Narrow-row soybeans can suppress weeds, but they also can contribute to the development of white mold caused by the fungal pathogen Sclerotinia sclerotiorum. This fungus has become a problem because it can thrive under the closed canopy of narrow-row and drilled soybeans. Severe infestations can reduce soybean yields by 50 to 70 percent, and the pathogen can survive for years in the soil as large, hard masses of fungal tissue called sclerotia.
|||Soybean white mold on stem.|
The pathogen can easily be identified by the white mycelium growing on the stem of the plant. Yellow and wilted leaves are often the first sign of infection. As S. sclerotiorum becomes established, it forms new wheat-kernel-sized sclerotia that return to soil via plant debris and tillage.
Although fungicides can control this pest, Iowa State University plant pathologists Charlie Martinson, XB Yang, and Luis del Rio have another approach--augmenting a parasitic fungus that attacks only S. sclerotiorum in the soil to eradicate the long-lived sclerotia. For the last 3 years, with support from the Leopold Center, these researchers have been studying the antagonistic fungus Sporidesmium sclerotivorum to determine how it might be used in Iowa as a biological control against soybean white mold.
This particular mycoparasite or fungal parasite was developed by USDA biocontrol scientists to control a related disease on lettuce. Since beginning the project in Iowa, Martinson says they have found native forms of S. sclerotivorum that are equally lethal to white mold. In 1997, nearly 800 soil samples that were analyzed at the ISU Plant Disease Clinic were subsampled for this native parasitic fungus; nine of the samples representing eight counties across Iowa contained the native form of S. sclerotivorum. If this native form proves equal to or better than the USDA form, Iowa may have a ready-made defense against the destructive white mold fungus. Research on biocontrol of white mold is taking several avenues to answer the questions associated with the white mold fungus and its antagonist S. sclerotivorum.
How does the USDA strain of S. sclerotivorum perform against white mold under Iowa conditions, and do Iowa native strains of the antagonist perform as well or better under Iowa conditions? What are the conditions conducive to the antagonist's growth? Martinson says the USDA strain of S. sclerotivorum does establish well in Iowa, but it appears to be moderately sensitive to some of Iowa's more alkaline soils. A native strain of S. sclerotivorum, thus, might have an advantage by being adapted to alkaline soils.
Researchers also are studying whether application of the antagonist in the spring or in the fall is more effective. Preliminary data show that spring may be better, but final data will not be available until later this fall.
How does S. sclerotivorum spread within a field? Although there are many anecdotal observations, no hard data exist for quantifying how the white mold parasite spreads within a field. Martinson says its ability to move in water runoff has posed some challenges in setting up replicated test plots. To more accurately test the spread of S. sclerotivorum, the project has set up spot plots--sites placed in cooperator fields with no replications nearby. Martinson says that several of the farmer cooperators have yield monitoring equipment, thus making it easy to calculate the effect of the pathogen's and mycoparasite's effects on yield.
Are there other native parasitic organisms that could fight white mold, and if so, what is their ecology? Martinson says that because white mold has been a problem in Iowa, there is some speculation that there are other organisms capable of destroying the white mold sclerotia. Researchers are isolating and testing several of these organisms in the laboratory to see how they perform. To date, none appear to perform as well as S. sclerotivorum.
If proven effective, can S. sclerotivorum be brought to market availability as a biocontrol agent? Martinson says numerous producers have been asking about white mold control. After Leopold Center support for the project concludes this year, the project will receive additional support from the Iowa Soybean Promotion Board to collect more information about biocontrol of this serious soybean pest.
Sclerotinia is a serious pest on other crops worldwide, including sunflower and beans. When the population of its antagonist builds up over time, however, the disease can be eradicated. The question remains whether a commercial firm would find production of the biocontrol agent profitable because once the mycoparasite is established, sales could conceivably fall off significantly.
If S. sclerotivorum proves to be an effective way to control soybean white mold, Martinson sees potential for its use in precision application systems as well as in organic regimes. With a low-impact tool to fight white mold, the narrow-row and low-tillage systems that are so beneficial for fighting soil erosion may gain additional effectiveness.
This article was modified from a longer version that was printed in the Fall 1998 issue of the Leopold Letter, vol. 10, no. 3. The project was funded in part by the Leopold Center for Sustainable Agriculture. The full-length article can be accessed at http://www.leopold.iastate.edu/centers/leopold/whitemoldstory.html 
This article originally appeared on pages 185-186 of the IC-480(24) -- November 9, 1998 issue.