A biotype of johnsongrass cross resistant to clethodim, sethoxydim, quizalofop-P, and fluazifop-P was identified in several fields in Washington County, MS. Absorption, translocation, and metabolism studies using ¹⁴C-clethodim and acetyl-coenzyme A carboxylase (ACCase) activity assays were conducted to determine the resistance mechanism. Absorption of ¹⁴C-clethodim was higher in the resistant than the susceptible biotype 4 hours after treatment (HAT), but at 24, 48, and 72 HAT, similar levels of radioactivity were detected in both johnsongrass biotypes. Consequently, resistant plants had more radioactivity present in the treated leaves at 4 and 24 HAT. However, there was no difference between resistant and susceptible biotypes in the translocation of ¹⁴C out of the treated leaf at 4, 8, 24, 48, and 72 HAT as a percentage of total absorbed. Metabolism of clethodim was similar in the resistant and susceptible biotypes. There was no difference in the specific activity of ACCase from the susceptible and resistant johnsongrass biotypes (means of 0.221 and 0.223 nmol mg⁻¹ protein min⁻¹, respectively). ACCase from the susceptible biotype was sensitive to clethodim, with an I₅₀ value of 0.29 μM clethodim. The ACCase enzyme from the resistant biotype was less sensitive, with an I₅₀ value of 1.32 μM clethodim. The resultant R/S ratio for clethodim was 4.5. These results indicate that resistance to clethodim in this johnsongrass biotype resulted from an altered ACCase enzyme that confers resistance to clethodim.

Nomenclature: Clethodim; johnsongrass, Sorghum halepense (L.) Pers., SORHA.

This article discusses the concept of relative potency of herbicides in bioassays where individual dose–response curves can be similar or nonsimilar, often denoted parallel and nonparallel curves, and have different upper and lower limits. The relative potency is constant for similar dose–response curves and measures the relative horizontal displacement of curves of a similar shape along the dose axis on a logarithmic scale. The concept of similar dose–response curves has been used extensively to assess results from herbicide experiments, for example, with the purpose of adjusting herbicide doses to environmental conditions, formulations, and adjuvants. However, deeming dose–response curves similar when they are not may greatly affect the calculation of the relative potency at response levels such as effective dosage (ED)₉₀, which is relevant for effective weed control, or ED₁₀, which is used in crop tolerance studies. We present a method for calculating relative potencies between nonsimilar dose–response curves at any response level. It also is demonstrated that if the upper, lower, or both limits among response curves are substantially different, then the ED₅₀ or any other ED level cannot be used indiscriminately to compute the relative potency. Rather, the relative potency should be viewed as a function of the response level.

Biological and biochemical methods, based on glufosinate inhibitory effects on plant growth and nitrogen metabolism, were examined for their applications to detect this herbicide. Dose–response analysis of radicle growth inhibition showed that, among six vegetables tested, Chinese mustard and edible amaranth were the most sensitive to glufosinate. Field mustard and cruciferous Ching-Geeng were found to be more sensitive to this herbicide than the other four vegetables when three-leaf seedlings were tested in another bioassay. In three-leaf seedlings of Ching-Geeng, accumulation of ammonium, a biochemical marker for glufosinate toxicity because of its inhibition of glutamine synthetase, showed a linear regression to the log-transformed concentrations of glufosinate ranging from 7.5 × 10⁻⁵ to 1.5 × 10⁻³ M. For the detection of glufosinate lower than 7.5 × 10⁻⁶ M, a linear regression was observed between ammonium accumulation and the applied concentration, instead of the log-transformed value, of glufosinate. A similar relationship was observed between the accumulation in Ching-Geeng seedlings of glyoxylate, another biochemical marker, and glufosinate but with a narrower range than that for ammonium accumulation. The applicability of ammonium accumulation in three-leaf seedlings of Ching-Geeng to detect glufosinate residue in water and soil was confirmed by high-performance liquid chromatography (HPLC).

Nomenclature: Glufosinate; Chinese mustard, Brassica rapa L.; field mustard, Brassica campestris L.; cabbage, Brassica oleracea L. var. Capitata; Ching-Geeng, Brassica chinensis L., ‘Ching-Geeng’; edible amaranth, Amaranthus tricolor L.

