Roundup ready soybean:: Glyphosate effects on Fusarium solani root colonization and sudden death syndrome

During 1997, the first year of widespread use of glyphosate (N-[phosphonomethyl]glycine) on Roundup Ready (RR) soybean [Glycine mar (L.) Merr.] a severe sudden death syndrome (SDS) epidemic occurred and several RR cultivars were affected. Effects of glyphosate on colonization of soybean root by Fusarium solani (Mart.) Sacc f. sp. glycines (Fsg) and SDS were evaluated. Five RR cultivar pairs that contrasted for SDS resistance from maturity groups (MG) II to VI were evaluated with and without glyphosate application. The MG Ii and III cultivars were evaluated near Bloomington, Pontiac, and Mahomet in central Illinois and the MG IV, V, and VI cultivars were evaluated near Harrisburg, Ullin, and Valmeyer in southern Illinois. The Fsg root infection severity (IS), colony forming units per gram of root (CFU), SDS leaf scorch disease index (DX), and grain yield were determined. Across environments within each MG, there were no significant effects of glyphosate on IS, CFU, and DX. Significant differences were expected between cultivars but only observed in some MG. There was no significant effect of glyphosate on yield. Significant Glyphosate X Cultivar interactions occurred for yield in MG VI, in favor of the glyphosate sprayed subplots. In this study root colonization by Fsg and SDS leaf symptoms did not significantly increase following the application of glyphosate. Data from this study indicate that the development of SDS on RR soybean is influenced by genotype. Farmers planting RR soybean in Fsg infested fields are encouraged to select cultivars with resistance to SDS.