Published online by Cambridge University Press: 20 January 2017
Previous greenhouse studies with a noncommercial glyphosate-resistantsugarbeet variety indicated that susceptibility to Rhizoctonia crown androot rot could increase after glyphosate was applied. Greenhouse and fieldexperiments were conducted in 2008 and 2009 to determine if glyphosateinfluenced disease severity in potential commercially available varieties ofglyphosate-resistant sugarbeet. In the first greenhouse experiment in 2008,Hilleshög 9027RR, the most tolerant variety to Rhizoctonia crown and rootrot, exhibited an increase in disease severity when glyphosate was applied.There were no significant differences between herbicide treatments inHilleshög 9028RR, and glyphosate decreased disease severity in Hilleshög9032RR when compared with the no-herbicide treatment. Experiments conductedto determine if glyphosate influenced Rhizoctonia solanigrowth in vitro indicated that glyphosate did not increasethe radial growth of R. solani, except at 10× (190 µg ae ml−1) the normal rate of glyphosate plus ammonium sulfate(AMS). Field and additional greenhouse experiments were conducted using fourcommercial varieties. Differences in disease severity were observed whencomparing varieties, but glyphosate did not significantly influence theseverity of Rhizoctonia crown and root rot when compared with theno-herbicide control. Choosing a glyphosate-resistant sugarbeet variety withthe best demonstrated tolerance to Rhizoctonia crown and root rot is animportant factor in reducing disease severity and maintaining sugarbeetyield.