Reliable estimates of weed fecundity require determination under ranges of management practices such as differing crops and tillage systems. We measured components of reproductive output per plant (numbers of primary tillers, panicles, and seeds; and sizes of panicles) in three emergence cohorts of green foxtail and yellow foxtail growing among corn and soybean in moldboard plow (MP), chisel plow (CP), ridge till (RT), spring disk (SD), and no till (NT). Differences in emergence between crops and foxtail Cohorts 1, 2, and 3 were 5, 0, and −7 d, respectively. In MP, Cohort 1 of green foxtail produced 2.3 primary tillers and 5.6 panicles per plant, and Cohort 1 of yellow foxtail produced 4.6 primary tillers and 9.0 panicles per plant. Panicle size was variable for both species across tillage systems, crops, and cohorts both years. Green foxtail plants produced the most seeds per plant (3,811) in NT corn, and cohorts did not vary greatly, whereas fecundity was highly variable across tillage systems and cohorts in soybean, where it averaged 3,240 (± 388) seeds per plant. Green foxtail seed number per plant were closely related to panicle numbers per plant for each year in corn (r² = 0.90) and soybean (r² = 0.78), and the relationship did not vary among tillage systems. Yellow foxtail seed number per plant was closely related to panicle number per plant, and it was specific for each tillage system in corn (r² = 0.60 to 0.85) and soybean (r² = 0.65 to 0.92). Estimates for vegetative and reproductive growth were more reliable for green foxtail than for yellow foxtail across tillage systems, crops, cohorts, and years.

Nomenclature: Green foxtail, Setaria viridis (L.) Beauv. SETVI; yellow foxtail, Setaria pumila (Poir.) Roem. & Schult. [= Setaria glauca (L.) Beauv.] SETLU; corn, Zea mays L. ‘Pioneer 3893’; soybean Glycine max (L.) Merr. ‘Pioneer 9091’.

Hoary cress is a perennial herbaceous weed that has invaded agricultural and natural areas of western North America. Invasions are often composed of dense patches, and it is unclear whether clonal growth via lateral rhizomes or seedling recruitment is the dominant method of patch expansion. To study the clonal structure of this invasive, six patches from three USA populations (194 ramets) were analyzed with the use of Amplified Fragment Length Polymorphisms (AFLPs). Known siblings and clones were also included to ensure sufficient variation for discrimination between clonal and nonclonal ramets. Patches had low genet/ramet ratios (mean G/N = 0.25) and low diversity levels (mean D = 0.49) compared to similar clonal studies. Single genets represented 55–85% of the ramets sampled in a patch, and the largest genet was 38 m across. Hoary cress exhibits a strong bias toward patch-size increase from clonal reproduction rather than from seedling recruitment. Results indicate that biological control methods that focus on reducing or eliminating seed production would do little to stop expansion of a patch. Despite the domination of a patch by one or a few large genets, other smaller genets are able to persist or are occasionally recruited into dense areas of a patch.

Nomenclature: Hoary cress, Lepidium draba L. [= Cardaria draba (L.) Desv.], CADDR.

The primary objectives of this research were (1) to characterize intraspecific variation in Powell amaranth seed germination and emergence response to nitrogen fertilization, and (2) to evaluate whether germination and emergence characteristics varied between seeds from populations originating on organic vs. conventional vegetable farms. We hypothesized that nonherbicide–based weed management and use of slower-releasing forms of N on organic farms may have selected for seeds with lower dormancy and lower germination sensitivity to N fertilization than seeds from conventional farms. Seeds were collected from five conventional and five organic vegetable farms in central New York State. A second generation of seeds, produced under common greenhouse conditions and stored for at least 3 mo at 5 C was tested for both germination in petri dishes and emergence in the field under multiple rates of ammonium nitrate (NH₄NO₃). Both seed germination and emergence were greater for seeds originating from organic compared with conventional vegetable farms. However, seed responsiveness to fertilization did not vary significantly by habitat of origin. Reduced rates or split applications of NH₄NO₃ significantly reduced emergence in the field in 2003 but had no significant effect on emergence in 2004. Large interpopulation variation in germination and emergence patterns suggests that for Powell amaranth and similar weed species, (1) species-level models of emergence may not be very robust across different farms, and (2) the effectiveness of manipulating emergence through soil fertility practices is likely to vary substantially according to farm and year.

Nomenclature: Powell amaranth = green pigweed, Amaranthus powellii S. Wats. AMAPO.

Integrated management techniques for several U.S. winter wheat production regions have been proposed for jointed goatgrass control. These strategies may be improved by a greater understanding of the genetic and environmental influences on seed production and germination. Plants from six jointed goatgrass collections were grown in common garden nurseries at two Oregon locations over two growing seasons. Unbroken spikes from each collection were used to evaluate seed dormancy and quantify seed production by inflorescence position. Germination tests were conducted over 14 d using spikelets of dormant and after-ripened samples in growth chambers set to 25/15 C day/night temperatures and a 12-h photoperiod. Spikelet position on the spike affected germination of the secondary seed in dormant samples of jointed goatgrass Collections D and G. In contrast, spikelet position did not affect secondary seed germination in dormant samples of Collection B. Spikelet position did not influence secondary seed germination in nondormant samples of all three collections. Spikelet position affected germination of the primary positioned seed in dormant samples of Collection B, and in nondormant samples of Collections B, E, and H. Unbroken spikes from jointed goatgrass Collections B and D were used to quantify seed production per spikelet position on the spike and per floret position within the spikelet. Seed production by floret position depended on spikelet position on the spike. This relationship varied for spikes of different lengths and for samples from the two collections. Efforts to model the life history of jointed goatgrass and predict germination should be adjusted to account for floret position within the spikelet, spikelet position within the spike, and source population. We suggest that future dormancy and germination research include sampling seed from several weed populations and efforts be made to standardize germination tests according to seed position on the inflorescence.

