Canada thistle is resilient to many control tactics, especially in undisturbed sites. Such sites are suitable for slow acting biological control agents, such as the bacterium Pseudomonas syringae pv. tagetis (PST), because complete control is usually not required in the short term. A new method of introducing or intensifying PST infection of Canada thistle was investigated. Sap of naturally infected Canada thistle was extracted and applied in water plus Silwet L-77 organosilicone surfactant with a backpack sprayer to healthy Canada thistle plants in the field. Application variables of time of the season, spray volume, concentration, and frequency were studied. When practical field rates were applied, infected sap concentration and spray volume did not affect the level of disease observed. This suggests that PST applications could be practical at the field scale because a single application caused apical chlorosis. However, multiple applications proved beneficial because four consecutive weekly applications caused greater disease incidence (50%) than one or two applications (28% and 30%, respectively). Disease symptomology was greatest when PST was applied in mid-July rather than mid-June or mid-August. However, the levels of disease expression were not adequate to effectively suppress Canada thistle. Increased toxin production, either by finding ways to support higher PST populations or by selecting strains that produce more toxin per bacterial cell, would improve this system.

The effects of sethoxydim {2-[1-(ethoxyimino) butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} on the metabolic activity of excised root tips of corn (Zea mays, L. ‘Goldencrossbantam′) were studied under laboratory conditions. Uptake and incorporation of 14C-labeled thymidine, uridine, leucine, glucose, and acetic acid into cell constituents, as well as respiration, increased continuously with time progressions during the incubation period. Sethoxydim did not affect either the uptake of any 14C-precursor into or respiration of the root tip tissue. Although RNA and protein syntheses were not affected by the herbicide, DNA and cell wall syntheses were inhibited 120 min after treatment with sethoxydim. Incorporation of 14C-acetic acid into lipid fraction was inhibited by sethoxydim in a time- and concentration-dependent fashion. This inhibition was observed at a shorter time after sethoxydim treatment than that of any other 14C-precursor. The effect was not observed in the nonproliferative regions of corn roots, whereas cerulenin (a fatty acid synthase inhibitor) inhibited the incorporation of 14C-acetic acid both in proliferative and nonproliferative regions. It is suggested that the inhibition of lipid synthesis by sethoxydim does not play a major role in the mode of action of this herbicide. The effects of sethoxydim, including those on lipid metabolism, are closely associated with proliferative conditions of susceptible graminaceous plants.

Major differences in above- ground biomass and tuber production of yellow nutsedge (Cyperus esculentus L. # CYPES) were not observed when corn (Zea mays L. “CO-OP S265”) was seeded on different dates (1st, 2nd, 3rd and 4th week of May; and 1st week of June). The final seedbed was prepared just prior to each seeding date and this cultivation stimulated dormant tubers to sprout. As a result, a large population of yellow nutsedge emerged with the corn at all seeding dates. Because fertilizer was banded near the corn row, yellow nutsedge biomass, tuber dry weight and number of tubers were higher within corn rows than between rows. Tuber size was affected by seeding date and shifted toward smaller tubers within corn rows and larger tubers between the rows as the corn was sown late. The optimum seeding date of corn was in the 3rd week of May when the highest corn yield was obtained and yellow nutsedge growth was generally reduced.

