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Managing Sclerotinia in soybeans with Contans

Michael Wunsch, Michael Schaefer, Suanne Kallis, and Billy Kraft, NDSU Carrington Research Extension Center

Due to its unique biology, Sclerotinia stem rot (white mold) is a particularly good target for biological control. Unlike most fungal plant pathogens, Sclerotinia sclerotiorum, the fungus that causes Sclerotinia stem rot, does not produce spores on diseased tissues. Instead, spores of the fungus are produced when sclerotia (fungal resting structures in the soil) germinate to produce tiny mushroom-like structures called apothecia. Biological control agents that degrade the fungal resting structures in the soil have the capability of significantly reducing Sclerotinia spore production and thereby reducing disease development.

This multi-year project evaluated the efficacy of Contans WG, a commercial formulation of the biological control agent Coniothyrium minitans, for control of Sclerotinia stem rot of soybean. Application timing (fall versus spring) and application rate (1 versus 2 lb/ac) were tested, and the efficacy of Contans was compared to a single application of the foliar fungicide Endura. Contans was applied to the surface of a disked field in October and in May and was incorporated with a shallow cultivation the same day. Testing was conducted on a 34-acre field irrigated with a center pivot. In the first two years of this project, small plots were utilized, which each plot separated by 60 ft of untreated (first year) or Contans-treated (second year) ground. In the last two years of this project, testing was conducted with 1.16-acre plots, with all treatments imposed with commercial-scale equipment.

Across the four years that this project was conducted, applications of Contans were consistently associated with trends towards reduced numbers of apothecia (fruiting structures of the Sclerotinia fungus), but differences were generally not statistically significant. Fall applications of Contans generally performed better than spring applications. Yield responses to applications of Contans were modest. Rigorous assessment of the efficacy of Contans was hampered by movement of spores of the Sclerotinia fungus between plots (first year), low disease pressure (second and fourth years), and high variability in white mold disease pressure across the footprint of the research trial (third and fourth years).
Studying the efficacy of a biological control agent such as Contans, which is expected to reduce the production of spores of the Sclerotinia fungus, is difficult due to movement of Sclerotinia spores between plots. Results from this multi-year research project strongly suggest that applications of Contans can reduce apothecia production by the Sclerotinia fungus, but the impact of Contans applications on soybean yield under high white mold disease pressure remains unclear.