Nomenclature: Jointed goatgrass, Aegilops cylindrica Host AEGCY.

Laboratory and greenhouse studies were conducted to determine the effect of several environmental factors on seed germination and seedling emergence of Crofton weed. Seeds germinated over a range of 10–30 C, with optimum germination at 25 C. High temperature markedly restricted germination, with no germination occurring at 35 C. Crofton weed was moderately photoblastic, with 17% germination occurring in the dark. Crofton weed germinated in a narrow range of pH (5–7). Maximum germination (94%) was observed in distilled water at pH 5.7. Germination was totally inhibited at osmotic stress higher than −0.7 MPa. Germination was greater than 65% at less than 100 mM NaCl, with no germination at 300 mM NaCl. Maximum emergence occurred when seeds were planted on the soil surface. No seedlings emerged when seeds were planted 1.5 cm deep. These results suggest that the future range of Crofton weed in China will be restricted largely to the Yunnan-Guizhou Plateau, which includes major parts of Yunnan and Guizhou provinces, the southwestern part of Sichuan province, and the western part of Guangxi province. Crofton weed also tends to be a sporadic problem in other regions, where the climatic and edaphic conditions are suitable for the seed germination.

Nomenclature: Crofton weed (Eupatorium adenophorum ‘Spreng’).

Alien weed species rank among the most important threats to conservation of biodiversity, making understanding the extent to which protected natural areas are vulnerable to invasion by weeds pivotal in long-term maintenance and conservation of biodiversity. We investigated the potential geographic range of the invasive paleotropical weed, smooth crotalaria, in protected natural areas across Brazil. The ecological niche dimensions of smooth crotalaria in Africa (its putative original distribution) were modeled using a genetic algorithm. Models for the native range and their projections to South America showed good predictive ability when challenged with independent occurrence data. All Brazilian protected natural areas were predicted as highly vulnerable to invasion by this species. However, smooth crotalaria appears more likely to occur in open (savanna-like vegetation, such as cerrado and pantanal) and highly fragmented (Atlantic forest) areas than in extensive closed forests (Amazon). Management suggestions and research priorities are outlined based on these results.

Nomenclature: smooth crotalaria, Crotalaria pallida Ait. CVTMU.

A study at Fairbanks, AK, was started in 1984 to determine soil seed longevity of 17 weed species. Seeds were buried in mesh bags 2 and 15 cm deep and were recovered 0.7, 1.7, 2.7, 3.7, 4.7, 6.7, 9.7, and 19.7 yr later. Viability was determined by germination and tetrazolium tests. Seed viability data were fit to an exponential model, separately for each depth, and the likelihood-ratio test was used to determine whether seed-viability decline was affected by burial depth. Depth of burial had a significant effect on viability decline of prostrate knotweed, marsh yellowcress, bluejoint reedgrass, and wild oat. By 19.7 years after burial (YAB), all seeds of common hempnettle, quackgrass, wild oat, foxtail barley, and bluejoint reedgrass were dead. Seeds of 12 other species were still viable: corn spurry (0.1%), prostrate knotweed (0.3% at 2 cm, 0.8% at 15 cm), flixweed (0.5%), pineapple-weed (0.6%), shepherd's-purse (1.3%), wild buckwheat (1.5%), common chickweed (1.6%), rough cinquefoil (1.8%), common lambsquarters (3.0%), Pennsylvania smartweed (3.3%), marsh yellowcress (8.5% at 2 cm, 0.3% at 15 cm), and American dragonhead (62.2%). Seed dormancy at 19.7 YAB was very low for all species (< 4%) except for American dragonhead, common lambsquarters, Pennsylvania smartweed, and shepherd's-purse, which had seed dormancies of 100, 27, 25, and 38%, respectively. Seed longevity was not increased by cold, subarctic temperatures.

Nomenclature:  American dragonhead, Dracocephalum parviflorum Nutt. DRAPA; bluejoint reedgrass, Calamagrostis canadensis (Michx.) Beauv. CLMCD; common chickweed, Stellaria media (L.) Vill. STEME; Common hempnettle, Galeopsis tetrahit L. GAETE; common lambsquarters, Chenopodium album L. CHEAL; corn spurry, Spergula arvensis L. SPRAR; flixweed, Descurainia sophia (L.) Webb ex Prantl DESSO; foxtail barley, Hordeum jubatum L. HORJU; marsh yellowcress, Rorippa islandica (Oeder) Borbas RORIS; Pennsylvania smartweed, Polygonum pensylvanicum L. POLPY; pineapple-weed, Matricaria matricarioides (Less.) C.L. Porter MATMT; prostrate knotweed, Polygonum aviculare L. POLAV; quackgrass, Elytrigia repens (L.) Nevski AGRRE; rough cinquefoil, Potentilla norvegica L. PTLNO; shepherd's-purse, Capsella bursa-pastoris (L.) Medicus CAPBP; wild buckwheat, Polygonum convolvulus L. POLCO; wild oat, Avena fatua L. AVEFA.

