis)
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Foliage, stalks, ears and cobs
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Warm (20-32°C)
and damp
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Race 0: Elongated tan lesions
between veins, with buff-to-brown borders; only on leaves
Race T: Tan, spindle-shaped or elliptical lesions
with yellow-green or chlorotic halos, which turn dark reddish-brown
border. On leaves, stalks, ears, and cobs
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On/in infected plant debris. In/on seeds (Race T). Dispersed
by wind and/or splashing rain. Transmitted by infected seeds (T).
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Use corn with normal cytoplasm and resistant hybrids and
varieties. Plow down crop debris. Use fungicides also labeled for Northern
corn leaf blight.
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Northern leaf blight
Setosphaeria turcica
(syn. Helminthosporium turcicum) (anamorph Exserohilum
turcicum)
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Foliage, occasionally ears
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Moderate temp. (18-27°C)
and heavy dews; several races occur (1, 2, & 3), but only 1 and 2
identified in New York
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Long elliptical (cigar shaped) grayish-green or tan lesions
2.5-15 cm long. First on lower leaves, progressing upward on the plant.
Severe leaf infection causes coalescing of lesions and gray appearance
like frost or drought injury. Infection during early stage of growth may
cause heavy loss in ear fill. When severe, plants are killed prematurely.
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On/in infected plant debris. Harbored by some weeds (ex.
(Johnsongrass, sudangrass, etc.)
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Because of race situation, use resistant hybrids; most
resistant to race 1 but not race 2; spraying with fungicides may be necessary;
eradicate weed hosts and plant debris.
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Northern corn leaf spot
Cochliobolus carbonum (syn. Helminthosporium
carbonum)
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Foliage & ears
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Moderate temperature & high relative humidity; sporulates
abundantly in damp weather
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Race 1: Oval to circular tan spots with concentric zones.
Race 2: Oblong, chocolate-colored spots; rare; both races
readily attack ears causing a black felty mold over the kernels.
Race 3: Narrow linear lesions up to 15-20 cm long; lesions
are grayish-tan, surrounded by a light to darkly pigmented border.
Race 4: Similar to race 2, but with concentric zones of
sporulation
Race 0: Produces small flecks on immature leaves, and
is not considered to be important
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On many grasses and weeds; on/in seeds; dispersed by wind
and seeds.
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Use of resistant hybrids and varieties.
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Disease and causal agent
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Plant parts affected
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Conditions favorable for the disease
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Appearance and
effect
of the disease
|
Survival
and
spread
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Control
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Eyespot
Kabatiella zeae
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Leaves and leaf sheathes, outer husks, kernels (when severe)
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Cool, humid weather
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Small translucent, round to oval lesions, 1-4mm in diameter
with yellowish halos that may fuse to form large necrotic areas; for field
corn rapid drying of foliage reduces grain yield and silage quality
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Overwinters in corn debris; dispersed by wind and splashing
rain.
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Use less susceptible or tolerant hybrids and varieties;
use rotation or plowing or both.
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Anthracnose leaf blight
Colletotrichum graminicola
(teleomorph Glomerella graminicola)
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Seedling and foliage; may also cause severe root &
stalk rot (field corn)
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High temp. & extended humid periods of cloudy weather;
free water necessary for spore dispersion and germination.
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Small, oval to elongated water-soaked spots on leaves;
the lesions are semi-transparent; spots enlarge, become tan at center
with reddish-brown or yellow-orange border; diseased leaves wither and
die within a few days; fruiting bodies may develop in the lesion with
dark setae.
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On corn debris and seeds and other grass hosts; spores
dispersed by wind and splashing rain or seeds.
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Use resistant hybrids and varieties; rotation important;
plow down crop residue and strive for balanced soil fertility.
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Gray leaf spot
Cercospora zeae-maydis
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Affects foliage some time after anthesis
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Prolonged periods of dew, fog and cloudy weather.
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Appears as rectangular lesions, which become tan and then
gray in color. The sharp parallel edges and opacity of mature lesions
are diagnostic
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Corn is the only known host. Common when corn follows
corn and diseased crop residues remain on the soil surface.
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Hybrids differ in their susceptibility. Fall burial of
crop residues and crop rotations are important.
