Poa annua control with norflurazon was different between two field trials in 1999. At the field site with suspected diuron-resistant P. annua, control was less than at the site with known diuron-susceptible P. annua. Diuron-resistant and -susceptible biotypes of P. annua were subjected to a range of norflurazon rates in the greenhouse. The diuron-resistant P. annua biotype (R/S : of 2.5) was collected in 1995 from a Lolium perenne L. seed production field near Tangent, OR. The R/S : for norflurazon in these experiments was 4.8; thus, confirming that diuron-resistant P. annua is resistant to norflurazon.

The extensive use of propanil for weed control in Oryza sativa over the past 30 yr has led to the development of propanil-resistant Echinochloa crus-galli. This problem was first identified in one Arkansas county in 1990. Since that time, 171 resistant populations have been identified in 20 counties. Random amplified polymorphic DNA (RAPD) analysis was used to assess genetic variation among populations of propanil-resistant and -susceptible E. crus-galli in Arkansas and to elucidate the origin of resistance and its dispersal. Sixteen E. crus-galli populations from seven Arkansas counties and one population from Mississippi were analyzed using 13 primers, which amplified a total of 159 repeatable fragments. Genetic distance was calculated among the populations, and the populations were clustered. The analysis revealed two distinct clusters with an average between-cluster genetic distance of 0.436 and average within-cluster distances of 0.012 and 0.029. These clusters may represent different Echinochloa species. Each cluster contained at least one resistant population and populations from different regions of the state. In addition, several resistant populations from different regions of the state were genetically nearly identical. Thus, it is likely that resistant biotypes have spread in Arkansas by seed dispersal and by independent mutation events. This suggests that there are multiple origins of the resistance, and simple control of weed seed dispersal will not stop the spread of resistance.

Growth chamber and laboratory experiments were conducted to evaluate foliar absorption and translocation of the isopropylamine (Ipa) or trimethylsulfonium (Tms) salts of glyphosate in Abutilon theophrasti and Setaria faberi. Glyphosate was applied at 250 g ai ha−1. Absorption of 14C-glyphosate in both formulations increased similarly in both species over 72 h. Seventy-two hours after treatment (HAT), A. theophrasti absorbed 26% of applied 14C of both formulations, whereas S. faberi absorbed 43% of both formulations. The addition of 1% ammonium sulfate (AMS) to both formulations increased the amount of herbicide absorbed to 35% in A. theophrasti and 60% in S. faberi 72 HAT. Translocation of absorbed 14C-glyphosate in both formulations (with or without AMS) was greater for S. faberi than A. theophrasti 72 HAT. For A. theophrasti, 17% of 14C absorbed in Ipa and Tms formulations were translocated out of the treated leaf, whereas additions of AMS increased the amount translocated to 25 and 23% for Ipa and Tms, respectively. The largest portion of translocated 14C moved to tissues above the treated leaf. For S. faberi, 37% of absorbed 14C in both Ipa and Tms formulations were translocated out of the treated leaf 72 HAT, whereas 42% translocated when 1% AMS was added to both formulations. In S. faberi, the largest portion of 14C-glyphosate translocated to tissues below the treated leaf. In summary, no differences were found between Ipa and Tms formulations in both weed species.

The effect of three spray variables (droplet number, active ingredient concentration, and droplet size) on uptake and translocation of formulated 14C-triclopyr ester was studied in greenhouse-grown Populus tremuloides seedlings. The dose per plant in all treatments was held constant. In all experiments, absorption (as a percentage of dose applied) was much greater than translocation (as percentage of dose absorbed). Absorption and translocation decreased as concentration (ai) was increased and droplet number decreased. Absorption and translocation also decreased as droplet number decreased and droplet size increased. When concentration (ai) was increased and droplet size decreased, absorption again decreased but to a much lesser extent than in the other two experiments; there was no significant effect on translocation. A time-course experiment indicated that uptake rate began to decrease within 1.5 h of application. The rate of decrease was greater at the higher concentration (ai), suggesting that the decrease was associated with contact injury. A model to integrate the application parameters and translocation gave a high correlation between dose per unit droplet stain circumference and translocation.