Tank-mix combinations of PRE and POST herbicides were applied to control emerged large crabgrass and goosegrass in common bermudagrass turf. Large crabgrass control was better when MSMA at 2.2 kg ai ha-1 was tank mixed with either pendimethalin at 1.7 kg ai ha-1 (≤ 85%) or dithiopyr at 0.3 kg ai ha-1 (≤ 94%), when compared with MSMA at 2.2 kg ai ha-1 (≤ 50%) alone. When MSMA plus metribuzin at 2.2 + 0.14 kg ai ha-1 was tank mixed with pendimethalin in 1992, large crabgrass control was better (≥ 96%) than when MSMA plus metribuzin was applied alone (≤ 73%). Quinclorac applied alone at 0.8 kg ai ha-1 controlled ≥ 94% large crabgrass, but control with diclofop was 32%. There was no increase in control when either quinclorac or diclofop was tank mixed with any PRE herbicide. Methyl ester of diclofop applied alone at 1.1 kg ai ha-1 provided 97% control of goosegrass at one location, but control was ≤ 62% at two other locations. Goosegrass control was better when diclofop at 1.1 kg ha-1 was tank mixed with dithiopyr at 0.6 kg ha-1 (≥ 77%) than when either diclofop (≤ 62%) or dithiopyr (≤ 50%) was applied alone. Goosegrass control was consistently better across years from combinations of dithiopyr plus diclofop at 0.6 + 1.1 kg ha-1 (82%) than from combinations of MSMA plus metribuzin with dithiopyr (34%) or pendimethalin (53%). Common bermudagrass injury at Griffin-Spalding Golf Course in 1991 was higher at 1 wk after combination treatments of MSMA plus metribuzin with dithiopyr (46 to 54%) or pendimethalin (50%) than with MSMA plus metribuzin applied alone (26%). Injury from the combinations was higher from single MSMA plus metribuzin treatment for 4 wk. Similar results occurred at the Georgia Station in 1992 from combinations of dithiopyr with MSMA plus metribuzin.

The cucumber (Cucumis sativus L.) accession PI 390244 tolerated up to 0.56 kg/ha atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] whereas most cultivars and accessions were killed by 0.14 kg/ha. The possible mechanisms for tolerance were investigated by comparing growth rates, effects on photosystem II in isolated chloroplasts, and differences in uptake and translocation of 14C-atrazine between tolerant PI 390244 and the susceptible cultivar Marketmore 70. PI 390244 had significantly more leaf area than Marketmore 70, but less root biomass. Isolated chloroplast preparations from both cultivars were equally susceptible to the herbicide. Although PI 390244 absorbed more total 14C-atrazine, it translocated less radioactivity from the roots. While the concentration of 14C found in the PI 390244 roots was 10 to 30% greater than in Marketmore 70, the amount of radioactivity in the cotyledons, and first and second true leaves of Marketmore 70 were 53, 34, and 55% higher, respectively, after 48 h.

Because environmental factors facilitating the breaking of dormancy of seed of catclaw mimosa (Mimosa pigra L. ♯ MIMPI) under field conditions were unknown, the effect of constant temperatures (2 to 44 C) on germination of scarified and unscarified seeds, and the effect of diurnally fluctuating temperatures on unscarified seeds were examined. Temperatures fluctuated diurnally by 10 and 20 C in these latter experiments. Germination was less than 10% at all constant temperatures except 44 C, which had 40% germination. Scarification increased germination to 100% at constant temperatures greater than 12 C. Germination of unscarified seed was significantly lower (0 to 23%) when day/night temperature ranges fluctuated by 10 C than when they fluctuated by 20 C. Optimum temperature for germination was 40/20 C, where 93% germination occurred within 4 days.

Considerable differences in rate of root elongation were observed. Species studied in order of decreasing rate root elongation were cocklebur (Xanthium pensylvanicum Wallr.), palmer amaranth (Amaranthus palmeri S. Wats.), barnyardgrass (Echinochloa crusgalli (L.) Beauv.), buffalobur (Solanum rostratum Dunal), kochia (Kochia scoparia (L.) Schrad.), crabgrass (Digitaria sanguinalis (L.) Scop.), sorghum (Sorghum vulgare L., var. RS 610), Russian thistle (Salsola kali L., var. tenuifolia Tausch), and tumblegrass (Schedonnardus paniculatus (Nutt.) Trel.). In 15 days, cocklebur and tumblegrass roots grew 33 and 11 in, respectively. With all species, the rate of root elongation always exceeded the rate of height increase. There was poor correlation between rate of root elongation and weed height, top weight, and root weight.