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Downy mildew
(Crazy top)
Sclerophthora macrospora
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All corn types including sorghum; fungus infects systemically
at seedling stage, but symptoms most obvious at top of plant because of
malformed stalks, shortened internodes and tops
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Soilborne fungus requires soil moisture at the seedling
stage for infection to occur.
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Symptoms vary but consist first of excessive tillering,
rolling and twisting of upper leaves; most characteristic symptom is proliferation
of the tassel leading to a leaf structure and suggestive of herbicide
injury; this phyllody may also appear in the ears.
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The fungus infects by motile zoospores, which encyst on
the roots and produce germ tubes. Fungus overwinters by oospores; infection
requires flooded soils soon after planting or before plants reach the
4 to 5 leaf stage.
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Provide adequate soil drainage or avoid plating in low,
wet areas; crop rotation and clean plowing are only slightly effective
control measures.
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Disease and causal agent
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Plant parts affected
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Conditions favorable for the disease
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Appearance and
effect of the disease
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Survival
and spread
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Control
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Common smut
Ustilago maydis
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Seedling infection rare because of warm temp. requirement;
foliage, ears, stems, and tassels are affected
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Rain and humid weather for initial infection, then dry,
sunny weather; high N levels.
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First indication as leaf infection (small bumps) occurs
in leaf whorl stage; large, fleshy galls covered by silvery-white membrane
ruptures to expose black mass of spores; reduced yield and difficulty
in handling for processing corn is a problem.
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Spores survive in soil; crop rotation of questionable
value; windblown spores from debris lodge in nodes and growing tips.
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Steady growth of corn reduces amount of smut; excess nitrogen
and mechanical injuries increases prevalence; choose resistant or tolerant
varieties; removal of galls before rupturing beneficial in gardens
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Head smut
Sphacelotheca reiliana
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All types of corn, also sorghum and sudangrass, from seedling
to ears and tassels
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High concentration of teliospores, soil temp. of 21-28°C,
and moderate to low soil moisture favors seedling infection; low nitrogen
levels heavy soils, slow to dry out in Spring; causes crusting and impedes
emergence
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Seedling infection through rootlets or coleoptile from
spores on seed or in soil, followed by systemic distribution; first symptoms
when tassels emerge with each sorus covered by grayish-white periderm
which ruptures to reveal dusty, black teliospores; if on tassels, then
almost always on ears and may cause multiple ears at same or successive
nodes; ears completely converted into fungal sori with only vascular strands
remaining
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Seedborne spores provide initial inoculum, soilborne teliospores
thereafter remain viable even through digestive tract of animals.
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Resistant or tolerant varieties; seed treatments with
fungicides to prevent introduction; requires long rotations once teliospores
occur in soil.
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Common corn rust
Puccinia sorhgi
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May occur on most plant parts but mainly on leaves
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Favored by moderate temp., frequent heavy dews and high
nitrogen
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Develops soon after silking; oval pustules containing
reddish-orange spores occur on both leaf surfaces; premature leaf senescence
reduces yields and quality due to sugar drop.
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Urediniospores cause reinfection; pustules turn black
in fall with winter teliospores, which infect Oxalis (wood sorrel)
weed host; primary inoculum blown into north from southern locations.
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White varieties generally more susceptible but some yellow
hybrids equally susceptible; choose tolerant varieties and consider fungicide
sprays.
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Barley yellow dwarf luteovirus (BYDV) and Cereal yellow
dwarf polerovirus (CYDV)
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Leaves display color symptoms; plants and ears are stunted
if plants are infected early
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Depends on the survival of specific aphid vectors for
both viruses plus occurrence in grain crops (barley, oats, wheat) and
wild grasses
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Prominent yellow or purple coloration of leaves occurs
with either BYDV or CYDV depending on the particular variety infected.
Early infection results in shortened plants and ears, and blanking of
the tips of ears
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Transmitted by aphids in a persistent manner. Requires
24-48 hours for acquisition (can be as short at 15-30 min.) and then hours
to circulate throughout the aphid before aphid can retransmit the virus
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Avoid planting corn next to overwintered small grains.
Insecticides can be used to reduce secondary virus spread within the crop.
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