A field in which Brassica napus volunteers were not controlled by several applications of glyphosate was investigated in 1998. This field had been planted with glufosinate-resistant and imidazolinone-resistant B. napus in 1997 and was adjacent to a field that had grown glyphosate-resistant B. napus. Mature volunteer B. napus were collected on a 50- by 100-m grid in the field. Progeny from 34 volunteers were sprayed with glyphosate at 440 g ae ha−1, and the survivors were sprayed with either glufosinate or imazethapyr at 400 or 50 g ai ha−1, respectively. Where seed numbers permitted (14 volunteers), seedlings were also sprayed sequentially with glyphosate, glufosinate, and imazethapyr, at 440 g ae ha−1, 400 g ai ha−1, and 50 g ai ha−1, respectively. In total, 15 volunteers had progeny that were between 66 and 82% resistant to glyphosate, consistent with the predicted 3:1 resistant : susceptible ratio. Volunteer B. napus plants with glyphosate-resistant seedlings were most common close to the putative pollen source; however, a plant with glyphosate-resistant progeny was collected 500 m from the adjacent field edge. Seedlings from all nine volunteers collected from the glufosinate-resistant area showed multiple resistance to glyphosate and glufosinate, whereas seedlings from 10 of 20 volunteers collected from the imidazolinone-resistant area showed resistance to imazethapyr and glyphosate. DNA extraction and restriction fragment length polymorphism (RFLP) analysis of seedlings confirmed that mature B. napus volunteers were hybrids resulting from pollen transfer rather than inadvertent seed movement between fields. Two seedlings from the 924 screened were resistant to all three herbicides. Progeny from these self-pollinated individuals were resistant to glyphosate and glufosinate at the predicted 3:1 resistant : susceptible ratio and resistant to imazethapyr at the predicted 15:1 resistant : susceptible ratio. Sequential crossing of three herbicide-resistant varieties is the most likely explanation for the observed multiple herbicide resistance. Integrated management techniques, including suitable crop and herbicide rotations, herbicide mixtures, and nonchemical controls should be used to reduce the incidence and negative effect of B. napus volunteers with multiple herbicide resistance.

Rapid detection methods were developed for discriminating between resistant (R) and susceptible (S) biotypes of Echinochloa colona to either propanil or fenoxaprop-P at all growth stages. In the pregerminated seed assay for fenoxaprop-P, seeds were placed on 1.0% agar containing a range of concentrations of herbicides and kept under humid conditions. For propanil, pregerminated seeds were placed on moist filter paper in the lid of a petri dish and, when one leaf had developed, the lid was inverted for 1 min into propanil solutions at a range of concentrations. For the fenoxaprop-P and propanil test, seedling length and fresh weight were measured after 1 wk. For juvenile plants with four-leaf to one-tiller, shoots and roots were trimmed and placed in 20-ml glass tubes containing 0.2% (wt/v) agar and a range of concentrations of herbicides. Shoot extension and weight were recorded after 7 d. Larger plants with several small tillers were also assayed by this method. Tillers were removed from larger plants and were evaluated as described for juvenile plants. At later growth stages from ear emergence to flowering, excised stem node segments (8 cm) were soaked in water to stimulate rooting. Rooted nodes were placed in 30-ml glass bottles containing 0.2% agar with a range of concentrations of herbicides, and the test was conducted as described for juvenile plants but with the final assessment of new shoot extension and weight being recorded after 10 d. Discrimination between R and S biotypes was possible on the basis of GR50 values for shoot length and fresh weight in all testing methods. With few exceptions, GR50 values for the length of new shoot were very similar to those for the new shoot fresh weight. We concluded that all testing methods in our study provided reliable and quick discrimination between biotypes for both propanil and fenoxaprop-P susceptibility, covering various growth stages from seed to flowering stage. Trimming plants before herbicide treatment gives a rapid method of discrimination by measuring not only newly grown shoot fresh weight but also shoot length.

