The cool, wet soils have resulted in some predictable problems with corn emergence, many of which include infection by fungi. Pythium and Rhizoctonia have been prominent this year in fields with emergence problems.
As plants move beyond the seedling stage, some of the seedling infections linger and cause more problems. In particular, infection of seedlings by Fusarium species can persist and move into the crown and stalk tissues. Several species of Fusarium commonly can be isolated from crowns of corn plants at any growth stage.
Stalk rot developing from a crown infection.
Normally, the crown tissue of healthy corn plants is slightly darker than surrounding tissue. The presence of Fusarium is indicated by a yellow-to-brown discoloration that sometimes is not distinguishable from an uninfected plant. In these cases, there are usually no other symptoms. In other cases, the discoloration is more severe and the crown tissue can become substantially rotted. These plants can exhibit various symptoms, including stunting, leaf discoloration, wilting, and even death. Extensive decay of this part of the plant cuts off the connection between leaves and roots and/or directly destroys the growing point.
Stressful growing conditions cause plants to become more susceptible to this type of infection. Last year, under similar weather conditions, we saw many cases in which plants were killed when they were anywhere from 3-4 leaves to knee-high. If hot, dry conditions occur after crowns are infected (which happened last year), this can be a death sentence for the plants. The decayed crown cannot support the moisture needs of the plant. If we experience this type of weather, watch for plants that wilt and die suddenly.
Crown infections can persist even longer, eventually resulting in stalk rot. We saw this happen last year in western Iowa. Some fields that lost plants to crown rot in June eventually were destroyed by stalk rot in August. If you observe crown rot problems in a field, watch for stalk rot later.
Because Fusarium species are so common in crop residue and soil, and resistance to this type of infection is not available, there are few practical control measures. Fungicidal seed treatment will not persist long enough to help in this situation. If predisposing stresses can be identified, alleviating them will reduce losses to Fusarium. When plants are stunted but crowns are not completely decayed, cultivation may promote the development of the nodal root system and help the plants recover.
This article originally appeared on page 111 of the IC-476(16) -- July 1, 1996 issue.