20 MIN READ

Identification of Southern Area Corn Diseases

February 22, 2021

The potential for corn seeds and plants to become infected with a disease is greatly determined by genetics, use of fungicidal seed treatments, environmental conditions, time in the growing season, and the presence of casual agents (fungal or bacterial pathogens, viral vectors, and nematodes). The warm, humid southern environment is very favorable for the development and propagation of many corn diseases.

Newly planted seeds are immediately subjected to pathogenic fungi and bacteria that inhabit the soil. In general, these pathogens have a greater potential for causing damage when soils are wet and cold; however, some pathogens prefer warm conditions. Infected seedlings may show signs of damping-off or rotting, stunting, yellowing leaves or loss of color, and deformities.

Susceptibility to diseases depends largely on the level of genetic resistance to a disease. Some seed products can have nearly complete resistance while others are very susceptible. When selecting seed products for planting, the historical disease presence should be considered, and seed selections made accordingly.

Foliar fungal diseases may become apparent anytime during the season depending on the presence of the pathogen. Many fungal pathogens can overwinter on or in infected residue and can contact plants via splashing rain or wind. Pathogens that are unable to overwinter in the southern areas of the United States can be carried in from Mexico, the Caribbean Islands, and South America on wind currents. Hurricanes have been known to bring diseases into new areas. Depending on the disease, foliar fungicides may provide control.

Bacterial diseases can also overwinter on infected residue. The bacterium can contact plants through water movement in the soil, splashing rain, wind-driven rain, and mechanical means such as cultivating when plants are wet. Infection occurs when bacterium enter the plant through wounds or natural tissue openings such as stomates.

Viral diseases rely on a vector such as aphids or thrips to infect a plant. As the vector feeds, infected juices are injected into plant tissue. Scouting for insects that have the potential to vector viral diseases and applying a timely insecticide may help protect plants from becoming infected via insect feeding. Caution should be taken not to spray insecticides without proper identification as many beneficial insects can be killed.

Additional harvest-time diseases include stalk and ear rots. Stalk rots can result in lodging that can diminish harvest speed, increase harvesting equipment wear and tear, decrease grain quality, and increase the potential for personal injury. Ear rots can affect grain quality, test weight, and marketability. Some ear rots produce toxins that can be harmful or even lethal to livestock and humans.

Nematodes are often discussed under the disease umbrella; however, they are microscopic parasitic worms that live outside (ectoparasitic) or within (endoparasitic) corn roots. Nematode infestations may go unnoticed, particularly under favorable growing conditions, or cause stunting, nutrient deficiencies, and create entrance wounds for other pathogens.

Click below on Seed and Seedling, Foliar, Viral, Stalk, Ear Rot, or Corn Nematodes to identify and learn about the corn diseases on your farm.

Identification, Characteristics, and Diagnosis:

  • Caused by soil-borne Diplodia (Stenocarpella) species.

  • May cause seed to rot prior to germination or cause preemergence or postemergence seedling blight.

  • Infected seeds are discolored, soft, and may be hard to find because of rapid decomposition.

  • Seedlings infected prior to emergence have discolored, wet-appearing, and rotted coleoptiles and primary roots. Symptoms resemble those associated with Pythium (Figure 4).

  • Seedlings infected after emergence are yellow, wilted, and generally die because the root system is rotted. Sunken lesions can appear on the mesocotyl.

  • More severe in wet soils, low lying areas, and compacted soils.

  • Favored by low soil temperatures (50 to 55° F).

Management:

  • Utilize fungicide-protected seed.

  • Plant when soil conditions and temperature are favorable for rapid emergence.

  • Monitor planting depth.

Fusarium-infected corn roots. Note discoloration on the mesocotyl
Figure 1. Fusarium-infected corn roots. Note discoloration on the mesocotyl.

Identification, Characteristics, and Diagnosis:

  • Caused by at least six Fusarium species.

  • Initially, infected seedling roots and the mesocotyl may be shriveled and have tan to reddish-brown lesions, which later become brownish to dark black and rotted (Figure 1).

  • Favored by wet and cool (55° F or lower) soils.

  • Most notable in early planted and reduced tillage fields.

Management:

  • Utilize fungicide-protected seed.

  • Plant into warm soils that are favorably dry.

  • Planting depth should not be too deep.

Penicilium Seedling Injury
Figure 2. Corn seedling roots infected with Penicillium fungi. Picture courtesy of William M. Brown, Jr., Bugwood.org
Penicillium leaf blight
Figure 3. Corn seedling with symptoms of Penicillium seedling blight. Picture courtesy of William M. Brown Jr., Bugwood.org.

Identification, Characteristics, and Diagnosis:

  • Caused by the fungus Penicillium oxalicum.

  • A bluish-green mold develops on seed causing roots to deteriorate (Figure 2).

  • Seedling leaves turn yellow and may die (Figure 3).

  • Favored by wet and cool (55° F or lower) soils.

  • Most notable in early planted and reduced tillage fields.

Management:

  • Utilize fungicide-protected seed.

  • Plant into warm soils that are favorably dry.

  • Planting depth should not be too deep.


Pythium Darkening of Mesocotyl
Figure 4. Darkened mesocotyl near the soil surface caused by Pythium. Picture courtesy of Don White, University of Illinois

Identification, Characteristics, and Diagnosis:

  • Caused by several Pythium species.

  • Favored by wet and cool (55° F or lower) soils.

  • Symptoms include dark, slimy lesions that cause roots or the mesocotyl to shrivel and the outer layer of roots may be rotted while inner tissues remain white and intact (Figure 4).

  • Most notable in early planted and reduced tillage fields.

  • Can infect anytime from planting to mid-season.

Management:

  • Utilize fungicide-protected seed.

  • Plant into warm soils that are favorably dry.

  • Planting depth should not be too deep.

Rhizoctonia seedling blight. Inoculated with Rhizoctonia slani AG-4 HG-II
Figure 5. Corn seedlings inoculated with Rhizoctonia solani AG-4 HG-II. Picture courtesy of Srikanth Kodati, University of Nebraska.

Identification, Characteristics, and Diagnosis:

  • Caused by the fungus Rhizoctonia solani.

  • Initial symptoms include brown lesions on the mesocotyl and roots of seedlings and young plants (Figure 5).

  • Later symptoms on older plants include reddish-brown, sunken cankers on crown and brace roots.

  • Plants may be stunted, chlorotic, or have no above ground symptoms.

  • Occurs more often in irrigated fields.

  • Favored by temperatures between 46 to 82° F.

Management:

  • Utilize fungicide-protected seed.

  • Plant into warm soils that are favorably dry.

  • Planting depth should not be too deep.

Sources:

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