Submitted to: Pest Management Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/2/2006
Publication Date: 12/1/2006
Citation: Larson, R.L., Hill, A.L., Fenwick, A.L., Kniss, A., Hanson, L.E., Miller, S. 2006. Influence of glyphosate on rhizoctonia and fusarium root rot in sugar beet. Pest Management Science 62:1182-1192 (2006); DOI: 10.1002/ps.1297 Interpretive Summary: Glyphosate application on Roundup Ready sugarbeet leads to an increase in root rot disease severity caused by two fungal pathogens of sugarbeet in greenhouse studies. The increase in disease does not appear to be fungal-mediated, since in vitro studies showed no impact of glyphosate on fungal growth or reproduction with any fungal isolates tested. It was concluded the increase in disease is a result of a change in plant metabolism following glyphosate application. Glyphosate inhibits an enzyme in the shikimic acid pathway, which gives rise to essential amino acids and plant defensive compounds. Glyphosate resistant plants contain a replacement enzyme that is insensitive to glyphosate, however in laboratory studies there was a detectable level of enzyme inhibition without imparting an impact on plant growth in the Roundup resistant plants. The slight inhibition may not be significant enough to prevent essential amino acid production needed for normal growth, but may impact the availability of substrates for the production of downstream products used in plant defense. Future directions include examination of plant defensive compound production following glyphosate application.
Technical Abstract: This study tests the effect of glyphosate application on disease severity of glyphosate resistant sugar beet and examines whether the increase in disease in fungal- or plant-mediated. In greenhouse studies of glyphosate resistant sugar beet, increased disease severity was observed following glyphosate application and inoculation with certain isolates of Rhizoctonia solani Kuhn and Fusarium oxysporum Schlecht. Emend Snyd. & Hans. Significant increases in disease severity were noted for R. solani AG-2-2 isolate R-9 and moderately virulent F. oxysporum isolate FOB13 on both cultivars tested, regardless of the duration between glyphosate application and pathogen challenge, but not with highly virulent F. oxysporum isolate F-19 or an isolate of R. solani AG-4. The increase in disease does not appear to be fungal mediated, since in vitro studies showed no positive impact of glyphosate on fungal growth or overwintering structure production or germination for either pathogen. Studies of glyphosate impact on sugar beet physiology showed shikimic acid accumulation is tissue specific and the rate of accumulation is greatly reduced in resistant cultivars when compared to a susceptible cultivar. The results indicate precautions need to be taken when some soilborne diseases are present if weed management for sugar beet is to include post-emergence glyphosate treatments.