Issue No. 6, Article 3/April 29, 2005
Corn Seed and Seedling Diseases
Much of the corn has been planted earlier than normal in Illinois this year, with about 64% in the ground on April 25. Warm weather and good soil conditions have allowed the planting to be much ahead of average. This is a good thing in most respects, but it is still April, and as we have been reminded in the last week, weather conditions can be cold and wet this time of the year. Not only can we see frost damage to corn, as has occurred in some areas, but these conditions also may favor seed and seedling diseases of corn. Where conditions are relatively warm and dry, seed and seedling diseases will have minimal impact and seed/plant loss may not exceed the 5% to 10% loss that we may normally expect. However, slightly higher losses may occur in large areas, and major losses can occur in scattered fields (as reports suggest each year) when prolonged cool and/or wet conditions in April and May favor seed and seedling diseases.
Harsh environmental conditions favor seed and seedling diseases. The incidence and severity of seed and seedling diseases of corn are influenced by low soil temperatures, high soil moisture, location, seed quality (such as cracked or infected seed), soil compaction, slow emergence and growth, the hybrid/inbred, fertilizer burn, herbicide injury, crusted soil, high temperatures (Penicillium infection), and high popu-lations of flea beetles (Stewart's wilt). Common favorable conditions for seedling diseases are cool, wet, and compacted soil. For example, Pythium seed and seedling rot is favored by wet and cool soil conditions.
Recognizing problems with corn seed and seedling diseases can aid diagnosis. Disease damage may appear similar to other environmental or chemical stress, but symptoms can help to diagnose a disease problem. General effects of corn seed and seedling diseases include reduced emergence, slow growth and stunting of plants in a random or circular pattern, wilting, chlorosis/yellowing, and postemergence damping-off. Specific symptoms of seed and root infections include rotted seed and seedlings before or after emergence; red/yellow discoloration of leaves; complete or partially rotted roots with firm or soft, brown reddish to gray lesions or decay; discolored and soft coeleoptile; death of leaf tips; wilting; and sunken or discolored lesions on mesocotyl. Leaf spots and streaks can result from anthracnose and Stewart's wilt infections.
Seed and seedling diseases can affect the corn crop in various ways. Seed and seedling diseases probably have a minor impact on corn in most Illinois fields in a typical year. This is due to average conditions that don't favor seed and seedling diseases, the use of good quality corn seed, and widespread use of fungicidal corn seed treatments. However, when soil is cool and wet after planting, emergence and growth are slow, or poor quality seed is planted, diseases can become a problem. The main effects may be reduced plant populations and stunting. Chronic, nonlethal infection at the seedling stage may also cause damage that affects the plants after the seedling stage.
Many different soilborne and seedborne pathogens can affect corn seeds and seedlings. Some common fungal pathogens that cause one or more of the symptoms noted above are Fusarium, Pythium, Stenocarpella (Diplodia), Rhizoctonia, Colletotrichum, and Penicillium. Our recent work in Illinois suggests that Rhizoctonia is not a widespread problem on corn in Illinois. Bacterial pathogens that also can affect corn seedlings in Illinois include Erwinia [Pantoea], which causes Stewart's wilt, and Pseudomonas, which causes holcus spot. In addition, nematodes can severely damage corn seedlings in some parts of Illinois, especially, but not exclusively, in sandy soils. Most of these pathogens are probably soilborne, which means they survive from year to year in the soil, often on infested residue remaining from a previous year's crop. Some pathogens are seedborne, such as Fusarium, Aspergillis, and Penicillium.
Fungicidal seed treatments for corn help with protection. Most corn seed sold in Illinois is treated with a combination of two or more fungicides to provide protection from seed and seeding diseases. The common fungicidal seed treatments used are of two main groups: one (ApronXL and Allegiance) is most effective against Pythium; the other (Maxim, Captan, Dynasty, and Trilex are examples) protects against the other fungi. These fungicides are typically most effective for only about 2 to 3 weeks after planting, depending on soil water content and temperature.--Dean Malvick