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1 October 2001 Identification of a Johnsongrass (Sorghum halepense) Biotype Resistant to Aryloxyphenoxypropionate and Cyclohexanedione Herbicides in Virginia
KEVIN W. BRADLEY, EDWARD S. HAGOOD
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Abstract

Field and greenhouse dose-response experiments were conducted to investigate the potential for resistance in a johnsongrass biotype from New Kent County, VA that survived repeated applications of quizalofop and quizalofop-P. During 1996 and 1997, foliar injury (30 to 60%) was initially observed on the johnsongrass at the New Kent field site, but this biotype eventually recovered and survived applications of fluazifop-P, quizalofop-P, and sethoxydim at twice the recommended field use rates. However, applications of clethodim at twice the recommended field use rates during 1997 provided essentially complete control of the New Kent johnsongrass biotype. In greenhouse dose-response experiments, the amount of quizalofop-P required to inhibit shoot growth by 50% (GR50) was 13.6 g/ha in the New Kent johnsongrass biotype and 0.8 g/ha in the susceptible johnsongrass biotype. In response to sethoxydim, the GR50 for the New Kent biotype was 122.5 g/ha while that of the susceptible biotype was 21.6 g/ha. Additionally, the New Kent biotype was least sensitive to fluazifop-P, which provided a GR50 value of 148.7 g/ha for the New Kent biotype and 5.1 g/ha for the susceptible biotype. As in the field trials, the New Kent biotype was sensitive to clethodim, which provided a GR50 value of 49.8 g/ha and 69.1 g/ha for the susceptible biotype. These values indicate that the New Kent biotype was 17 times more resistant to quizalofop-P, 5.7 times more resistant to sethoxydim, and 29.5 times more resistant to fluazifop-P than the susceptible biotype, and that the New Kent and susceptible johnsongrass biotypes are equally sensitive to clethodim. These GR50 values for the New Kent johnsongrass biotype are inconsistent with the much higher GR50 values most commonly observed in graminicide-resistant weed biotypes, and suggest a mechanism of resistance other than an insensitive ACCase in the New Kent johnsongrass biotype.

Nomenclature: Clethodim; fluazifop-P; quizalofop-P; sethoxydim; johnsongrass, Sorghum halepense (L.) Pers. #3 SORHA; soybean, Glycine max (L.) Merr.

Additional index words: Cross-resistance, herbicide resistance, ACCase inhibitors, graminicides.

Abbreviations: ACCase, acetyl coenzyme-A carboxylase; APP, aryloxyphenoxypropionate; CHD, cyclohexanedione; DAT, days after treatment; GR50, dosage giving 50% reduction in shoot dry weight; WAT, weeks after treatment.

KEVIN W. BRADLEY and EDWARD S. HAGOOD "Identification of a Johnsongrass (Sorghum halepense) Biotype Resistant to Aryloxyphenoxypropionate and Cyclohexanedione Herbicides in Virginia," Weed Technology 15(4), 623-627, (1 October 2001). https://doi.org/10.1614/0890-037X(2001)015[0623:IOAJSH]2.0.CO;2
Published: 1 October 2001
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