Submitted to: Crop Protection Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/23/2013
Publication Date: 8/22/2013
Citation: Mengistu, A., Reddy, K.N., Bellaloui, N., Walker, E.R., Kelly, H.M. 2013. Effects of glyphosate on Macrophomina phaseolina in vitro and its effects on disease severity of soybean in the field. Crop Protection Journal. 54:23-28. Interpretive Summary: Charcoal rot is a disease of soybean caused by the fungus (mold) Macrophomina phaseolina that causes significant economic yield loss in the United States and around the world. Glyphosate, sold commercially as Roundup, is a non-selective, broad-spectrum herbicide most extensively used on genetically modified glyphosate-resistant soybean cultivars. Reports have shown significant disease increase in soybean associated with the wide-spread application of glyphosate. In this study, we evaluated the effect of increasing rates of glyphosate on the growth of the charcoal rot mold in the laboratory and also made single and sequential applications of glyphosate at three-leaf (V3) and six-leaf (V6) growth stages in tilled fields in Mississippi and in no-tilled field in Tennessee. The research showed that glyphosate has the ability to inhibit growth of the charcoal rot mold in culture. Field application of glyphosate however, did not enhance or reduce the severity of charcoal rot in no-till plots in TN. However, charcoal rot severity was suppressed in tilled soils in MS when single application at V3 or V6 rather than when sequential applications were made. This is preliminary evidence that charcoal rot severity may be reduced if glyphosate is applied only once in tilled fields.
Technical Abstract: Laboratory and field studies were conducted to assess the effects of glyphosate on Macrophomina phaseolina culture growth in vitro and the disease severity of charcoal rot in soybean at Stoneville, MS and Jackson, TN. Glyphosate inhibited M. phaseolina growth in a linear dose dependent manner when technical grade glyphosate acid (GlyCry) was used; however, growth was inhibited in an exponential dose dependent manner when a commercial formulation as glyphosate-potassium salt (Gly-K salt) was used. The glyphosate GR50 values (glyphosate concentration required to cause a 50% reduction) in culture radial growth ranged from 0.25 to 9.94 mM among the M. Phaseolina isolates, temperatures, and formulations. The three isolates differed in response to various concentrations across the three temperature regimes. Among the three isolates, TN 410 was the most sensitive for both GlyCry (GR50=7.74 mM) and Gly-K salt (GR50=0.25 mM) at 30 °C. This research indicates that glyphosate has the ability to inhibit growth of M. phaseolina in culture in vitro. The preliminary field studies demonstrated that application of glyphosate to glyphosate-resistant soybeans did not enhance or reduce the severity of charcoal rot in no-till but had some suppressing effect in tilled environments when single applications were made at growth stage V3 or V6 in MS.