Assessment of the disease response of glyphosate resistant spring wheat genotypes to soilborne fungal pathogens

Glyphosate resistant (GR) wheat permits post-emergence weed control and maintains the environmental benefits associated with no-till crop production. However, yield gains may be lost due to increased activity of soilborne pathogens on glyphosate treated weeds. The objective of this study was to determine the risks of incorporating GR wheat into no-till production systems. Near isogenic lines (NILs), with and without glyphosate resistance, were evaluated under controlled-environment and no-till field conditions. Three research projects were conducted to address this objective. (1) Growth chamber experiments determined that the disease response of GR wheat genotypes were not negatively affected by transformation with glyphosate resistance, and the disease severity of Pythium ultimum, Rhizoctonia solani, R. oryzae, and G. graminis var. tritici (Ggt) did not increase on GR genotypes after application of glyphosate. (2) Field trials were conducted under no-till production conditions at three agroclimatic zones of eastern Washington in 2002 and 2003. Rhizoctonia spp. or Ggt inoculated oats were direct seeded into field plots with barley before planting NILs. Glyphosate treated GR genotypes were significantly (P ≤ 0.05) stunted in Rhizoctonia spp. and Ggt inoculated plots at Pullman in 2003. Severe incidence of stripe rust (Puccinia striiformis), noted at Pullman in 2002 and 2003, decreased when glyphosate was applied to stripe rust susceptible GR Bobwhite. (3) Glyphosate-volunteer weed management systems were evaluated in growth chamber experiments to assess the transmission of P. ultimum, R. solani, R. oryzae, or Ggt from glyphosate treated volunteer to GR wheat. Pre-plant and post-emergence management systems significantly (P ≤ 0.05) decreased height, tiller production, and root length for R. solani and Ggt infested soils. GR genotypes for no-volunteer systems were not affected by pathogen treatment. The primary conclusions from this work are: (1) Applying glyphosate does not enhance susceptibility of GR wheat to soilborne pathogens; and (2) plant health of GR wheat is compromised when glyphosate is applied to control GS weeds in Ggt and R. solani infested soils.