Phytoremediation depends on the ability of plants to tolerate and assimilate contaminants. We have been interested in the use of common ornamental plants to ameliorate the effects of pesticide waste on golf courses and ornamental plant nurseries. This research characterized the interaction between two ornamentals, Acorus gramenius and Pontederia cordata, and the herbicide simazine. Simazine tolerance levels for the ornamentals were determined by exposing plants to 0, 0.01, 0.03, 0.1, 0.3, 1.0, or 3.0 mg L−1 in aqueous nutrient media for 7 d. Response endpoints included fresh weight gains for both species after 7 d exposure and 7 d postexposure. Quantum efficiency of P. cordata was also measured using dark-adapted (Fv/Fm) and light-adapted (fluorescence yields) plants. Simazine uptake and distribution within each plant was determined by exposing plants to [14C]-simazine in nutrient media (0.24 mg L−1) for 1, 3, 5, or 7 d. Plant tissues were combusted and analyzed by liquid scintillation counting. Pontederia cordata fresh weight gains were reduced 76 and 70% by 1.0 and 3.0 mg L−1, respectively, after 7 d exposure. Acorus gramenius fresh weight gains were reduced 103, 124, and 144% at 0.3, 1.0, and 3.0 mg L−1 following 7 d exposure, respectively. Photosynthetic efficiency measured on dark-adapted P. cordata (Fv/Fm) was reduced 21, 47, and 71% by 0.3, 1.0, and 3.0 mg L−1, respectively, whereas photosynthetic efficiency measured on light-adapted plants (fluorescence yields) were reduced 9, 25, 59, 87, and 96%, respectively, by 0.03, 0.1, 0.3, 1, and 3 mg L−1. Simazine activity in solution was reduced 45 and 34% over 7 d with A. gramenius and P. cordata, respectively. By day 7, activity was distributed throughout the plants, but predominantly in the leaves. Uptake of simazine was correlated with water uptake throughout the 7 d. These results suggest that A. gramenius and P. cordata may be good candidates for incorporation in a phytoremediation scheme for simazine.

Two Glycine max near-isolines, GL2415 (glyphosate sensitive) and GL2600RR (glyphosate resistant), were compared for susceptibility to Sclerotinia sclerotiorum, the fungus that causes white mold disease (also known as Sclerotinia stem rot). A formulated isopropylamine salt of glyphosate (RU) at 840, 1,680, and 2,520 g ae ha−1; the RU formulation blank containing only adjuvants (RUFB) at 0.4% (v/v); organosilicone at 0.3% (v/v); a crop oil concentrate at 1.0% (v/v); and a nonionic surfactant at 0.5% (v/v) were evaluated for their influence on V5 G. max susceptibility to S. sclerotiorum in the field. The same treatments plus a formulated isopropylamine salt of glyphosate lacking adjuvants at 2,520 g ae ha−1 and a formulated trimethylsulfonium salt of glyphosate at 2,520 g ae ha−1 were applied to GL2600RR G. max in the greenhouse. Results from these studies indicated no differences in fungal growth, disease development, or G. max yield for any of these treatments. To determine if any interaction occurred between the chemicals, plants, and S. sclerotiorum, leaflets from both cultivars were dipped into and immediately removed from solutions of the RU at 4, 7, and 10% (v/v) and the RUFB and adjuvants at the rates described. The resulting lesion developments were similar across both cultivars but not across chemical treatments. Lesion developments were smaller on leaflets treated with RUFB and RU, but larger on leaflets treated with the other three adjuvants. Sclerotinia sclerotiorum mycelia growing on potato dextrose agar were inhibited by high concentrations of RU and of RUFB at 100 mM ae glyphosate and by the three adjuvants at 1% (v/v). Formulated glyphosate lacking adjuvants did not inhibit mycelial growth on potato dextrose agar. The glyphosate resistance trait did not appear to be associated with the susceptibility of G. max to S. sclerotiorum. Neither the glyphosate resistance trait nor the applications of glyphosate and adjuvants influenced G. max susceptibility to S. sclerotiorum.

