Solanum ptycanthum plants putatively resistant to acetolactate synthase (ALS) inhibitors were identified in a Wisconsin Glycine max field in 1999. Three- to four-leaf-stage S. ptycanthum plants in the greenhouse were 150, 120, and 5.9-fold resistant to imazethapyr, imazamox, and primisulfuron, respectively, compared with susceptible plants. In vivo ALS was 170- and less than 20-fold more resistant to imazethapyr and primisulfuron, respectively. These results suggested that the S. ptycanthum accession was highly resistant to imazethapyr and imazamox, and that resistance was associated with insensitive ALS. This is the first confirmed occurrence worldwide of S. ptycanthum resistance to ALS inhibitors.

Laboratory experiments were conducted to determine the effects of application factors and standing Triticum aestivum stubble on herbicide spray deposition and efficacy in a simulated no-till environment. Spray deposition on weeds was reduced in the presence of stubble, and deposition losses on Amaranthus hybridus were greater than those on Setaria faberi. Spray penetration through stubble was significantly enhanced with electrostatic charging of a fine hydraulic spray. The combination of 45 kV electrostatic charge and 50 cm nozzle spacing produced maximum spray deposition on weeds and resulted in a 96% and 345% increase in deposition on A. hybridus and S. faberi, respectively, compared to the uncharged controls. Deposit reduction from standing stubble was greater at travel speeds of 16 km h−1 (36 to 52%) than 8 km h−1 (9 to 38%). On a dry weight and plant density basis, weeds retained more spray than was retained by stubble, yet stubble, at average densities, was capable of capturing 9 to 12% of total applied spray dose per unit area. Bounce studies of individual droplets of water or imazethapyr plus adjuvant mixture demonstrated that T. aestivum straw had a general affinity for all spray droplets, exhibiting no rebound even for 800-µm water droplets. Setaria faberi foliage exhibited poor retention of droplets: both 350- and 800-µm water droplets as well as 800-µm droplets of imazethapyr plus adjuvant mixture rebounded. Only 350-µm herbicide mixture droplets were retained by S. faberi. Amaranthus hybridus retained all droplets. In broadcast spraying, British Crop Protection Council “Medium” quality sprays were poorly retained by S. faberi compared to “Fine” sprays, whereas A. hybridus retained both sprays equally well. However, imazethapyr spray deposits resulting from coarser sprays were more efficacious on S. faberi than fine spray deposits, a difference that was not observed for A. hybridus.

Time-dependent adsorption of imazethapyr was studied in the laboratory on a sandy loam soil at 16% moisture for 30 d. Soil pH was adjusted to 4.5 to 6.8. Concentration of imazethapyr in soil water declined rapidly within the first day of incubation for all soil pH levels, indicating rapid initial adsorption. However, the concentration of imazethapyr in the soil water continued to decline slowly over time. Even at pH 6.8, where batch equilibrium studies at high solution to soil ratios indicated low adsorption, only 22% of the applied imazethapyr was in solution 10 d after treatment (DAT), and 10% 30 DAT. Soil column mobility studies also showed a rapid decline in mobility as time after application increased. The amount of imazethapyr remaining in soil columns rapidly increased within 4 to 10 DAT, even at pH 7.8, where low adsorption is predicted. Imazethapyr was stable during the time of these experiments, so degradation to less soluble or more tightly bound products is not the reason for the increased adsorption. These results indicate that batch equilibrium studies that use slurries with high solution to soil ratios (often 2:1 or higher) and short equilibration times may not accurately predict mobility of acidic herbicides in the field, where moisture content is lower.

The fungal protein Nep1, produced by Fusarium oxysporum f. sp. erythroxyli in liquid culture, caused extensive necrosis to Centaurea maculosa when water solutions of Nep1 (5 µg ml−1) and an organosilicone surfactant (1,1,1,3,5,5,5-heptamethyltrisiloxanyl propyl-methoxy-poly[ethylene oxide]) were applied as foliar sprays. Nep1 did not cause necrosis when applied with a nonionic surfactant or organosilicone surfactant plus unrefined corn oil. Plant age, protein concentration, organosilicone surfactant concentration, and the presence of a dew period influenced the amount of necrosis caused by Nep1. The addition of an 18-h dew period after treatment resulted in an increase of 10% or more in foliar necrosis at the 0.313 and 1.25 µg ml−1 (0.40 and 1.62 g ai ha−1) Nep1 concentrations. Increasing the spray volume from 129 ml m−2 (1,291.3 L ha−1) to 516 ml m−2 (5,165.2 L ha−1) more than doubled the amount of foliar necrosis caused by the 0.313 µg ml−1 (0.40 g ai ha−1 vs. 1.62 g ai ha−1) Nep1 concentration. A maximum necrosis rating of 95% was reached by 1.25 µg ml−1 Nep1 applied at 516 ml m−2 (6.46 g ai ha−1) followed by an 18-h dew period. Nep1 (6.46 g ai ha−1) remained active when coapplied to Centaurea maculosa with the herbicides 2,4-D or glyphosate (0.13 to 2.58 kg ai ha−1), causing foliar necrosis prior to the herbicides killing Centaurea maculosa. An increase in the organosilicone surfactant concentration from 1 to 2 ml ai L−1 was required to achieve levels of Nep1-induced necrosis on Centaurea maculosa acclimated to direct sun comparable to levels achieved on greenhouse-grown plants. Repeated application of Nep1 (6.48 g ai ha−1) 3 wk after an initial treatment (6.48 g ai ha−1) prevented the recovery of acclimated Centaurea maculosa. Greater damage was caused to acclimated Centaurea maculosa when Nep1 was applied near the middle of the day (80% necrosis at 10:00 A.M. and 85% necrosis at 2:00 P.M.) compared to early or late in the day (25% necrosis at 6:00 A.M. and 10% necrosis at 6:00 P.M.).