Threehorn bedstraw is an important dicotyledonous weed in southern Australia that is particularly difficult to control in pulse crops. Knowledge of the germination ecology of this weed would facilitate development of effective weed-control programs. Experiments were conducted to study the germination of two populations, Roseworthy Campus (RC) and Yorke Peninsula (YP), of threehorn bedstraw from South Australia. In the absence of chilling, seeds germinated only in the darkness. Germination was considerably higher under an alternating day/night temperature range of 13/7 C compared with 20/12 or 25/15 C day/night temperature. Germination was inhibited by light; however, when seeds were subsequently transferred to complete darkness they germinated readily. Potassium nitrate (0.005 M KNO₃) and gibberellic acid (0.001 M GA₃) stimulated germination in the darkness in both populations. This concentration of KNO₃ increased germination of the RC and YP populations from 26 and 37% to 56 and 68%, respectively; however, higher concentrations of KNO₃ inhibited germination. GA₃ added in combination with KNO₃ further increased germination to 81 and 94%, respectively. Germination was also promoted by cold-stratification treatment (5 C). Complete germination (100%) was achieved within 4 wk of cold stratification, when seeds were incubated in sand. In the field, seedling recruitment of both populations was higher under minimum tillage (25 to 27%) than no-till (15 to 18%) conditions, reflecting greater exposure of seeds to light under no-till systems.

Nomenclature: Threehorn bedstraw, Galium tricornutum Dandy, GALTC.

Garden huckleberry, a member of the Solanaceae family and a close relative of black nightshade, is an exotic plant introduced from Africa. Information on garden huckleberry response to a new environment and to herbicides is useful for determining the potential of this species to become an invasive weed, predicting the potential range of this species in the United States, and developing an optimum garden huckleberry management program. Germination and survival of garden huckleberry seed, as affected by environmental factors, were studied under greenhouse and controlled-environment growth chamber conditions. Garden huckleberry seed became viable between 2 and 3 wk after anthesis and was nondormant when separated from fresh berries. Garden huckleberry seed was not photoblastic and germinated equally well under both a 14-h photoperiod and continuous darkness. Seed germinated at constant temperatures from 17 to 35 C, with optimum germination between 22 and 30 C. Germination of garden huckleberry seed markedly declined as the osmotic potential of the germination medium decreased. The optimum pH for germination of garden huckleberry was between 5 and 9. Paraquat, dicamba, and bromoxynil provided excellent garden huckleberry control (95 to 100%); atrazine and glyphosate were more phytotoxic (90%) than imazamox (80%); and acifluorfen and foramsulfuron gave inadequate control.

Nomenclature: Aciflurofen; atrazine; bromoxynil; dicamba; foramsulfuron; glyphosate; imazamox; paraquat; black nightshade, Solanum nigrum L. SOLNI; garden huckleberry, Solanum melanocerasium All.

Controlled-environment experiments were conducted to predict the dispersal distance of horseweed seed. Seed were released from a fixed height and collected at three distances from the introduction point along a 6-m wind tunnel. Dispersal potential was assessed at wind speeds of 8 and 16 km hr⁻¹ and release heights of 50.8 and 76.2 cm. In separate experiments, settlement velocity was determined to be 0.323 m sec⁻¹ (SD = 0.0687). These data were used to parameterize a mechanistic model and compared to a quantile extrapolation (QE) of wind-tunnel results. The QE method predicted a greater mean dispersal distance than the mechanistic model, with large disparities between maximum dispersal distances. Quantile extrapolation predicted dispersal distances over 100 m, whereas the mechanistic model predicted a maximum distance of approximately 30 m. Air turbulence within the wind tunnel and complex dynamics of seed flight may have contributed to the discrepancy between models. Predicting the mean and numerical distribution of seed dispersal distance is crucial when estimating the spread of wind-dispersed seed and for the design of a field-sampling protocol. Although controlled-environment experiments lack the wind variability present in natural systems, predictions from wind-tunnel studies provide a better first approximation of dispersal distance than the mechanistic model. Field experiments designed on the basis of these outcomes are more likely to capture the true dispersal distribution. This should provide more accurate data to inform management decisions for wind-dispersed species.

Nomenclature: Horseweed, Conyza canadensis L. Cronq., ERICA.