Selections of wild oats (Avena fatua. L.) had varying degrees of resistance to isopropyl N-phenylcarbamate (IPC). Strains with gray glabrous lemmas and non-dormant seed tended to be more resistant to IPC than those with brown pubescent lemmas and dormant seed.

Rush skeletonweed (Chondrilla juncea L.), a serious agricultural pest in Australia, is increasing in the western United States. Persistence in established areas is encouraged by the resprouting of all portions of the taproot and wind borne achenes. Invasion to new areas is reduced by short achene life, the germination pattern of young seedlings, and the lack of shade tolerance. Fall applied 4-amino-3,5,6-trichloropicolinic acid (picloram) and 2-methoxy-3,6-dichlorobenzoic acid (dicamba) were effective for chemical control of spot infestations.

Old World climbing fern is one of the most invasive plants in natural areas of central and southern Florida. The fern spreads across the landscape by wind-blown spores and invades isolated and undisturbed habitats such as interior portions of the Florida Everglades. Land managers in Florida have reported that multiple herbicide treatments are required to control the fern, which could indicate that herbicides do not translocate throughout the plant in long-established populations. We conducted a greenhouse study to determine the absorption and translocation patterns in Old World climbing fern using the three herbicides most commonly used for management of this plant by land managers in Florida. Using 14C-labeled herbicides, we evaluated absorption and translocation of glyphosate (2.25 kg ai ha−1), metsulfuron (0.10 kg ai ha−1), and triclopyr (1.68 kg ai ha−1) in Old World climbing fern using five different application scenarios (cut-and-spray, basal spray, 25% foliar spray, 50% foliar spray, and 100% foliar spray). Triclopyr was absorbed to the greatest extent (60.3%) of applied radioactive compounds compared to glyphosate (31.2%) and metsulfuron (19.8%). The majority of radioactivity remained in treated leaves for all herbicides with only small percentages of the absorbed radioactivity being detected in other plant parts. All three herbicides translocated acropetally and basipitally to some extent. Radioactivity, for the most part, translocated evenly throughout the plants but the greatest amount of radioactivity derived from triclopyr occurred in rhizomes when the cut-and-spray and basal applications were used. The radioactivity in rhizomes derived from glyphosate was greater in those treated using cut-and-spray. Based on autoradiographs, there was limited horizontal movement of any herbicide in the rhizomes of Old World climbing fern which could explain why resprouts are observed several weeks following treatment.

Dalapon–2–C14, applied as droplets, was employed in tracer studies by autoradiography and counting. That dalapon remained non-metabolized throughout the experiment was confirmed by chromatography. Surfactants greatly enhanced (cuticular) penetration. Small amounts of herbicide were sorbed almost instantaneously (15–30 seconds). Movement away from the treated area “showed” or “appeared as” first a diffusional pattern, but soon became channelized in veinlets and larger vascular bundles. Dalapon applied off the midvein did not enter the midvein in appreciable quantities during basipetal transport. Dalapon which entered the midrib originally remained highly concentrated in this channel. During transport, some dalapon was retained or retarded by tissues through which it passed. Retention was greatest in the basal sections (intercalary meristem regions) of young grass blades. A 10-fold build-up of dalapon was demonstrated following drop application off the main vein, but not after treatment on the midrib. Results are explicable on the basis of the known anatomy of Zea and the translocation of dalapon with assimilates.

