ABSTRACT

Organic arsenical resistant (R-biotype) common cocklebur (Xanthium strumarium L.) was first reported in Lee County, SC and later confirmed in Lee, Calhoun, and Claredon Counties, SC. Subsequently, R-biotypes have been confirmed in AL and MS. Three MS counties that border TN were reporting common cocklebur resistance to MSMA in 1991. These reports, coupled with the failure of several producers to control common cocklebur, prompted this study to determine if the R-biotype was present in TN and if present to develop a management strategy. Common cocklebur seed were obtained from cotton fields in Madison, Crockett, Fayette and Tipton (two locations) counties. These seed along with 24 selections from AR were planted in the greenhouse and in the field at Jackson, TN and were sprayed initially with MSMA (with surfactant) at 2.0 lb ai/A. Surviving plants in the field received a second application of MSMA. Plants were 3 to 5 inches tall and with four to five true leaves when treated. Percent control was determined 12 days after treatment. The five selections from TN and two from AR were identified as R-biotypes.

Field studies were also conducted at three sites where R-biotypes were collected to further verify resistance and to evaluate the potential for control with some herbicides under development. Neither MSMA nor DSMA at 4.0 and 3.6 lb ai/A, respectively controlled common cocklebur at these sites. In some instances leaves were desiccated, but new growth occurred from lateral and terminal buds. Post-directed bromoxynil at 0.38 lb ai/A or glyphosate at 0.5 lb ai/A controlled common cocklebur >90%, but injured cotton where contact with the plant occurred DPX PE350 postemergence at 0.06 lb ai/A controlled common cocklebur >80% without injury to cotton. Unfortunately, DPX PE350 is not yet available. In the meantime, arsenical resistant common cocklebur in cotton can be managed by rotating to other crops where practical. If this is not possible, then one must strive for a height differential with soil-applied herbicides and utilize an early post-directed spray other than MSMA/DSMA. Escapes in the row should be removed to prevent cotton yield loss and cocklebur seed production. Should DPX PE350, glyphosate resistant or bromoxynil resistant cotton become available, arsenical resistant common cocklebur could be easily controlled.