Studies were conducted to determine the effect of in-row eastern black nightshade establishment and removal timings in plasticulture tomato on tomato yield loss and nightshade berry production and seed viability. Eastern black nightshade was transplanted at 1, 2, 3, 4, 5, 6, and 12 wk after tomato planting (WAP) and remained until tomato harvest, or was established at tomato planting and removed at 2, 3, 4, 5, 6, 8, and 12 WAP to determine the critical weed-free periods. Eastern black nightshade seed viability increased with berry size and with length of establishment or removal time. The critical weed-free period to avoid viable nightshade seed production was 3–6 WAP. Tomato yield decreased with early weed establishment or with delayed time of weed removal. The critical weed-free period to avoid greater than 20% tomato yield loss for the sum weight of extra large and jumbo grades was 28 to 50 d after tomato transplanting.

Nomenclature: Eastern black nightshade, Solanum ptycanthum Dun. SOLPT; tomato, Lycopersicon esculentum.

Prickly lettuce is an annual weed that germinates in both the fall and the spring. It is often found in no-till soybeans and winter wheat in Ontario, Canada, as well as along the edges of fields. Field studies were conducted from 2001 to 2004 to estimate crop yield losses, and to characterize the phenology and seed production of prickly lettuce in relation to time of emergence. Prickly lettuce had a large impact on soybean yield, with losses of 60 to 80% at densities of 50 plants m⁻² or more. Prickly lettuce density estimated to cause a 10% soybean yield loss varied from 0.2 plants m⁻² in 2002 to 1.2 plants m⁻² in 2003 and 2004. In winter wheat, prickly lettuce at densities up to 200 plants m⁻² caused no detectable yield loss in this study. Plants that emerged in the fall generally were larger, flowered earlier. and produced more seeds than those emerging in spring, but size and fecundity were strongly density-dependent. The number of flowers produced per plant could be estimated from the height of the main stem. Seed production per plant ranged from 2,200 to 67,000 in soybeans, and up to 87,000 in a noncrop area at the edge of the field. Winter wheat harvest interrupted prickly lettuce flowering, and only about 25 to 30% of the plants present in the wheat crop survived harvest and flowered in untreated stubble. These plants produced less than 4,000 seeds per plant. Postharvest control with glyphosate, mowing, or cultivation prevented prickly lettuce seed production in wheat stubble. This study suggests that prickly lettuce populations could build up quickly in continuous no-till soybeans, but rotation with winter wheat and control of plants at the edge of the field would help to limit population growth.

Nomenclature: Prickly lettuce, Lactuca serriola L. LACSE; soybean, Glycine max (L.) Merr.; wheat, Triticum aestivum L.

Field experiments were conducted to determine density-dependent effects of eastern black nightshade season-long interference on tomato-yield loss when growing in-row with staked plasticulture tomato. Eastern black nightshade was transplanted at densities of zero, one, two, three, four, or five plants per crop plant hole in the plastic. Eastern black nightshade densities of one to five reduced the number and weight of larger fruit grades (threes, extra larges, jumbos, marketables, totals) similarly but did not reduce yields of smaller fruit grades (culls, mediums, and larges) from the weed-free. Eastern black nightshade reduced percent yield loss of jumbo grade, the premium grade, which could be predicted by a rectangular hyperbola model. The value ($ ha⁻¹) of jumbo fruit and the value of the sum of large, extra large, and jumbo grade was reduced at densities of eastern black nightshade as low as one plant per hole.

Nomenclature: Eastern black nightshade, Solanum ptycanthum (Dun.) SOLPT; tomato, Lycopersicon esculentum L.

To develop more effective pest-management strategies, it is essential to understand how different pests interact with each other and the crop. Field studies were conducted in 2003 and 2004 at two Nebraska locations to determine the effects of early-season crop defoliation on the critical time for weed removal (CTWR) in narrow-row soybean. Three soybean defoliation levels were selected to simulate 0, 30, and 60% leaf tissue removal by the bean leaf beetle. Weeds were allowed to compete with the crop until V2, V4, V6, R3, and R5 growth stages. There were also season-long weedy and weed-free treatments. Results indicated that the CTWR in soybean occurred earlier as defoliation levels increased from 0 to 60%. The CTWR occurred at V3, V2, and V1 growth stage for 0, 30, and 60% defoliation levels, respectively. Overall, 60% defoliation resulted in earlier CTWR by at least 14 d. Yield losses from defoliation and weed interference were primarily associated with a reduction in number of pods per plant⁻¹.

Nomenclature: Soybean, Glycine max L. Merr.

The influence of management practices at a system level is rarely studied in weed science, even though weed communities respond to the cumulative effect of farm management systems. On-farm visits and detailed grower surveys were used to objectively classify 59 Indiana tomato fields into management systems. Fields were chosen to represent a range of practices used to grow conventional and organic tomatoes. Multivariate statistical analyses identified five distinct management systems based primarily on differences in hours spent hand-weeding, use of plastic mulch, irrigation, row spacing, and whether tomatoes were staked. Farmers generally reported many more hours of hand-weeding for organically managed fields than for fields in the other groups. This finding may reflect a trade-off between the use of herbicides and the need for hand-weeding. However, some organically managed fields were grouped with conventional fresh market fields, suggesting that management practices besides herbicide inputs can be used to reduce hand-weeding. Although some fresh market fields used to produce organic or conventional tomatoes had similar management systems, there was little overlap between fields in fresh market or processing tomato production. Further research is needed to determine underlying relationships among management systems and weed control in Indiana tomato production.