Seedbank persistence in canola seeds is related to their potential to develop secondary dormancy. This can result in volunteer weed problems many years after canola production. The potential to be induced into secondary dormancy is controlled by both the canola genetics and the environment of the mother plant. However, the effect of time of harvesting on secondary dormancy potential is not known. The objective of this study was to determine the effect of harvest timing on potential to develop seed dormancy in canola. Six harvest samples were collected weekly from two canola genotypes (5440 and 5020) starting from 10 to 20% seed color change on the main stem until they were fully ripened. Freshly harvested seeds of 5440 and 5020 showed 13 and 16% primary dormancy at 32 and 33 d after flowering (DAF), respectively, but dormancy decreased with harvest timings and no dormancy was observed when seeds were fully mature (78 DAF). After dormancy induction, 10% of 5440 seeds were dormant at 32 DAF, but 94% of seeds were dormant at 78 DAF. Similarly, 70% of 5020 seeds were dormant at 33 DAF, but 90% of seeds were dormant at 68 DAF. Thus, seeds had lower potential to secondary dormancy at early development but have a high potential to secondary dormancy induction at full maturity. This study suggests that windrowing these canola genotypes at the recommended time (60% seed color change on the main stem) may reduce ability of the seed to develop secondary dormancy and thus reduce the persistence of seeds in the soil seedbank.

The effects of three postemergence herbicides that inhibit carotenoid biosynthesis were tested on field dodder. Flurochloridone, sulcotrione, and mesotrione treatments led to bleaching symptoms in field dodder stems. The effect of flurochloridone was rapid; 2 d after treatment (DAT) the stem was bleached and contained only 2% β-carotene, with a massive accumulation of phytoene in comparison with the control. However, flurochloridone treatment did not inhibit stem elongation, and full recovery of pigment composition at newly elongated stems was recorded 6 DAT. The effects of sulcotrione and mesotrione were similar, but the recovery was slower than with flurochloridone. The developing stems were fully bleached 6 DAT, with no detectable β-carotene, and subsequently no recovery was observed. All three herbicides led to mass destruction of the plastids in the parenchyma cells of the cortex and pith tissue. This disruption of the plastids was associated with depletion in starch content. Sulcotrione and mesotrione treatments reduced field dodder biomass accumulation, whereas flurochloridone delayed biomass accumulation by 6 d only. The differences in phytotoxicity between flurochloridone, sulcotrione, and mesotrione might be due to their different translocation patterns. Flurochloridone is xylem-mobile and is therefore localized at the applied area because of the low transpiration rate of field dodder. Sulcotrione and mesotrione are phloem-mobile and tend to accumulate in sinks such as the primordial tissue. The massive destruction of the amyloplasts and the rapid decline in starch content in response to all the herbicide treatments may indicate the possible involvement of carotenoids in stabilizing the amyloplast outer membrane in field dodder.

Accurate and precise estimation of weed seed bank populations is critical to studying weed seed bank demographics. Research was conducted over 3 site-years in Montana to (1) examine the spatial distribution of wild oat seed banks on a small scale (1 m2 plots), (2) compare wild oat seed bank density sample means and precision between two soil samplers, and (3) predict the sample area needed to quantify a range of wild oat seed bank densities at several levels of precision. Seed bank sample means obtained with a large sampler (10- by 10-cm box) were greater than means obtained with a small sampler (4.4- or 3.8-cm-diam cylinder) for 15 of 18 seed banks. There was no clear advantage in precision (SE/mean) when sampling seed banks using a large number of small soil samples rather than using a small number of large soil samples. Furthermore, at very low seed bank densities, using a small number of large samples gave better precision. Precision improved as sample number increased for each seed bank at each site-year. High-density seed banks tended to have better precision than low-density seed banks at any given sample number. Seed banks had an aggregated spatial distribution when sampled with either soil sampler. As the desired precision level decreases (becomes more precise), the predicted sample area required increases greatly. A seed bank containing 6,000 seeds m−2 has a predicted sample area of 0.5, 0.7, 1.3, and 2.9% of the total area to obtain precision levels of 0.5, 0.4, 0.3, and 0.2, respectively.

