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Title: ASSESSMENT OF TWO NON-DESTRUCTIVE ASSAYS FOR DETECTING GLYPHOSATE RESISTANCE IN CONYZA CANADENSIS

Author
item Koger Iii, Clifford
item Shaner, Dale
item Henry, William
item NADLER-HASSAR, TOM - COLORADO STATE UNIV
item THOMAS, WALTER - N CAROLINA STATE UNIV
item WILCUT, JOHN - N CAROLINA STATE UNIV

Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/26/2005
Publication Date: 7/1/2005
Citation: Koger III, C.H., Shaner, D.L., Henry, W.B., Nadler-Hassar, T., Thomas, W.E., and Wilcut, J.W. 2005. Assessment of two non-destructive assays for detecting glyphosate resistance in conyza canadensis. Weed Science. 53(4):438-445.

Interpretive Summary: Repetitive use of glyphosate (Roundup) for over 20 years has led to the development of glyphosate resistant weeds in six plant species worldwide. The need exists for an easy and rapid assay for confirming resistance to glyphosate in suspected weeds. Two assays were tested for their ability to confirm glyphosate resistance in glyphosate-resistant and susceptible corn, cotton and soybean; as well as known and suspected glyphosate-resistant horseweed populations. Both assays were able to differentiate resistant from susceptible plants of crops and horseweed. Both assays can be used as a rapid screening tool for confirming resistance in suspected populations, which can help in developing alternative weed management strategies for glyphosate resistant weeds.

Technical Abstract: Two rapid, non-destructive assays were developed and tested for their potential in differentiating glyphosate-resistant from glyphosate-susceptible biotypes of horseweed. In one assay, leaves of glyphosate-resistant and -susceptible corn, cotton, and soybean plants as well as glyphosate-resistant and -susceptible horseweed plants were dipped in solutions of 0, 300, 600, and 1200 mg ae L-1 glyphosate for 3 d and subsequent injury was evaluated. In the second assay, the sensitivity of the EPSPS enzyme was evaluated in vivo by incubating excised leaf-disc tissue from the same plants used in the first assay in 0.7, 1.3, 2.6, 5.3, 10.6, 21.1, 42.3, and 84.5 mg ae L-1 glyphosate solutions for 16 h and measuring shikimate levels with a spectrophotometer. The leaf-dip assay differentiated between glyphosate-resistant and -susceptible crops and horseweed biotypes. The 600 mg L-1 rate of glyphosate was more consistent in differentiating resistant and susceptible plants compared with the 300 and 1200 mg L-1 rates. The in vivo EPSPS assay detected significant differences between susceptible and glyphosate-resistant plants of all species. Shikimate accumulated in a glyphosate dose-dependent manner in leaf discs from susceptible crops, but shikimate did not accumulate in leaf discs from resistant crops and levels were similar to nontreated leaf-discs. Shikimate accumulated at high (> 21.1 mg ae L-1) concentrations of glyphosate in leaf discs from all horseweed biotypes. Shikimate accumulated at low glyphosate concentrations (< 10.6 mg L-1) in leaf discs from susceptible horseweed biotypes but not in resistant biotypes. Both assays were able to differentiate resistant from susceptible biotypes of horseweed and may have utility for screening other weed populations for resistance to glyphosate.