Stalk Rots (fungi Fusarium moniliforme, Macrophomina phaseoli, Helminthosporium rostratum, Diplodia maydis and other fungi)

Infection occurs directly through root systems or stalks, resulting in weakened, discolored stalks. Infection with some fungi begins within 30 days after planting. Infection with Fusarium spp. will result in rotted tissue having whitish pink to red discoloration. Stalk rot caused by Diplodia mavdis is usually brown to gray in color. Stalk rot caused by Helminthosporium rostratum will be brown to green in color.

Fusarium stalk rot

Stalk rot of corn, health stalk, charcoal stalk rot

Nematodes (stubby-root Trichodorus spp., root-knot Meloidogvne spp., sting Belonolaimus longicaudatus)

Stubby-root (Trichodorus spp.) causes poor root growth and stunting. Roots appear stubby due to nematode attack of growing point which inhibits cell division and elongation. Secondary branching behind root tip often gives a brush effect since these root-tips are also injured shortly after development. Symptoms are often confused with injury caused by other nematodes, herbicide residues and acid soils.

Sting and stubby root nematode injury to corn roots

Virus Disease Complex

Maize chlorotic dwarf virus (MCDV) and maize dwarf mosaic virus (MDMV) are often confused with other growth problems since they cause stunting, yellowing and reddening of foliage. These symptoms along with the presence of johnsongrass—alternate host for these viruses—are usually sufficient to diagnose the virus complex. While each virus disease may be found alone, they often infect the same plant and are difficult to distinguish. MCDV produces a deeper, reddish discoloration of upper leaves than does MDMV which is more likely to cause yellow color and a mosaic pattern of light and dark green color. MCDV is also characterized by more bunching or shortening of internodes. These viruses are transmitted from johnsongrass to corn by aphids (MDMV) and leafhoppers (MCDV).

Maize dwarf mosaic virus

Southern Rust (fungus Puccinia polvsora)
Common Rust (fungus P. sorghi)

Pustules of Southern rust first appear in small patches which may eventually cover most of the foliage. They  are covered by a thin, skin-like layer of leaf epidermis. These pustules break open the leaf surface to expose the orange uredospores. Common rust (P. sorghi) produces a larger, brick-red pustule which is elongated and breaks through leaf epidermis more quickly after infection.

Common corn rust

Northern and Southern Leaf Blight (fungi Helminthosporium turcicum; H. maydis)

Northern corn leaf blight (NCLB) produces long (1-6 inch, 2.5-15.0 cm), eliptical or cigar-shaped tan colored lesions which are grayish-green at first. Lower leaves are usually attacked first and the disease is favored by lower than normal temperatures and high moisture or heavy dew. Yield loss is expected if the disease is established before silking. The chlorotic lesions on lower leaves indicate hybrid resistance where fungus sporulation and necrosis of tissue are inhibited. Southern corn leaf blight (SCLB) causes much smaller lesions (1/4 x 3/4 inch, 6 x 8mm) which have parallel sides and are tan in color.  SCLB is also favored by warmer temperatures than NCLB. Race T lesions are larger and spindle-shaped or eliptical with yellowish halos. Race O attacks only leaves, but race T may attack stalks, sheaths, husks, shanks, ears and cobs of hybrid having Texas male sterile cytoplasm. Such hybrids were abandoned due to susceptibility.

Northern corn leaf blight, top, susceptible variety; bottom, resistant variety

Ear Rots (fungi Fusarium spp., Aspergillus flavus)

Fusarium ear and kernel rot causes a pink to reddish-brown discoloration of individual kernels or groups of kernels. A powdery or cottony-pink growth may develop on infected kernels, especially around channels made by insect larvae.

Aspergillus flavus causes a greenish-yellow mold growth on the surface of infected grain. Most infected corn characteristically emits a bright blue-greenish-yellow fluorescence when exposed to high intensity, ultraviolet light (black light). However, this does not necessarily mean that the poisonous by-product aflatoxin, has been produced by the fungus. Laboratory assays are required to determine levels of contamination. Field molding and aflatoxin contamination have been associated with production stresses and insect damaged ears.

Fusarium kernal rot

Aspergillus flavus kernal rot

Bacterial Leaf Blight (bacterium Pseudomonas avenae)

Bacterial leaf blight produces narrow, parallel-sided streaks on upper leaves which run together to give plants a scorched appearance when infection is severe. Lesions are water soaked at first, becoming tan and eventually shredding leaf tissue. Damage is usually restricted to localized spots within fields since it is spread only in water droplets by blowing rain or field machinery. Bacteria are soilborne, and require several rainy days where water stands in whorl long enough for incubation and infection to occur. Clear weather usually halts further infection/spread and results in plant recovery.  Vasey grass is a weed host for the bacterium.

Bacterial streak