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Title: A BIOTYPE OF COMMON WATERHEMP (AMARANTHUS RUDIS) RESISTANT TO TRIAZINE AND ALS HERBICIDES

Author
item FOES, MATTHEW - UNIV OF ILLINOIS
item LIU, LIXIN - UNIV OF ILLINOIS
item TRANEL, PATRICK - UNIV OF ILLINOIS
item WAX, LOYD
item STOLLER, EDWARD

Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/25/1998
Publication Date: N/A
Citation: N/A

Interpretive Summary: There are recent reports from growers that several classes of herbicides, normally effective in controlling common waterhemp in fields, now fail to control this weed satisfactorily. This observation led us to conduct greenhouse and laboratory studies that tested the response of this weed to herbicides. We tested a selection of a reported 'hard-to-control' common waterhemp from Bond County Illinois and found that it was cross resistant to several ALS (acetolactase synthase) inhibiting herbicides such as imazethapyr, thifensulfuron, and flumetsulam. In addition, this selection also displayed a high level of multiple resistance to the photosynthesis-inhibiting triazine herbicide atrazine. Our laboratory studies revealed that the physiological basis for the resistance in the plant was a change at each site of these herbicides' action that rendered the herbicide ineffective. Further, by applying molecular techniques we determined the exact locations on the gene that caused amino acid changes in the enzymes at the sites of herbicide action and resulted in the lack of herbicide activity in the field. Results of this study are valuable to growers, extension personnel and researchers that are involved in weed management.

Technical Abstract: A common waterhemp biotype not controlled by triazine or acetolactate synthase (ALS) inhibiting herbicides was isolated from a field in Bond County, Illinois in the fall of 1996. Greenhouse and laboratory experiments were used to determine resistance and cross resistance patterns among three ALS inhibiting herbicides and atrazine. Based on whole plant responses, Bond County waterhemp required over one thousand times more imazethapyr to reduce growth 50% as compared with a susceptible biotype. Cross resistance to thifensulfuron, a sulfonylurea, and flumetsulam, a triazolopyrimidine sulfonanilide, was also detected in the resistant waterhemp biotype. Based on in vivo enzyme assays, ALS from Bond County waterhemp relative to the susceptible biotype was 20, >8.4, and 68 fold resistant to imazethapyr, thifensulfuron, and flumetsulam, respectively. Whole plant efficacy trials indicated that Bond County waterhemp also was highly resistant to atrazine. An application rate corresponding to over 20 kg ha-1 of atrazine was needed to retard growth 50%. Results from chlorophyll fluorescence assays revealed that a 100 nM atrazine solution inhibited photosynthesis in the susceptible biotype whereas 10 M atrazine did not affect chlorophyll fluorescence in the resistant biotype. Regions of the genes encoding ALS and the D1 protein were sequenced to determine the molecular basis for resistance. Triazine and ALS herbicide resistance was conferred by, respectively, a glycine for serine substitution at residue 264 of the D1 protein and a leucine for tryptophan substitution at residue 577 of ALS.