Use of harvest and tillage to minimize soybean diseases

Three diseases were prevalent in Iowa soybean fields this year: Rhizoctonia root rot, white mold, and brown stem rot. Rhizoctonia root rot was a major mid-season disease problem, while white mold and brown stem rot were prevalent later in the season because of cool weather in August and September. White mold also was more widespread than in 1994.

Recently I've been getting two questions:

  • 1) What can I do at harvest to manage soybean diseases?, and
  • 2) Does tillage help reduce the risk of these diseases?

These are good questions to ask at this time of year because certain soybean diseases can be spread during harvest, and fall tillage can break the cycles of many diseases. You can minimize certain disease problems in the next soybean crop by following proper harvest and tillage practices with your current crop. In this article, I will review the effects of these two practices on each disease.

Rhizoctonia root rot

The fungus that causes this disease grows and survives in the top 2 inches of the soil surface. It infects only the root and lower portion of the soybean stem about 1 inch from the soil surface. Combines do not cut into stems where they are infested and, therefore, this disease does not spread when infested fields are harvested. Tillage practices, however, greatly influence the survival of this fungus because of the surface-growing nature of this pathogen. In no-till drill soybeans where soils are not disturbed, fungus colonies continue to expand. Any tillage¬either chisel or disking¬can break soil surface structures and destroy Rhizoctonia colonies. Cultivation during the growing season is an effective way to minimize the disease. In drill-planted soybeans where no mid-season cultivation is done, the disease sometimes becomes severe because the fungus continues to grow. If you want to control this disease with limited tillage, you only need to till areas where the disease was found during summer months.

Brown stem rot

In Iowa, the brown stem rot pathogen is present almost anywhere soybeans are grown. It doesn't help to manage the harvest to prevent the spread of this disease, but tillage may reduce disease risks for the next soybean crop. The fungus that causes brown stem rot must survive in soybean residues; burying residues from infested plants accelerates the decomposition process, and therefore, destroys the pathogen. If your fields had severe brown stem rot this year and you want to use tillage to manage the disease, fall tillage, along with corn rotation, can reduce your disease risk. Remember, tillage is not the only option to manage this disease. There are many brown stem rot-resistant varieties to use if you are a no-tiller.

White mold

Both harvest and tillage can affect this disease. White mold fungus produces sclerotia that can survive numerous conditions. Sclerotia are present in soybean stems and debris, which can be blown from one part of the field to another during harvest, or it can be carried by combines from one field to another at harvest. Fields severely infested with white mold should be harvested last. If this is not feasible, clean debris from combines after harvesting fields infested with white mold. If the disease is present in one or two patches in a field, always harvest these areas last, which can help prevent the spread of infested crop debris to other parts of soybean field.

Soybeans killed by white mold.

If you save seeds to plant your next crop of soybeans, make sure your fields are free of white mold. Using seeds with white mold sclerotia can be a disaster-a sad lesson learned by several growers I know. Harvest also is the time to mark spots where soybeans are severely damaged by white mold. This information is useful if you think you might use chemicals to control white mold in the future.

White mold sclerotia in a soybean stem.

Tillage affects the survival of white mold, but there are no good recommendations for disease management because of this disease's unique features. Sclerotia can survive in deep soil up to seven years. Only sclerotia within 2 inches of the soil surface can germinate and release spores to infect soybeans. Burying infested residues by mold-board plow can prevent germination of the sclerotia. But the effectiveness in controlling white mold by burying sclerotia is affected by cultivation and tillage in following seasons. For example, if you cultivate, disk, or use a chisel plow in corn the following season, you bring some sclerotia to the soil surface. Further, if soybean fields have already been infested by white mold in past seasons, the sclerotia may be mixed in the soil. Fall tillage of these fields would bury new sclerotia in deep soil, but uncover old sclerotia.

This article originally appeared on pages 176-177 of the IC-476(24) -- October 7, 1996 issue.

Updated 10/06/1996 - 1:00pm