The noxious weed Benghal dayflower has become a severely troublesome agricultural weed in Georgia in the southeastern Unite States, and there are indications that it is moving northward. Benghal dayflower is glyphosate tolerant and possesses a high degree of reproductive elasticity, making it a formidable threat in many crop systems. The purpose of these experiments was to develop the first temperature response profiles for Benghal dayflower, and use them to evaluate whether temperature might limit its northward invasion into North Carolina and adjacent states on the U.S. east coast. Experiments focused on vegetative and early reproductive growth, stages considered crucial for establishment and competitiveness. Exposure to a range of aerial temperatures revealed that Benghal dayflower growth and production of aerial and subterranean reproductive structures were maximized at 30 C, with sharp declines occurring at cooler temperatures. When exposed to differing root temperatures in hydroponics, with a constant aerial temperature, Benghal dayflower growth did not show the same cool temperature sensitivity, but reproductive performance declined when temperatures decreased below about 29 C. The root temperature responses of several other weed species known to thrive in the climate of this geographic area also were determined. Growth of sicklepod, hemp sesbania, and jimsonweed was more sensitive than Benghal dayflower to cool temperatures, whereas the growth response of velvetleaf was similar. Based on the comparison of the Benghal dayflower temperature responses in controlled environments to (1) seasonal air and soil temperatures in the field, and (2) the temperature responses of agronomic weeds known to thrive in the region, it is concluded that cool temperatures will not restrain the northward spread of Benghal dayflower into North Carolina.

Absorption and translocation of 14C-metriflufen {2-[4-(4-trifluoromethylphenoxy)phenoxy] propanoic acid} in johnsongrass [Sorghum halepense (L.) Pers.] and soybean [Glycine max (L.) Merr. ‘Lee 68’] were evaluated under different environmental conditions. At 40% relative humidity (RH), an increase in air temperature from 18 to 35 C increased translocation of the 14C in johnsongrass more than four-fold following application of 14C-metriflufen to an area on the third leaf. At 100% RH, translocation of the 14C was twice as great at 27 C as at 18 C, but translocation at 35 C was intermediate. In the combined analysis, translocation in johnsongrass was no better at 100% RH than at 40% RH. Regardless of level of RH, most translocation in johnsongrass at 18 and 27 C was toward the distal half of the treated leaf, but at 35 C most translocation was toward the proximal half of the treated leaf. Translocation of 14C in soybeans increased more than four-fold as air temperature was increased from 18 C to 35 C, and more translocation occurred at 100% RH than at 40% RH at 18, 27, and 35 C. More than 80% of the 14C-metriflufen remained in the treated leaf or on the surface of treated soybean leaves 48 h after application. More of the applied radioactivity was recovered from soybean plants than from johnsongrass plants.

Water, especially water used for irrigation, has been adjudged for many years an important agent in the dissemination of weed seeds. Egginton and Robbins (7) found a total of 81 different species of weeds in 156 weed seed catches from three different ditches during 1918 and 1919 and determined that the number of weed seeds passing a given point on a ditch which averaged 12 feet in width during a period of 24 hours may reach several millions. Hope (11) analyzed a considerable number of weed seed catches from irrigation channels during 1924 and 1925. He computed that 1,674,030 seeds of 13 different weed species which occurred on the surface and to a depth of 1.57 inches in one 10-foot ditch passed during a 24-hour period on September 2, 1925. Further computations illustrated that the flow of weed seeds would amount to a deposition of 170,800 seeds per acre of land in one 6-inch irrigation. The latter computations were based on the assumption that the seeds were distributed uniformly over the cross section of the ditch at the point of catch.