Nomenclature: Tomato, Solanum lycopersicum L.

Laboratory and greenhouse experiments were conducted to determine the allelopathic potential of centipedegrass. Germination and growth of indicator species were evaluated in soil leachates, leaf debris, and aqueous leaf extracts of centipedegrass. Centipedegrass soil leachates did not inhibit annual bluegrass, goosegrass, henbit, large crabgrass, or perennial ryegrass germination compared with the nonfertilized control. Incorporated centipedegrass leaf debris did not reduce lettuce germination, shoot weight, or root weight compared with the control. However, shoot and root dry weights of radish were reduced with increasing rates of centipedegrass leaf debris. Six and 9 mg debris g⁻¹ soil reduced radish shoot weight by 49 and 64%, respectively, compared with the control. Aqueous leaf extracts of centipedegrass reduced lettuce germination; however, radicle and hypocotyl length were similar to the control. These data do not conclusively suggest centipedegrass has widespread allelopathic activity; however, significant reductions in shoot and root weight of radish with increasing centipedegrass leaf debris demonstrate a pattern of inhibition of one species against another, which fulfills a requirement of allelopathic interactions.

Nomenclature: Annual bluegrass, Poa annua L. POAAN; goosegrass, Eleusine indica (L.) Gaertn. ELEIN; henbit, Lamium amplexicaule L. LAMAM; large crabgrass, Digitaria sanguinalis (L.) Scop. DIGSA; centipedegrass, Eremochloa ophiuroides (Munro) Hack.; lettuce, Lactuca sativa L.; perennial ryegrass, Lolium perenne L.; radish, Raphanus sativus L.

Common lambsquarters is an important annual weed of many crops world-wide. Ascochyta caulina is a plant pathogenic fungus that, under natural conditions, causes necrotic spots on the leaves and stems of Chenopodium species. The objective of this study was to evaluate the effect of weed growth stage, relative humidity, dew period, and temperature on the infection of A. caulina isolates against common lambsquarters. In greenhouse experiments, replicated groups of common lambsquarters plants were sprayed with different isolates of A. caulina 2, 3, 4, 5, and 6 wk after emergence. Both disease severity and pathogen-induced dry weight reduction decreased with plant age. The efficacy of all isolates tested was reduced by high leaf-to-air vapor-pressure deficit. Disease severity was more responsive to relative humidity than temperature. However, a minimum dew period of 6 h was required to cause significant disease severity in common lambsquarters. Among all tested A. caulina isolates, W90–1 gave the highest disease scores under all conditions, with the exception of temperatures ≤15 C.

Nomenclature: Ascochyta caulina (P. Karst) v.d. Aa & v Kest.; common lambsquarters, Chenopodium album L. CHEAL.

The fungal plant pathogen Microsphaeropsis amaranthi is virulent against a number of key weeds in the Amaranthaceae, including common waterhemp, and is under investigation as a bioherbicide. Common waterhemp has become a key weed in midwestern crop production systems and is a good target for a bioherbicide that could be integrated into weed management systems. We investigated the direct effects of a range of chemical herbicides and adjuvants upon conidia of M. amaranthi and found that many herbicides and most adjuvants were strongly inhibitory to germination. On the other hand, M. amaranthi was compatible with a selection of postemergence herbicides commonly used in midwestern weed management systems, including carfentrazone, chloransulam, and imazethapyr. Most glyphosate products suppressed or abolished germination of M. amaranthi conidia, but by testing adjuvants commonly used in glyphosate products and technical-grade glyphosate salts, it was revealed that this inhibition was due to formulation additives and not the active ingredient. When glyphosate and conidia of M. amaranthi were sprayed onto common waterhemp seedlings, the herbicide predisposed plants to infection by M. amaranthi. When M. amaranthi was applied 1 to 3 d after glyphosate, the glyphosate rate required to control common waterhemp was reduced by half. Similar results were observed on clones propagated from a common waterhemp plant resistant to glyphosate. When M. amaranthi was applied to seedlings 2 d before glyphosate, the efficacy of the herbicide was reduced. These findings demonstrate that positive interactions between herbicides and M. amaranthi exist but reveal practical difficulties that may limit the integration of the strategy in the field.

Nomenclature: Microsphaeropsis amaranthi (Ell. & Barth.); common waterhemp, Amaranthus rudis Sauer, AMATA.

