Management of Congo grass cover crop affects timing of Sclerotinia sclerotiorum carpogenic germination and decay of soybean stem rot

Abstract

The effects of cover crop management on the carpogenic germination of Sclerotinia sclerotiorum sclerotia and stem rot disease of soybean were investigated during three seasons (2008/2009 to 2010/2011) in a commercial farm of Goiás State, Brazil. Congo grass (Urochloa ruziziensis) annually grown during the fall at different densities (150, 300, 450 and 600 units of pure live seeds) was evaluated as cover crop and compared with a spontaneous weeds treatment. The germination of sclerotia was evaluated during cover crop growth in the spring season and in the summer, during soybean flowering. In general, apothecia density, disease incidence and disease severity index assessed at the 5.2 and 5.5 stages decreased with the increase of cover crop density. Trichoderma harzianum doses did not affect disease variables Soybean yield was not affected by any treatment. Principal component analysis clustered grass-soybean treatments apart from the spontaneous weeds-soybean sequence in all seasons. Carpogenic germination under the Congo grass canopy was negatively associated with apothecia density and disease during soybean growth. Mulching with Congo grass stimulates early carpogenic germination of S. sclerotiorum, enhancing soil disinfestation rather quickly. Adjustments in soybean crop stand are suggested to match improved disease management with higher soybean yields.