Early planting and warm weather in April, followed by an extended period of cool, wet weather and frost in May, has created stress for corn seedlings throughout much of Minnesota. As of May 9, 94% of the corn in Minnesota was planted and 32% had emerged (compared to the five-year averages of 64% planted and 9% emerged). While the stress of the cool conditions and frost may only set back the corn slightly, it could also set up a situation for seedling disease in some areas. Keep in mind that many factors other than disease result in poor stands, reduced growth and low vigor. In most years and in most fields, seedling diseases are a minor problem, but this year they may be more common than usual.
Seedling disease damage may appear similar to damage from other environmental, insect, or chemical problems. Some general and specific symptoms, however, can help to diagnose whether a problem is due to a disease. Suspect plants should be carefully removed from the soil and examined for the following types of symptoms:
- Reduced emergence
- Slow growth and stunting of plants in a random or circular pattern
- Yellow/reddish discoloration of leaves
- Postemergence damping-off
Specific symptoms of seed and root infections include:
- Rotted seedlings before or after emergence
- Complete or partially rotted roots with firm or soft, brown to gray lesions
- Discolored and soft coleoptile
- Sunken or discolored lesions on the mesocotyl
The severity and incidence of corn seedling diseases can be increased by low soil temperatures, high soil moisture, poor seed quality, slow emergence and growth of seedlings, fertilizer and herbicide injury and crusted soil. Common conditions that favor seedling diseases are wet, cool and compacted soil. For example, seedling rot caused by Pythium is favored by wet and cool soil.
A variety of different pathogens can cause seedling diseases of corn. Some common fungal pathogens that cause one or more of the symptoms noted above are Fusarium, Pythium, Stenocarpella (Diplodia), Rhizoctonia and Colletotrichum. Bacterial pathogens rarely cause severe seedling disease problems in Minnesota, but one example is Pseudomonas,which causes holcus spot. In addition, nematodes can severely damage corn seedlings, especially but not exclusively in sandy soils. Most of these pathogens are soilborne, which means they survive from year to year in the soil or on residue in the soil remaining from a previous year's crop. Some pathogens can also be seedborne such as Fusariumand Aspergillus.
Most commercial corn seed is treated with two or more fungicides to provide protection from seedling diseases for at least two to three weeks after planting. Two main groups of common fungicidal seed treatments are routinely used. The first group – which includes ApronXL, Allegiance and other types of metalaxyl – are most effective against Pythium. The second group of fungicides – which includes Maxim, Dynasty, Stamina and Trilex as examples – protect against most of the other fungi. Corn seedlings will need some time to recover after the warm and sunny weather returns, and fields should be scouted and plants sampled where emergence is low and plants are stunted or wilting to determine if disease is a problem.