SMALLFLOWER UMBRELLA SEDGE (Cyperus difformis) with GROUP B/2 resistance: (INHIBITION OF ACETOLACTATE SYNTHASE ) Inhibition of Acetolactate Synthase
MUTATION: PROLINE 197 to HISTIDINE
Smallflower Umbrella Sedge (Cyperus difformis) is a monocot plant in the cyperaceae family. A single amino acid substitution from Proline 197 to Histidine has led to resistance to Inhibition of Acetolactate Synthase as indicated in the table below.		Smallflower Umbrella Sedge

Chemical Family	Example Herbicide	Resistance Level
Imidazolinones	Imazethapyr	Resistant > 10 fold
Pyrimidinyl benzoates	Bispyribac-Na	Resistant > 10 fold
Sulfonylureas	Chlorsulfuron	Resistant > 10 fold
Triazolopyrimidine - Type 1	Chloransulam-methyl	Resistant > 10 fold
Triazolinones	Flucarbazone-Na	Not Determined

REFERENCES

Tehranchian, P., D. S. Riar, J. K. Norsworthy, V. Nandula, S. McElroy, S. Chen, and R. C. Scott. 2015. ALS-resistant smallflower umbrella sedge (Cyperus difformis) in Arkansas rice: physiological and molecular basis of resistance. Weed Science 63 : 561 - 568.

Smallflower umbrella sedge is a problematic weed in direct-seeded rice in the midsouthern United States. It recently has evolved resistance to the acetolactate synthase (ALS) –inhibiting herbicide halosulfuron in Arkansas rice. Studies were conducted (1) to determine if the resistant biotype is cross resistant to other ALS-inhibiting herbicides, (2) to evaluate alternative herbicide control options, and (3) to determine the mechanism of resistance. Whole-plant bioassay revealed that halosulfuron-resistant plants were not controlled by bispyribac–sodium, imazamox, and penoxsulam at the labeled field rate of each herbicide. The level of resistance to these herbicides, based on the lethal dose needed to kill 50% of plants (LD₅₀) was ≥ 15-fold compared to a susceptible biotype. Both biotypes were controlled >96% with bentazon and propanil and ≤ 23% with quinclorac, thiobencarb, and 2,4-D. Hence, effective control measures exist; albeit, the number of herbicide options appear limited. Based on in vitro ALS enzyme assays, altered target site is the mechanism of resistance to halosulfuron and imazamox. Massively parallel sequencing with the use of the Illumina HiSeq detected an amino acid substitution of Pro₁₉₇-to-His in the resistant biotype that is consistent with ALS-inhibiting herbicide resistance in other weed species..

This case was entered by Patrick Tranel Email: tranel@illinois.edu

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