Pennsylvania smartweed [Persicaria pensylvanica (L.) M. Gómez] is a common weed of rice (Oryza sativa L.) in the midsouthern United States and has recently become a concern for farmers because of reduced tillage systems. Acetolactate synthase (ALS) inhibitors have been extensively used for controlling smartweeds in imidazolinone-resistant and conventional rice. In the present study, we confirmed resistance to commonly used ALS inhibitors in rice and characterized the underlying resistance mechanism in a P. pensylvanica biotype from southeast Missouri. A dose–response experiment was conducted in the greenhouse using bensulfuron-methyl, imazethapyr, and bispyribac-sodium to determine the resistance index (resistance/susceptibility [R/S]) based on GR50 estimates. The target-site ALS gene was amplified from R and S plants, and sequences were analyzed for mutations known to confer ALS-inhibitor resistance. The P. pensylvanica biotype in question was found to be resistant to bensulfuron-methyl (R/S=2,330), imazethapyr (R/S=12), and bispyribac-sodium (R/S=6). Sequencing of the ALS gene from R plants revealed two previously known mutations (Pro-197-Ser, Ala-122-Ser) conferring resistance to sulfonylureas and imidazolinones. This is the first report of ALS-inhibitor resistance in P. pensylvanica.

Weed control with thiameturon applied postemergence at 8 to 32 g ai/ha varied with species, growth stage, addition of surfactant, and environmental conditions. Pennsylvania smartweed control was excellent, velvetleaf control ranged from poor to excellent, and common lambsquarters control was poor with thiameturon applied without surfactant. When surfactant was added to the thiameturon spray mixture, velvetleaf and common lambsquarters control was >85% at 8 g/ha when weeds were growing actively and at 16 g/ha when weeds were under moisture stress. However, in weed-free studies, corn yields were reduced by 35 kg/ha for each 1 g/ha thiameturon applied in 1986 and by 10 kg/ha for each 1 g/ha thiameturon applied in 1987 when surfactant was added to the spray mixture. When no surfactant was added, yields were reduced 12 kg/ha for each 1 g/ha thiameturon applied in 1986 but were not reduced in 1987.

Field experiments were conducted in 1993, 1994, and 1995 to determine the effects of glufosinate rate and application timing on giant foxtail, common lambsquarters, common cocklebur, and Pennsylvania smartweed control in absence of a crop. Glufosinate at 140 g ai/ha controlled less than 80% of the weed species evaluated. When glufosinate rate was increased to 420 g/ha and applied to 10-cm giant foxtail, control was greater than 80% all 3 yr of the study. Applications made to 10-cm plants resulted in 80% or greater control for common cocklebur all 3 yr and Pennsylvania smartweed 2 of the 3 yr with 420 and 560 g/ha, respectively. Common lambsquarters was the most tolerant species evaluated and was not consistently controlled acceptably (> 80%), even with glufosinate at rates of 560 g/ha. Control with glufosinate at 420 or 560 g/ha was most effective when applied at the 10-cm weed height compared either to the 5- or 15-cm weed height.