At war with white mold: biological control could offer promising approach
By E. Anne Larson
Communications specialist
Crops pests are a wily lot. When researchers attack one problem area, invariably another problem pops up somewhere else. Such is the result of the move during the past several years to narrow-row and reduced- or no-till soybeans as a means to suppress weeds and reduce soil erosion.
While these practices have gone a long way toward addressing production concerns, they've also contributed to a resurgence of soybean stem rot (or white mold, 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" (sklare-oh-shee-a).
How white mold develops
When soils remain damp for extended periods of time, which occurs more frequently after the soybean canopy closes, the sclerotia extend small, mushroom-like structures called "apothecia" above the soil. These structures spew millions of spores that sometimes travel as much as 100 to 200 feet into the air. The spores that land on decaying soybean blossoms take hold and emit a toxin into the plant that kills the plant tissue and quickly paves the way for invasion into the stem. In lab tests, the white mold fungus can grow 1/2" to 1" per day. Temperatures under 85 degrees F. are most conducive to white mold, thus the most serious infestations in Iowa occur north of Interstate 80.
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 the S. sclerotiorum becomes established, it forms new wheat kernel-sized sclerotia that return to soil via plant debris and tillage, where they can remain viable for five to seven years.
Natural antagonist may be key
While fungicides exist to fight this pest, Iowa State University plant pathologists Charlie Martinson, X.B. 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 past three years, with support from the Leopold Center, the 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 (a 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 went through the ISU Plant Disease Clinic were sub-sampled for this native parasitic fungus; nine of the samples representing eight counties across Iowa contained the native form of the mycoparasite. If this native variety proves equal to or better than the USDA form, Iowa may have a ready-made defense against the destructive white mold fungus.
Currently, the research on biocontrol of white mold is taking several avenues to answer the many questions associated with the fungus and its antagonist, S. sclerotivorum:
Question 1
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 appears to be moderately sensitive to some of Iowa's more alkaline soils. That's where a native strain of the biological control agent might have an advantage.
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.
Question 2
How does the antagonist S. sclerotivorum spread within a field?
While 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 the mycoparasite, the project has set up "spot plots"-sites placed in cooperator fields with no other 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.
Question 3
Are there other native parasitic organisms that could fight soybean stem rot, and what is their ecology (how did they get here and where can they be found)?
Martinson says that since white mold has been a problem in the state before, 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 laboratory settings to see how they perform. To date, none appear to perform quite as well as S. sclerotivorum.
Question 4
If proven effective, can this biocontrol be brought to market availability?
Martinson says numerous producers have been asking about soybean stem rot 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 further information about biocontrol of this serious soybean pest.
Possible effects
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.
Should S. sclerotivorum prove to be an effective way to control soybean stem rot, Martinson sees potential for the antagonist to be useful in precision application systems as well as organic regimes. With a low-impact tool to fight soybean stem rot, the narrow-row and low-tillage systems that are so beneficial for fighting soil erosion may gain additional effectiveness.
Narrow-row and reduced- or no-till soybeans can suppress weeds and reduce erosion, but conditions are ripe for development of white mold (also called Sclerotina stem rot). The most common symptoms are white mycelium growing on the stem and yellow or wilted leaves.
When soils remain damp for extended periods of time, white mold sclerotia extend small, mushroom-like structures called apothecia above the soil. The apothecia spread spores that emit a toxin, which spreads quickly and can kill soybean plants.
Return to the Fall 1998 Leopold Letter Index
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