Field studies were conducted in three site-years to measure no-till Glycine max yield loss in relation to Apocynum cannabinum vegetative shoot density. Apocynum cannabinum densities of 28 to 40 shoots m−2 reduced predicted G. max yield 58 to 75% and 62 to 94% with the rectangular hyperbolic and linear regression models, respectively. Differences between locations were attributed to rainfall and temperatures, with delayed G. max canopy closure and higher yield loss where soil moisture remained high and temperatures were relatively cool. Application of these predictive G. max yield loss equations to field populations of A. cannabinum showed that between 19 and 36% and 20 and 29% G. max yield loss could be expected from within A. cannabinum patches for the rectangular hyperbolic and linear regression models, respectively. The rectangular hyperbolic regression model appeared to describe the relation between G. max yield loss and A. cannabinum density accurately; however, the model appeared to be dominated by the initial linear phase. This may indicate a lack of high levels of intraspecific competition among A. cannabinum shoots. The results of this study indicate that there is a strong linear relation between G. max yield loss and A. cannabinum shoot density. We conclude that the biological basis for the use of the rectangular hyperbolic model for creeping perennial weeds is questionable.

Secale cereale is a serious weed problem in winter Triticum aestivum–producing regions. The interference relationships and economic thresholds of S. cereale in winter T. aestivum in Colorado, Kansas, Nebraska, and Wyoming were determined over 4 yr. Winter T. aestivum density was held constant at recommended planting densities for each site. Target S. cereale densities were 0, 5, 10, 25, 50, or 100 plants m−2. Secale cereale–winter T. aestivum interference relationships across locations and years were determined using a negative hyperbolic yield loss function. Two parameters—I, which represents the percent yield loss as S. cereale density approaches zero, and A, the maximum percent yield loss as S. cereale density increases—were estimated for each data set using nonlinear regression. Parameter I was more stable among years within locations than among locations within years, whereas maximum percentage yield loss was more stable across locations and years. Environmental conditions appeared to have a role in the stability of these relationships. Parameter estimates for I and A were incorporated into a second model to determine economic thresholds. On average, threshold values were between 4 and 5 S. cereale plants m−2; however, the large variation in these threshold values signifies considerable risk in making economic weed management decisions based upon these values.

There is little information published on patch expansion of perennial weeds and none for Apocynum cannabinum. Studies were conducted to measure the between-season and in-season expansion patterns of natural A. cannabinum patches over three growing seasons. Regression analysis indicated strong relations between patch area in consecutive years 1996 to 1997 (r2 = 0.81) and 1997 to 1998 (r2 = 0.76). Patches less than 20 m2 in 1996 increased in area by more than 100% in 1997 during a fallow season. However, patches decreased in size 6 to 51% between 1997 and 1998 when Glycine max was grown. Evidence suggested that a late-season mowing of the A. cannabinum patches in 1997 contributed more to the decline in patch area than competition from G. max during the 1998 season. The relations between patch area and growing degree units (r2 = 0.97) indicated that greater than 89% of the terminal patch expansion occurred prior to the accumulation of 435 growing degree units (GDU) (June 19, 1997; May 31, 1998; June 9, 30-yr average), with minimal patch expansion between 435 and 1,000 GDU. Patches were at 50% of their final area on May 27, 1997, and May 14, 1998, a time when only 22% of the A. cannabinum population had emerged (r2 = 0.99). Knowledge of patch size and expansion could help growers time weed scouting, to account for the later emergence patterns of this species, as well as assist in timing appropriate weed management efforts. This information could also be used in conjunction with aerial photographs to project potential patch size for site-specific management of this weed.

Rhizobacteria isolated from the rhizospheres of dominant weed species in six representative cropping systems and one native prairie ecosystem in mid-Missouri were screened for phytotoxicity on Lactuca sativa seedlings and their host plants in the laboratory. The proportions of deleterious rhizobacteria (DRB) were compared among different cropping systems to determine possible effects of crop management practices on the occurrence of DRB. Phytotoxicity screening on L. sativa seedlings revealed that an integrated crop management system with a Zea mays–Glycine max–Triticum aestivum cover crop rotation under no-tillage had the highest proportion of DRB at 25.3%, followed by an organic farming system with continuous Fragaria virginiana (strawberry) and organic amendments under minimum tillage at 22.9%. A continuous cool-season grass–legume meadow with no agrochemical inputs had the lowest proportion of DRB at 13%. Crop management practices that maintained high soil organic matter had higher proportions of DRB compared to cropping systems with lower organic matter. Phytotoxicity screening on host plants greatly reduced the proportion of rhizobacteria characterized as DRB, likely because of the high sensitivity of L. sativa seedlings to phytotoxins. Although screening on L. sativa is an effective method to detect phytotoxic rhizobacteria, our research indicates that it is essential to test selected cultures on their host weed species for accurate assessment of their occurrence in the field. Using this approach, we found that crop management practices influence the occurrence of DRB naturally associated with weed seedlings. Results suggest that crop production systems can be developed to favor soil microorganisms such as DRB that affect weed growth and thereby become important considerations in overall weed management.

