Don't Use Tillage for Control of Soybean White Mold

FOR IMMEDIATE RELEASE
October 1, 2009

Soybean producers in northern Illinois have experienced problems with a disease called sclerotinia stem rot, or white mold, this year. The disease is most severe in years that have wet, cool conditions during the flowering period, especially in fields or portions of fields that have restricted air movement, such as drilled or other field situations where the canopy has closed quickly.

The disease is characterized by dead plants which appear in midseason and have a white fungal mold growing on dead tissue of the stem, often just above the soil line. Small black fungal structures known as sclerotia can be found on and inside plants. The most diagnostic in-field symptom is the white mold, however. The disease can last for years in the soil in the hard black masses of sclerotia.

"Over the years, some producers have asked about using tillage to bury soybean residue as a practice to help control the disease. But, tillage is not a desired option for many producers and may not be that effective in controlling the disease," says John Church, University of Illinois Extension Educator, Natural Resources, Rockford. Burying infested residue can cause the organisms to die, since they require air for survival. However, tilling solely for the purpose of controlling this disease is not effective, since fungal spores can become present regardless of the tillage method in a single field. Seed and possibly soil movement can spread sclerotia. Their spores are spread to other plants through air movement. Sclerotia send small, mushroom-like structures above the soil, which can spew millions of spores. The spores invade other plants through decaying soybean blossoms and then invade the stem.

There are a few practices that producers can use that may reduce the potential for the disease in individual fields. Choose varieties that have shown less disease symptoms in the field during previous years. No soybean varieties are completely resistant to white mold, but some are less susceptible than others. Since the white mold fungus can be seed borne, planting bin-run seed is not recommended. Avoid drilling in fields with a history of white mold. In areas where white mold is a severe problem, wider (30-inch) row spacing may reduce the disease's impact. However, because wider spacing can impact the yield potential of soybeans, it is recommended only in areas where severe white mold is observed frequently. This year many fields planted in 30 inch rows still had the disease present since the conditions this year were conducive for the pest.

Currently, two foliar fungicides are registered for control of white mold in soybeans. In the University of Illinois trials, fungicide treatments were effective at controlling white mold, but timing is critical and must be applied during bloom stage. Also, the yield benefit of controlling white mold with fungicides is only fully realized when disease incidence is moderately high.

A biological control product containing a parasite of the white mold fungus's sclerotia is also being studied in University of Illinois research trials. North Dakota State University research indicated it is effective at colonizing and killing sclerotia. Biological control should not be applied to flowering soybean plants, but instead it should be applied to the soil in the fall after harvest or in the spring prior to planting.

Despite control methods, the most determining factor for sclerotinia white mold is still the weather. If drier, warmer conditions exist at the time of infection, there will be less disease.