We are just beginning the time of year when disease management takes place. Most disease management tactics for 1999 will be performed before the crop is planted. These tactics include planning for rotation, tillage, and variety selection. The most immediate decision is whether fall tillage is appropriate for managing disease problems. The trick is to find a balance between disease management and soil conservation.
Many corn and soybean pathogens survive in crop residue; therefore, their populations can be reduced by tillage operations. In buried residue, pathogens must compete with soil microorganisms for use of their food source (the residue), which is often rapidly decomposed. Pathogen survival also is reduced because of direct attack by antagonistic soil microorganisms. On the soil surface, residue and the pathogens in the residue last longer. Fall tillage is typically more effective than spring tillage because the residue is buried for a longer duration before planting. Pathogen survival is usually shortest if residue is completely buried; however, partial burying by chopping, disking, or chiseling also is beneficial. It is feasible to perform tillage that will reduce pathogen survival and still leave enough residue to retard soil erosion.
The effects of tillage on some of the important corn and soybean diseases are outlined below. You should check ISU Extension publications for more information on the effects of tillage on specific diseases.
Seedling diseases or damping-off. Tillage reduces seedling diseases by allowing the soil to warm faster. Spring tillage can be as effective as fall tillage. Zone or ridge tillage are good options.
Corn leaf diseases. Tillage reduces the severity of leaf diseases, except for rusts and Stewart's wilt, which are not affected by tillage.
Corn stalk rots. Tillage does not consistently reduce stalk rots. In dry years, tillage can sometimes increase stalk rot problems by increasing moisture stress on the plants. When stalk rots occur as a result of stress from a leaf disease, however, there is a benefit to tillage.
Corn ear rots. Tillage can reduce the occurrence of some ear rots (Gibberella, Diplodia) but not others (Fusarium, Aspergillus).
White mold of soybean. White mold sclerotia (survival structures) buried 2 inches or more under the soil will not germinate, but they survive for about 7 years and may be brought back to the surface by subsequent tillage operations.
Soybean cyst nematode (SCN). Tillage is not recommended to manage SCN. Tillage operations can spread the disease within and between fields.
Phytophthora and Rhizoctonia root rots. Phytophthora and Rhizoctonia root rot diseases are reduced by tillage.
Sudden death syndrome. Sudden death syndrome of soybean is reduced by tillage, including ridge tillage.
There are many factors that influence tillage decisions, and a grower must decide on priorities for individual fields. In fields that had serious disease problems in 1998, disease management for 1999 should be a high priority. Fall tillage reduces the risk of many diseases in any field, but the disease risks must be weighed against the disadvantages of tillage (such as soil erosion). In many fields, diseases can be well managed by other means, particularly resistance and rotation. Fields that have a history of difficult-to-manage disease problems should be considered for fall tillage.
This article originally appeared on page 179 of the IC-480(23) -- October 12, 1996 issue.