The extensive use of foliar-applied protoporphyrinogen oxidase (PPO)-inhibiting herbicides indisputably contributes to the continued selection of waterhemp resistant to PPO-inhibiting herbicides (PPO-R). However, the role of soil-residual applications of PPO-inhibiting herbicides in the selection of PPO-R has been clouded by the efficacy that these herbicides have on PPO-R waterhemp. The aim of the present study was to understand if soil-residual PPO-inhibiting herbicides have the potential to influence the proportion of resistant waterhemp in emerging plants as herbicide concentrations diminish in the soil. Greenhouse and field experiments were conducted in a PRE dose-response experiment testing fomesafen or fomesafen plus s-metolachlor in the presence of mixed seed populations of PPO-R and -susceptible (S) waterhemp. The first 20 (greenhouse) or 10 (field) waterhemp plants that survived the residual herbicide treatment were sampled for genotypic analysis to detect the presence of the allele responsible for PPO-R in waterhemp (ΔG210). Relative to the nontreated control, the highest rate of fomesafen increased the frequency of resistance (FOR) by 70% in the greenhouse experiments and 20% in the field experiments. The addition of s-metolachlor did not reduce the fomesafen-induced increase in the FOR in the surviving plants from field or greenhouse experiments. However, the additional herbicide from an alternate site of action (s-metolachlor) substantially improved soil-residual herbicide efficacy over fomesafen alone, which limited the number of waterhemp plants surviving the herbicide application. Thus, the selection for resistance can be delayed with the addition of a herbicide from an alternative site of action by postponing and/or reducing waterhemp emergence. These data strongly suggest that soil-residual PPO-inhibiting herbicides can influence the FOR in a field population, placing even greater importance on the implementation of best management practices such as full herbicide use rates and using herbicides from multiple sites of action to mitigate the risk of selecting for herbicide-resistant weed biotypes.

Greenhouse studies were conducted to determine the influence of phosphorus (P) concentrations on the growth of lettuce, smooth pigweed, and common purslane in monocultures and in mixtures and to determine the P-absorption rate of each species over time. For the P-competition studies, lettuce–smooth pigweed and lettuce–common purslane mixtures were established in P-less hydroponic solutions. Each lettuce–weed mixture was established separately. Concentrations of P were 10, 20, 40, 80, and 160 mg L−1. Lettuce to weed planting proportions were 2:0, 0:2, and 1:1. In the mixtures, biomass of common purslane increased sharply between 10 and 20 mg P L−1, depressing lettuce growth. No biomass changes were observed in smooth pigweed as P concentration increased. However, both weeds increased their P content within this range, depriving lettuce of this nutrient. Common purslane competed for P for its own growth, whereas smooth pigweed absorbed P luxuriously. For the P-absorption studies, roots of lettuce, smooth pigweed, and common purslane plants were submersed in a 20 mg P L−1 solution for 1, 2.5, 5, 10, 20, 40, 60, 90, 180, 360, 720, and 1,440 min. Common purslane was shown to be the most aggressive species for the nutrient, absorbing 50% of the content in 295 min, whereas lettuce and smooth pigweed needed 766 and 825 min to absorb 10 mg P L−1.

Previous reports have underscored antagonism that may result from mixing glyphosate and glufosinate across a wide range of application rates and for various broadleaf and grass weed species, but no investigation has been conducted to characterize glyphosate absorption and translocation when combined with glufosinate. The objectives of this study were to evaluate herbicide efficacy and assess herbicide interaction and physiological response with combinations of glyphosate and glufosinate on common lambsquarters, velvetleaf, and giant foxtail. Greenhouse studies to determine interaction resulted in strong and persistent antagonism between glyphosate at 110 and 220 g ae ha−1 and glufosinate at 20 or 40 g ae ha−1 in giant foxtail, whereas only transient and reduced antagonism was noted for velvetleaf and common lambsquarters. Combining glyphosate and glufosinate increased the maximum absorption of glyphosate by 9% and 23% in velvetleaf and giant foxtail, respectively, compared with glyphosate alone. In velvetleaf, averaged over time, only 2.6% of the applied radioactivity translocated out of the treated leaf when glufosinate was mixed with glyphosate compared with 9.9% when glyphosate was applied alone. In giant foxtail, 21.6% of the [14C]glyphosate translocated out of the treated leaf when glufosinate was mixed with glyphosate compared with 52.4% when glyphosate was applied alone. Conversely, no change in the radioactive pattern of translocation was noted for common lambsquarters. These results suggest that reduced translocation of glyphosate is the physiological mechanism responsible for the antagonism observed between glyphosate and glufosinate in giant foxtail and, to a lesser extent, in velvetleaf.