Most well-drained Mississippi Delta soils have been used for cotton production, but corn has recently become a desirable alternative crop, and subsequently, atrazine use has increased. Between 2000 and 2001, 21 surface soils (0 to 5 cm depth) with known management histories were collected from various sites in Leflore, Sunflower, and Washington counties of Mississippi. Atrazine degradation was assessed in 30-d laboratory studies using ¹⁴C-ring–labeled herbicide. Mineralization was extensive in all soils with a history of one to three atrazine applications with cumulative mineralization over 30 d ranging from 45 to 72%. In contrast, cumulative mineralization of atrazine from three soils with no atrazine history was only 5 to 10%. However, one soil with no history of atrazine application mineralized 54 and 29% of the atrazine in soils collected in 2000 and 2001, respectively. Methanol extracted 15 to 23% of the ¹⁴C-atrazine 7 d after treatment in soils having two applications within the past 6 yr, whereas 65 to 70% was extracted from no-history soils. First-order kinetic models indicated soil with 2 yr of atrazine exposure exhibited a half-life of less than 6 d. Most probable number (MPN) estimates of atrazine-ring mineralizing-microorganisms ranged from 450 to 7,200 propagules g⁻¹ in atrazine-exposed soils, and none were detected in soils with no history of atrazine use. Although most soils exhibited rapid atrazine mineralization, analysis of DNA isolated from these soils by direct or nested polymerase chain reaction (PCR) failed to amplify DNA sequences with primers for the atzA atrazine chlorohydrolase gene. These results indicate that microbial populations capable of accelerated atrazine degradation have developed in Mississippi Delta soils. This may reduce the weed control efficacy of atrazine but also reduce the potential for off-site movement. Studies are continuing to identify the genetic basis of atrazine degradation in these soils.

Nomenclature: Atrazine; cyanazine; DEA, de-ethyl atrazine; DIA, de-isopropyl atrazine; corn, Zea mays L.; cotton, Gossypium hirsutum L.

This article briefly reviews how long weed seeds can live in the soil and what happens to them during burial. Freshly matured seeds of many weeds are water-permeable, but those of others are water-impermeable. Water-permeable seeds may have morphological, physiological, or morphophysiological dormancy, with physiological dormancy (PD) being the one most commonly found in buried weed seeds. Further, nondeep PD is very common in buried weed seeds, and many of them exhibit annual dormancy cycles in response to seasonal temperature changes. The time of year when seeds are nondormant varies with the species, i.e., autumn, spring, or spring to summer. A light requirement for germination plays an important role in preventing nondormant seeds from germinating in the soil. To germinate, soil disturbance that exposes seeds to light must occur at a time of year when seeds are nondormant. Buried seeds of some species come out of dormancy and remain nondormant regardless of seasonal changes in environmental conditions; however, a light requirement for germination prevents them from germinating in the soil. Water-impermeable seeds have either physical dormancy (PY) or a combination of PY and PD, with PY being the most common, e.g., in members of the Fabaceae and Malvaceae. Seeds with PY have a water gap in the seed coat that opens in response to an environmental signal, thereby allowing water to enter. When disturbance brings seeds to the soil surface, temperatures that are higher than those in the soil can cause the water gap to open. Consequently, the water gap indirectly serves as a depth sensor. A challenge for the future is to use information about buried weed seeds to better manage weeds in crops, and modeling is an important step in that direction.

Weed seeds initiate most weed invasions of arable fields, yet there is relatively little information on the value of managing weed seed banks. Matrix population models were used to examine the relative importance of managing weed seed banks, in relation to other life stages, for four model weed species with varying life histories. Simulations for giant foxtail and common lambsquarters, summer annual weeds of arable fields; garlic mustard, an obligate biennial invasive weed of temperate forests; and Canada thistle, a perennial weed of pastures and arable fields, were run under conditions of varying population density and efficacy of seedling control. The models were subjected to elasticity analysis to determine what happened to weed populations when different life stages were targeted. Losses from the dormant seed bank were most important for summer annual weeds, of intermediate importance for biennial weeds, and of low importance for perennial weeds. More effort is needed to develop weed seed-bank management techniques for summer annual weed species as part of integrated weed management systems.

Nomenclature: Common lambsquarters, Chenopodium album L. CHEAL; giant foxtail, Setaria faberi Herrm. SETFA; garlic mustard, Alliaria petiolata (M. Bieb.) Cavara and Grande ALPET; Canada thistle, Cirsium arvense (L.) Scop. CIRAR.

To better understand seed predation and enhance weed seed losses in arable fields, we developed a conceptual model that integrates seed dispersal, seed burial, and seed demand, the three processes that determine the dynamics of summer annual weed seeds on the soil surface in late summer and autumn. Published and unpublished experimental data were used to parameterize a simulation model for a number of crop–weed combinations. Sensitivity analyses of models for giant foxtail in corn and soybean indicated that factors related to seed availability were more important in determining overall seed losses due to predation than those related to seed demand. Delaying harvest date and destroying unshed weed seeds collected at harvest emerged as promising strategies to reduce seed input into the seed bank. The role of plant debris in hiding weed seeds from predators was ambiguous and requires further investigation. Estimates of overall seed losses due to predation based on model simulations in various crops and cropping systems indicated that weed seed predation could serve as an important tool in ecological weed management.

