Cultivar and Herbicide Selection Affects Soybean Development and the Incidence of Sclerotinia Stem Rot

Glyphosate-resistant (GR) soybean [Glycine max (L.) Merr.] is produced on a majority of U.S. soybean hectares. Concerns have been expressed regarding interactions of herbicides such as glyphosate and Sclerotinia stem rot [Sclerotinia sclerotiorum (Lib.) de Bary]. Research evaluated the effect of herbicides on growth, phytoalexin production, incidence of Sclerotinia stem rot, and grain yield of GR and isogenic nonresistant cultivars in 1998 and 1999. Lactofen at 70 g a.i. ha⁻¹ delayed reproductive development, reduced leaf area index (LAI), reduced Sclerotinia stem rot lesion diameter 2 to 26 d after treatment (DAT), increased phytoalexin production 2 to 26 DAT, reduced disease severity index (DSI), and increased yield of ‘Great Lakes 2415’ (GL2415) by 510 kg ha⁻¹ compared with untreated soybean. Glyphosate at 840 g a.e. ha⁻¹ increased disease severity index in ‘Great Lakes 2600’ (GL2600) and ‘Pioneer 93B01’ (P93B01) GR compared with untreated soybean while yield of ‘Novartis S20-B9’ (S20-B9) GR treated with glyphosate was 650 kg ha⁻¹ greater than untreated soybean. Thifensulfuron at 4.5 g a.i. ha⁻¹ delayed reproductive development, reduced LAI, and did not affect phytoalexin production of ‘Novartis S 19-90’ (S 19-90) or S20-B9 GR but increased grain yield of ‘Novartis S 12-49’ (S 12-49) by 450 kg ha⁻¹ compared with untreated soybean. S 12-49 and GL2415 treated with thifensulfuron had lower disease severity index and increased yield compared with GR isolines. Reduction of disease severity index following lactofen may be attributed to increased phytoalexin production, reduced LAI, and delayed reproductive development; however, increased yield was observed for only one of eight cultivars. Producers can reduce Sclerotinia stem rot with postemergence herbicides, but soybean yield did not necessarily increase.