Alan
McNab
Professor of Plant Pathology |
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Stewart's
Bacterial Wilt of sweet corn, caused by the bacterium Xanthomonas
stewartii, is more severe on young plants than on older
plants. Symptoms appear first on leaves. Pale green to yellowish
streaks with wavy margins may extend the length of the leaf.
These streaks usually change from pale green to yellow or brown.
On young plants, brown discoloration and sometimes cavities
may form in the center of the stem near the soil line. Early-infected
plants may die; late-infected plants may die; late-infected
plants may be stunted or merely have streaked leaves. The disease-causing
bacterium overwinters in flea beetles and is spread to corn
when flea beetles feed on corn plants. Warm winter favor overwintering
of flea beetles and usually precede seasons when Stewart's
bacterial wilt is prevalent. Flea beetles carrying the bacteria
can be expected if the sum of the average monthly temperatures
(in ° F) for December (D), January (J), and February (F)
exceed 90. For example, if D=34, J=31, and F=29, then D+J+F=94,
flea beetles may be abundant, and Stewart's wilt could be a
problem. |
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Leaf Spots on sweet corn include northern corn leaf blight
(NCLB) caused by Helminthosporium turcicum, southern corn
leaf blight (SCLB) caused by Helminthosporium maydis yellow
leaf blight (YLB) caused by Phyllosticta maydis, antracnose
caused by Colletrichum graminicola, and bacterial leaf spot
caused by Pseudomonas alboprecipitans.
Size and color help distinguish the different fungal leaf spots from
each other. Specific environmental conditions also favor development of each
disease. For convenience of comparison, specific information on leaf spots is
summarized in the accompanying table.
Bacterial leaf spot lesions are white to straw-colored and are about
1/16 inch wide and several inches long. The bacteria also can cause stalk, shank,
and husk rot. Some southern grasses are hosts of this bacterium. Spread within
fields is believed to be by machinery movement. |
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Common Rust is caused by the fungus Puccinia
sorghi. Rust begins as oval to elongate sinnamon brown pustules
scattered over both surfaces of leaves. The pustules rupture and
expose dusty red spores and later black spores. The red spores
are spread by wind and can infect corn leaves directly; the black
spores are overwintering spores which germinate and indirectly
infect Oxalis an alternate host for the corn rust fungus.
In souther areas, red spores persist on corn from season to season,
thus initiating early-season disease development. In northern areas,
disease initiating depends on reintroduction of wind-borne red
spores from southern areas or depends on development of special
spores on the alternate host, Oxalis. Disease is promoted
by cool temperatures (61 to 70 ° F) and 100 percent relative
humidity. |
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Southern Corn Rust, caused by the fungus Puccinia
polysora, generally is confined to the southeastern area in
the United States. Souther rust pustules are lighter in color,
smaller, more circular, and do not break open as early as common
rust pustules. The souther rust fungus overwinter as spores on
infected corn plants in residue from diseased plants. Souther rust
is favored by high temperatures (80 ° F) and high relative humidity. |
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Smut, caused by the fungus Ustilago maydis,
is characterized by the presence of large fleshy irregular galls
on leaves, stems, ears, and tassles. Immature galls are white and
spongy; mature galls turn brown and contain dark powdery spores.
The smut fungus overwinters in soil. Smut development is favored
by dry conditions and by temperatures between 79 and 94 ° F. Disease
development is promoted by plant injury caused by hail, cultivation,
and insects. |
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Sourghum Downy Mildew, caused by Peronosclerospora
sorghi, affects corn and sourghum. The disease was first recognized
in Texas is the early 1960's and has spread as far as New Mexico,
Georgia, Illinois, and Indiana. Young plants are more susceptible
to infection than old plants; the fungus usually becomes systemic,
especially in young plants. Distinct yellow streaks, particularly
at the base of the leaves, are diagnostic symptoms in the field.
Tassel parts may become leaf-like on plants that are infected early.
A downy white mold may appear on yellow affected leaf areas. Disease
is promoted by warm and humid conditions. The Texas strain of the
fungus appears to be better adapted to the relatively low North
American temperatures (65 ° F best for reproduction; 50 to 68 ° F
best for spore germination) than strains in warmer parts of the
world. It is unknown how important this disease could become in
North America.
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Crazy Top is caused by a downy mildew fungus, Sclerophthora
macrospora. It is characterized by partial to complete proliferation
of the tassel; the normal flower parts continue to grow and become
small leaf-like structures. Other symptoms include development
of excessive ear shoots, increased number of internodes above ears
and in shanks, excessive tillering, suppressed tassel and ear formation,
and stunted narrow strap-like leathery leaves. The crazy top fungus
overwinters in soil as resistant oospores (fungus reproductive
structures). The disease is most prevalent in areas of fields where
soil is flooded or waterlogged for 1 to 2 days becore seedlings
are 4 to 6 inches tall. When the soil conditions are wet, the oospores
germinate to produce spores that can infect plants. The fungus
moves and grows systematically in plants. |
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Virus-Like Disease are caused by several different
viruses and mycoplasma; the diseases are characterized by symptoms
common to several of the diseases. This has led to confusion in disease
identification. Symptoms are helpful in the identification of a virus-like
diseases; however, positive identification requires additional procedures
conducted by virus-identification specialists.
Maize Dwarf Mosaic
Maize Chlorotic Dwarf
Corn Stunt
Wheat Streak Mosaic
Maize Streak
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Kernel Red Streak is caused by a toxin secreted
by the wheat curl mite when it is feeding on corn plants. Red streaks
appear in kernels. Usually the streaking is more pronounced in kernels
near the tip of an ear. White corn is less severely affected than
yellow corn. |
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