Rotation may not stop all fungi

Rotation is one of the major ways to manage disease. Rotation breaks the disease cycle and lowers the amount of pathogen in a field. However, we have found fields where stand reductions occur during both corn and soybean rotations. Our studies show that some soilborne fungi can attack both corn and soybeans in rotation fields.

Corn seedling infected by Rhizoctonia.

In the last three years, Pythium and Rhizoctonia were collected from diseased soybean plants in numerous Iowa soybean fields. The isolates were tested by inoculating them to corn and soybean separately. The results indicate that a high number of isolates from rotation fields can cause diseases in both crops.

Results indicate that rotation should not be considered as an effective control measure for Pythium and Rhizoctonia because some strains of the two fungi attack both crops. In fact, rotation may not only fail to reduce the populations of these two pathogens, it may even increase them. The reason is simple: continued use of a single rotation scheme may produce pressure toward selection of fungi pathogenic to both rotation crops. Rotation, however, is effective to control many other soilborne fungal diseases, such as soybean brown stem rot and corn seedling blight by Fusarium.

If you have experienced stand reduction of corn in past years, you should scout for soybean seedling blight if fields are wet after soybean planting. Knowing which diseases you experience this season will help you assess your risk for the next crop.

Always note if seedling blight occurs in fields that have been rotated between corn and soybean. The information you collect about seedling diseases in one crop should be useful for the next rotated crop. For instance, fields with substantial corn seedling diseases may be at a higher risk for seedling disease on next year's soybeans.

This article originally appeared on page 65 of the IC-478 (9) -- May 19, 1997 issue.

Updated 05/18/1997 - 1:00pm