Influence of glyphosate, crop volunteer and root pathogens on glyphosate-resistant wheat under controlled environmental conditions.

BACKGROUND The herbicide glyphosate has a synergistic effect on root disease because of increased susceptibility from reduced plant defenses resulting from the blockage of the shikimic acid pathway. Could glyphosate-resistant (GR) wheat cultivars and glyphosate application in-crop increase the risk of damage from soil-borne pathogens? Growth chamber experiments were conducted with two GR wheat lines and their corresponding glyphosate-sensitive (GS) parents and four pathogens (Rhizoctonia solani Kühn R. oryzae Ryker & Gooch, Gaeumannomyces graminis (Sacc.) v. Arx & J. Olivier var. tritici J. Walker and Pythium ultimum Trow). Treatments consisted of different herbicide timings and planting of crop volunteer to mimic management practices in the field. RESULTS GR cultivars were not inherently more susceptible to root pathogens than GS cultivars, and application of glyphosate did not increase root disease. When crop volunteer was grown in close proximity to GR cultivars, the timing of glyphosate application had a profound effect. In general, the longer the crop volunteer was left before killing with glyphosate, the greater was the competitive effect on the planted crop. Both R. solani and G. graminis var. tritici reduced plant height, number of tillers and root length of the GR cultivars in the presence of crop volunteer with glyphosate application. CONCLUSION To minimize the damaging effects of these pathogens, producers should apply glyphosate at least 2-3 weeks before planting GR wheat, as currently advised for GS cereals.