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Management of Sclerotinia sclerotiorum in soybean using the biofungicides Bacillus amyloliquefaciens and Coniothyrium minitans
Sclerotinia sclerotiorum is a soilborne pathogen of soybean that causes Sclerotinia stem rot, alternatively called white mold. Sclerotinia stem rot can cause significant yield losses under cool and wet environmental conditions. Two biofungicides, Coniothyrium minitans and Bacillus amyloliquefaciens, are currently available and labeled to limit or suppress S. sclerotiorum in soybean. These biofungicides can be applied in place of synthetic foliar fungicides to provide an alternative mode of action for the control of Sclerotinia stem rot. However, limited information is available regarding the efficacy of C. minitans and B. amyloliquefaciens as biocontrol agents of S. sclerotiorum in soybean and the sensitivity of the biofungicides biological activity on S. sclerotiorum to pesticides commonly used in soybean production systems. This research aims to provide management recommendations for S. sclerotiorum in soybean using C. minitans and B. amyloliquefaciens and to develop guidelines for how to incorporate the biofungicides into an established soybean pest management program. To assess the effectiveness of C. minitans and B. amyloliquefaciens as biocontrol agents of S. sclerotiorum dual culture, amended media, and soil plate assays were conducted along with experiments in the growth chamber and field. The presence of a distinct inhibition zone surrounding the B. amyloliquefaciens colony in the dual culture assay and the absence of mycelial growth on the media plates amended with B. amyloliquefaciens confirmed that the bacteria can control the mycelial growth of S. sclerotiorum through antibiosis. The absence of an inhibition zone surrounding the C. minitans isolate in the dual culture assay along with the degradation of sclerotia following treatment with C. minitans in the soil plate assay indicates an inability to limit the mycelial growth of S. sclerotiorum and confirms that the primary mode of action is mycoparasitism. In the growth chamber, B. amyloliquefaciens at 14.03 L/ha applied using the dip method significantly reduced Sclerotinia stem rot lesion length when compared to the non-treated control and resulted in the lowest lesion area under the disease progress curve (lAUDPC). When B. amyloliquefaciens and C. minitans were applied in the field, no differences were observed between treatments for soybean moisture, test weight, or yield. To evaluate the sensitivity of B. amyloliquefaciens and C. minitans biological activity on S. sclerotiorum to pesticides commonly used in soybean production systems a poison plate assay as well as soil plate, growth chamber, and field experiments were conducted. In the poison plate assay C. minitans was most sensitive to the preemergence herbicide flumioxazin and the synthetic fungicides boscalid and fluazinam, while B. amyloliquefaciens was sensitive only to the synthetic fungicide fluazinam. In the soil plate assay the mycoparasitic activity of C. minitans on sclerotia of S. sclerotiorum was sensitive to flumioxazin, metribuzin, glyphosate, picoxystrobin, and boscalid. In the controlled environment experiments, none of the pesticides tested decreased the efficacy of B. amyloliquefaciens. There were no significant interactions between C. minitans and B. amyloliquefaciens with preemergence herbicides, postemergence herbicides, and synthetic fungicides for soybean moisture, test weight, and yield. This research demonstrates that B. amyloliquefaciens and C. minitans are effective biocontrol agents of S. sclerotiorum in soybean. However, antagonistic relationships exist between the biofungicides and certain preemergence, postemergence, and synthetic fungicides used in soybean production systems.