Nomenclature: Giant foxtail, Setaria faberi Herrm. SETFA; corn, Zea mays L.; soybean, Glycine max (L.) Merr.

Reduction of seed-bank persistence is an important goal for weed management systems. Recent interest in more biological-based weed management strategies has led to closer examination of the role of soil microorganisms. Incidences of seed decay with certain weed species occur in the laboratory; however, their persistence in soil indicates the presence of yet-unknown factors in natural systems that regulate biological mechanisms of seed antagonism by soil microorganisms. A fundamental understanding of interactions between seeds and microorganisms will have important implications for future weed management systems targeting seed banks. Laboratory studies demonstrate susceptibility to seed decay among weed species, ranging from high (velvetleaf) to very low (giant ragweed). Microscopic examinations revealed dense microbial assemblages formed whenever seeds were exposed to soil microorganisms, regardless of whether the outcome was decay. Microbial communities associated with seeds of four weed species (woolly cupgrass, jimsonweed, Pennsylvania smartweed, and velvetleaf) were distinct from one another. The influence of seeds on microbial growth is hypothesized to be due to nutritional and surface-attachment opportunities. Data from velvetleaf seeds suggests that diverse assemblages of bacteria can mediate decay, whereas fungal associations may be more limited and specific to weed species. Though microbial decay of seeds presents clear opportunities for weed biocontrol, limited success is met when introducing exogenous microorganisms to natural systems. Alternatively, a conservation approach that promotes the function of indigenous natural enemies through habitat or cultural management may be more promising. A comprehensive ecological understanding of the system is needed to identify methods that enhance the activities of microorganisms. Herein, we provide a synthesis of the relevant literature available on seed microbiology; we describe some of the major challenges and opportunities encountered when studying the in situ relationships between seeds and microorganisms, and present examples from studies by the ARS Invasive Weed Management Unit.

Nomenclature: Giant ragweed, Ambrosia trifida L.; jimsonsweed, Datura stramonium L.; Pennsylvania smartweed, Polygonum pensylvanicum L.; velvetleaf, Abutilon theophrasti Medic.; woolly cupgrass, Eriochloa gracilis (Fourn) A. S. Hitchc.

Because herbicide and cultivation efficacy is generally density independent, seedling density following these weed control practices will be proportional to the density of germinable seeds in the seedbank. Most farmers would therefore benefit from management practices that reduce seed inputs, increase seed losses, and reduce the probability that remaining seeds establish. Germination, predation, and decay are the primary sources of loss to the seedbank that may respond to management. Farmers have long prepared stale seedbeds in which shallow soil disturbance encourages germination losses. Postdispersal seed predation by vertebrate and invertebrate granivores may cause high rates of seed mortality in a wide range of cropping systems, but seed dispersal asynchronous with predator activity, and seed burial, may limit the overall effect on the seedbank. Although seeds would seem to be an ideal carbon source for soil microorganisms, limited evidence from a study of wild oat suggests that decay may be less responsive to management than germination, and likely predation. A final management objective, supporting a program that aims to reduce seedbank inputs and increase losses, is to reduce the size of the effective seedbank through manipulation of residues and disturbance to reduce the probability of establishment. Incorporation of green manures generally reduces weed establishment, whereas larger-seeded or transplanted crops may better tolerate the residue-mediated changes in the chemical, biological, and physical properties of the soil surface environment. Evidence from no-till systems further support the hypothesis that changes in soil surface conditions may regulate the abundance of “safe sites” for weed establishment, thereby modulating the size of the effective seedbank.

Nomenclature: Wild oat, Avena fatua L. AVEFA.

Land managers typically use herbicides, biological controls, fire, grazing, and revegetation to manage and restore rangeland dominated by invasive plants. Without careful planning and implementation, these tools may temporarily control the weeds but may ultimately have minimal influence on ecological processes, fail over the long term, and lead to weed reinvasion. This can result from the lack of a broad ecological perspective. Successional management provides a process-based framework for weed ecologists to develop and test integrated weed management strategies and for land managers to organize implementation of these strategies in a way that adequately addresses ecological processes. This framework offers land managers practical methods for modifying ecological processes to direct plant community composition away from invasive species and toward desired plant assemblages. To date, successional management has not gained widespread application because, in part, it has not been conceptually linked to other successional models. Therefore, we illustrate how other successional models can be incorporated within the framework. Incorporating other prevailing successional models will further elucidate ecological processes, offer additional management strategies, and widen the possibilities for ecologically based management of rangeland weeds. Approaching management of weed-infested rangeland through this process-based framework will enable managers to implement strategies that maximize the likelihood of success because these methods will be integrated based on ecological principles. Successional management should be adjusted as we gain a better understanding of the factors that drive succession.