Field Corn Diseases

From Bugwoodwiki

Arnet, J.D.; Bertrand, P.; Crawford, J.; Ellis, HC; Lambert, W.; McGlohon, N.; Suber, F.; Thompson, S.; Womack, H.; Brown, E.A.; Evans, B.R. Insect and Disease Identification Guide for IPM in the Southeast. The University of Georgia. Cooperative Extension Service Bulletin 849. September 1981. 59p.

Stalk Rots (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 maydis is usually brown to gray in color. Stalk rot caused by Helminthosporium rostratum will be brown to green in color.

Charcoal stalk rot produces numerous black sclerotia which appear as tiny black dots on vascular strands and give a charred appearance. Tiny, pin-head size, black sclerotia usually attack plants approaching maturity, causing brown water-soaked lesions on roots which later turn black. Fungus spreads into lower stalk as plants mature and causes premature ripening, shredding and breaking off at the crown. Charcoal rot is usually associated with hot, dry condition; and adequate moisture decreases severity. This disease could be confused with black discoloration caused by secondary fungi in stalks originally rotted by Fusarium or other pathogens.

Nematodes (Trichodorus spp., root-knot Meloidogvne spp. and 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.

Root-knot (Meloidogyne spp.) injury is difficult to distinguish from that of other nematodes since galls may be absent or less evident than in more susceptible hosts. Meloidogyne incognita and M. arenaria are frequently associated with root injury and poor growth. The latter seems more likely to produce detectable galls.

Sting (Belonolaimus longicaudatus) nematodes are more common to deep sand type soils. Root injury and growth inhibition are usually more drastic than with other nematodes. Note severe reduction of root system and lack of feeder root development.

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).

Southern Rust (fungus Puccinia polysora) and 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.

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

Northern corn leaf blight (NCLB) produces long (1-6 inches, 2.5-15.0 cm), elliptical 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 8 mm) 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 elliptical 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.

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.

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.