Parasitization by Orobanche is a complex process, one that is mediated by host-derived chemical signals that control parasite seed germination and haustorium initiation and one that ultimately results in the union of two plant species. Experiments were conducted to characterize Orobanche parasitization of the model plant Arabidopsis thaliana and to begin to explore the role of host flavonoid metabolism in the interaction. Arabidopsis thaliana stimulated seed germination and allowed tubercle development of O. aegyptiaca and O. ramosa but did not significantly stimulate seeds of O. crenata, O. minor, or O. cernua. However, if Orobanche seeds were artificially stimulated, O. crenata and O. minor successfully established tubercles on A. thaliana. When compared to the recognized crop hosts, Daucus carota and Nicotiana tabacum, A. thaliana stimulated less O. aegyptiaca germination but allowed for formation of equivalent numbers of tubercles. These findings indicate that A. thaliana is not a large-scale producer of germination stimulant but is highly susceptible to the parasite once Orobanche seeds have germinated. Experiments comparing wild-type A. thaliana plants to mutant lines deficient in flavonoid biosynthesis revealed no differences in the ability to stimulate germination or to allow tubercle formation, indicating that host flavonoid production is not essential for Orobanche parasitization. The results of this work support the use of A. thaliana as a valuable host in understanding Orobanche parasitization.

Laboratory and field experiments were conducted to determine the effects of light, temperature, oxygen, and seed burial depth on seed germination and seed production of Eriochloa villosa without the interference of a crop. Eriochloa villosa seed germination did not respond to light, although it is possible that experimental conditions were not appropriate for this response to develop. Seeds germinated above 10 C and below 45 C, with germination above 85% occurring for temperatures between 20 and 40 C. Seed germination rate and total germination increased as the oxygen concentration increased to the ambient level. Eriochloa villosa seed emerged from 15 cm deep, with maximum emergence from 1 to 4 cm. In the field, emergence was dependent on tillage that affected the vertical distribution of seeds in the soil. Seeds emerged in the reduced-tillage field from between 0.5 and 9 cm deep in soil, with maximum emergence from 2 cm. A delay in planting greatly reduced the growth and seed production of E. villosa. Seed production under noncompetitive conditions was reduced by at least 75% when planting was delayed 6 to 8 wk. However, plants that emerged after July 7 still produced approximately 3,000 seeds when grown under conditions without intraspecific and interspecific competition.

Greenhouse and field studies were conducted to evaluate Oryza sativa response to the simulated carryover of fluometuron, imazethapyr, metolachlor, and norflurazon in dry- and water-seeded culture. In the greenhouse, fluometuron and norflurazon did not affect O. sativa emergence, but reduced O. sativa shoot and root dry weight at all the residue levels. Greater reduction in O. sativa emergence and shoot dry weight was observed with imazethapyr and metolachlor in dry- compared with water-seeded O. sativa. In the field, imazethapyr at 0.04 kg ai ha−1 injured dry-seeded O. sativa both years, but O. sativa yield was reduced only in the second year. Greater O. sativa injury and yield reduction were observed in dry- compared with water-seeded O. sativa with metolachlor at most residue levels. Norflurazon at 0.42 and 0.21 kg ai ha−1 caused greater O. sativa yield reduction in water-seeded O. sativa than in dry-seeded O. sativa in the first and second year, respectively. The results indicate that water-seeded O. sativa has less risk of being damaged by imazethapyr and metolachlor but is more susceptible to residues of norflurazon compared with dry-seeded O. sativa. Thus, choosing the right seeding method following a specific rotational crop herbicide could help to minimize the risk of O. sativa injury from herbicide carryover.

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.