- Cool, wet soil conditions can delay corn germination and emergence and predispose corn seedlings to infection by several species of fungi.
- Symptoms of corn seedling blights and root rots can be similar, making a field diagnosis difficult.
- Broad spectrum fungicide seed treatments offer protection against seedling disease pathogens but do not eliminate the risk of disease when environmental conditions favorable to disease development exist.
DISEASE PATHOGENS AND SYMPTOMS
Many different fungi are responsible for causing seedling diseases in corn. Bacteria, nematodes, and root-feeding insects can also play a role in causing disease and complicating diagnosis. Identification of specific seedling blights and root rots based on symptomology can be difficult because different pathogens often cause similar symptoms. Microscopic examination at a plant diagnostic clinic is the most reliable way to confirm a specific disease diagnosis.
Typical belowground symptoms of seedling disease include rotted seed that is soft and brown, seminal roots that have a wet and slimy appearance, and a mesocotyl with brown lesions or soft, water-soaked tissue. Aboveground symptoms include damping-off after emergence and seedlings that turn yellow, wilt, and die.
It is not uncommon to isolate multiple root-infecting fungi from one plant sample. Several common soilborne fungi such as Fusarium, Penicillium, Pythium, and Rhizoctonia are often isolated from infected seedlings and roots. Other fungi such as Aspergillus, Nigrospora, and Trichoderma may also cause seedling diseases in corn.1
Fusarium. At least six Fusarium species have been identified that cause seedling diseases and root rots in corn.2 Plant susceptibility to root rot increases when plants are under stress or injured by herbicide applications. Infected plants can have tan to reddish brown lesions and the root or mesocotyl may shrivel (Figure 1). Root symptoms range from very slight brown discoloration to dark black, completely rotted roots. This disease can occur under a wide range of temperature and moisture conditions. Root rots occurring after the seedling stage are often caused by Fusarium. Fusarium root rot can move into the base of the corn plant, resulting in crown and stalk rot after pollination and during grain fill.
Penicillium. The roots and mesocotyl of infected plants may be discolored and rotted. Sometimes a blue-green fungal growth can be seen on infected seeds. Symptoms of this seedling blight include browning of leaf tips. Entire infected plants may turn yellow and die, or remain discolored and stunted the remainder of the growing season (Figure 2). Penicillium tends to infect plants that have yet to develop their nodal root systems. This fungus is favored by high temperatures, which can inhibit other fungi.
Pythium. Several species of Pythium can rot the seed prior to germination or infect young seedlings before or after emergence. It is one of the most common fungi associated with seed rot and seedling blight of corn. This fungus is favored by high moisture and low temperatures and requires wet soils to produce infecting spores. Symptoms include dark, slimy lesions that cause the root or mesocotyl to shrivel. The outer cortex of the root may be rotted while the inner part, or stele, remains white and intact (Figure 3). Pythium can infect anytime between planting and midseason but is primarily a seedling problem. Strains of Pythium that are more adapted to warmer, wet conditions have developed in some locations.3 (A. Robertson, personal communication, April 2014)
Rhizoctonia. The most distinctive symptoms are reddish-brown sunken cankers, which form on crown and brace roots of large plants. Older plants may lodge due to a poor root system. Initial symptoms are brown lesions on the mesocotyl and roots of seedlings and young plants. Plants may develop distinct reddish-brown sunken cankers on roots. Plants may be stunted or chlorotic, but often there are no aboveground symptoms. Rhizoctonia can infect corn roots between 46° to 82° F and can also cause crown rot and brace root rot on older plants. This disease tends to be more severe in irrigated corn.
Cultural Practices. Wet and cool soil temperatures (less than 50° to 55° F) can delay seed germination and emergence and predispose corn seedlings to disease.
Seedlings become more susceptible to infection the longer a seed is in the ground before emergence and the more stress germinating corn endures. To help minimize seedling diseases, plant high quality seed at the appropriate planting depth and soil conditions to promote rapid germination and emergence. Fields that have a tendency to stay wet or have a history of seedling disease should not be targeted for early planting; these fields should be planted slightly later in the season when soil temperatures are more favorable for plant growth. Avoid both mechanical and herbicide injury to the seed, as these may influence the occurrence of seedling diseases.3
Seed Treatments. Most corn seed is treated with seed treatment fungicide, often with multiple active ingredients. These products can provide a level of protection against seedling blight pathogens, but may not eliminate all threats under severe environmental conditions that favor infection. All seed treatments have a limited period of activity, which is usually only for the first month after planting. Broad- spectrum seed treatments, such as Acceleron® Seed Applied Solutions, can help minimize the risk of developing corn seedling diseases.
1 Robertson, A. and Munkvold, G.. 2009. Check general root and mesocotyl health when assessing corn stands. Iowa State University Extension. http://www.extension.iastate.edu.
2 Jackson-Ziems, T. and Korus, K. 2013. Seedling diseases appearing in corn. University of Nebraska. http://cropwatch.unl.edu.
3 Sweets, L. and Wright, S. 2008. Corn Diseases. University of Missouri. http://ipm.missouri.edu Compendium of Corn Diseases. 1999. APS Press.
Web sources verified 03/28/16. 150421070624