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Glyphosate-resistant sugar beets became more susceptible to root rot when sprayed with glyphosate.

Manipulated Organism: Sugar Beet (Beta vulgaris).

Inserted Transgenes: CP4 EPSPS gene from Agrobacterium, which produces an herbicide-resistant version of the enzyme targeted by the herbicide glyphosate (Roundup).

Goal of This Study: Although glyphosate-resistant (GR) crops are not killed by glyphosate, application of the herbicide to GR soybeans has exacerbated certain fungal diseases. To investigate this problem with two GR sugar beet varieties (B4 and H16), researchers exposed plants to different pathogenic strains in a greenhouse setting.

Results of This Study:
  • The GR beet cultivar B4 showed significantly more injury (such as leaf discoloration and ultimately death) from the soil fungus Rhizoctonia solani isolate R-9 when sprayed with glyphosate using a surfactant spray as a control. Disease severity was not visibly altered by glyphosate for infection with R. solani isolate R-1411.

  • Both GR beet cultivars showed significantly more injury from the soil fungus Fusarium oxysporum isolate FOB13 when sprayed with glyphosate. Disease severity was not visibly altered by glyphosate for infection with F. oxysporum isolate F-19.

  • In both GR varieties, there was a significant increase in shikimic acid when 3-4 week old plants were sprayed with glyphosate, indicating only partial resistance to the herbicide. The authors suggest this might explain the disease observations because "[i]nhibition of the shikimic acid pathway could result in reduced defensive capabilities of sugar beet" (p. 1189).
Additional Comments: "Increases in Rhizoctonia root rot could increase the soil pathogen population and affect other susceptible crops in rotation with GR sugar beet, such as dry bean, soybean and corn" (p. 1190).

Source: Larson, R. L., A. L. Hill, A. Fenwick, A. R. Kniss et al. (2006). "Influence of Glyphosate on Rhizoctonia and Fusarium Root Rot in Sugar Beet," Pest Management Science vol. 62, pp. 1182-92.

Author Affiliations: USDA Agricultural Research Service, Fort Collins, CO; Department of Plant Sciences, University of Wyoming.

Funding: Not mentioned.

Product Status: U.S. commercial production of GR sugar beets began in 2008.

Copyright 2009 The Nature Institute.

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