By Pierce Paul and Peter Thomison, Ohio State University Extension
Corn harvest is progressing very slowly across the state as the crop is taking unusually long to dry down this year. The longer the crop stays in the field, there greater the risk of late-season diseases such as ear and stalk rots, especially if it continues to rain. Stalk rot often refers to a combination of several interrelated problems, including stalk breakage, stalk lodging, premature plant death, and root lodging. Several factors may contribute to stalk rot, including extreme weather conditions, inadequate fertilization, problems with nutrient uptake, insects, and diseases. For instance, when leaves above the ear are severely damaged (either by diseases, insects, or some environmental stress) well before grain-fill is complete, the plants often translocate sugars from the stalk to fill grain, causing them to become weak and predisposed to fungal infection. A number of fungal pathogens cause stalk rot, but the three most important in Ohio are Gibberella, Collectotrichum (anthracnose), and Fusarium.
Losses due to stalk rot vary from field to field and from one hybrid to another. Stalk rots may cause lodging, especially if the affected crop is not harvested promptly. However, it is not uncommon to walk corn fields where nearly every plant is upright yet nearly every plant is also showing stalk rot symptoms. Many hybrids have excellent rind strength, which contributes to plant standability even when the internal plant tissue has rotted or started to rot. However, strong rinds will not prevent lodging if harvest is delayed and the crop is subjected to weathering, e.g. strong winds and heavy rains.
A symptom common to all stalk rots is the deterioration of the inner stalk tissues so that one or more of the inner nodes can easily be compressed when squeezed between thumb and finger. It is possible by using this “squeeze test” to assess potential lodging. The “push” test is another way to predict lodging. Push the stalks at the ear level, 6 to 8 inches from the vertical. If the stalk breaks between the ear and the lowest node, stalk rot is usually present. To minimize stalk rot damage, harvest promptly after physiological maturity, even if you have to do so at a slightly higher moisture content (moisture in the lower 20s). Harvest delays will increase the risk of stalk lodging and grain yield losses, and slowdown the harvest operation. In addition, lodging may lead to ear rots and grain contamination with mycotoxins as ears come into contact with the soil and crop residue.