Twenty-nine s-triazines were evaluated as subsurface and surface preemergence sprays for the control of crabgrass in corn, grain sorghum, soybean, and cotton and as directed postemergence sprays in cotton. In preemergence experiments corn was tolerant to most chloro-alkylamino-s-triazines but was severely injured by most methoxy- and methylmercapto-alkylamino-s-triazines. 2-Chloro-4,6-bis(isopropylamino)-s-triazine [propazine], 2,4-bis(ethylamino)-6-trichloromethyl-5-triazine, and 2-chloro-4-isopropylamino-6-methylamino-s-triazine were about equally effective for selective preemeregnce weed control in grain sorghum. 2,4-Bis(ethylamino)-6-trichloromethyl-s-triazine and perhaps 2-methoxy-4,6-bis [(3-methoxy)propylamino]-s-triazine were sufficiently active and selective to suggest additional evaluation as preemergence herbicides for soybeans. Cotton exhibited tolerance to certain methylmercapto derivatives; 2,4-bis(isopropylamino)-6-methylmercapto-s-triazine [prometryne] as a preemergence and as a directed postemergence spray appeared especially promising for control of weeds in cotton. Generally methoxy- and methyl-mercapto-alkylamino-s-triazines as preemergence and postemergence sprays were more effective for controlling crabgrass than corresponding chloro-alkylamino-s-triazines. The initial and residual toxicities of 14 of the more promising s-triazines were compared in four soils of the southern United States. Initial effective levels of the methoxy and methylmercapto derivatives varied more among soils than those of corresponding chloro derivatives. In most cases methoxy s-triazines were more toxic to the fifth oat crop than corresponding chloro s-triazines whereas chloro s-triazines exhibited the greatest initial toxicities. Certain methylmercapto derivatives appeared fairly persistent also.

First of all I would like to bring greetings from the Canada Weed Committee (CWC). Under one name or another this Committee has met every year since 1929 when it was organized by a handful of workers to coordinate their tests with sodium chlorate, sodium arsenite, sulphuric acid, etc. The CWC still meets as a working planning committee to review current work, up-date recommendations and coordinate testing for the forthcoming year.

A greenhouse experiment used a replacement series design to compare the vegetative growth 6 wk after emergence in pure cultures and mixtures of winter wheat and Italian ryegrass, with phosphorus (P) levels recommended by soil testing. The planting proportions of wheat and Italian ryegrass were 100 and 0%, 75 and 25%, 50 and 50%, 25 and 75%, and 0 and 100%, respectively. There was no alleopathic interaction between the species. Both species in all pure and mixed cultures had substantially less growth in the low-P than in the recommended P treatment. However, the relative performance of the two species differed between P treatments. In the recommended P treatment in pure culture, Italian ryegrass had more tillers and greater root weight and length than wheat. Pure culture wheat in the low-P treatment exceeded pure culture Italian ryegrass in leaf area, weights of leaves, stems, and roots, and root length. Thus, the growth of wheat was inhibited less by P deficiency than the growth of Italian ryegrass in pure culture. In the 50:50 mixture of the recommended P treatment, wheat had greater leaf, stem, and root weights than Italian ryegrass. In the 50:50 mixture of the low-P treatment, the two species were very similar in growth, except that Italian ryegrass had about three times more tillers than did wheat. Whereas P deficiency limited the growth of wheat less than Italian ryegrass in pure culture, P deficiency did not affect the relative competitiveness of Italian ryegrass as much as wheat in mixed cultures. The ability of Italian ryegrass to compete with wheat when P was limiting may result from a difference in root growth. Italian ryegrass had a greater fresh root length to fresh root weight ratio than did wheat in the low-P treatment in pure culture and in the 50:50 mixture. The greater surface area of Italian ryegrass roots likely enhanced the competitiveness of Italian ryegrass relative to wheat under P-deficit conditions. Thus, the use of the recommended P nutrition from soil testing may be a key component to diminish Italian ryegrass competition in wheat fields.

Acetolactate synthase (ALS) inhibitors are widely used for POST control of sedges in turfgrass. A suspected resistant (R) biotype of annual sedge was collected from a bermudagrass turf in Georgia with a history of exclusive use of halosulfuron. Research was conducted to evaluate the resistance level of this biotype to halosulfuron, efficacy of ALS-inhibiting herbicides and other mechanisms of action for control, and the molecular and physiological basis for resistance. In greenhouse experiments, the halosulfuron rate required to reduce shoot biomass 50% in comparison with the nontreated at 8 wk after treatment (WAT) were 8 and > 1,120 g ai ha−1 for the S (susceptible) and R biotypes, respectively. Imazapic, sulfosulfuron, and trifloxysulfuron reduced biomass of the S biotype greater than 60% at 8 WAT, but biomass was reduced less than 20% for the R biotype. Glufosinate, glyphosate, MSMA, and sulfentrazone reduced shoot biomass of the R biotype by 93, 86, 97, and 45%, respectively. In laboratory experiments, the halosulfuron concentration required to inhibit ALS activity by 50% in excised leaf tissues was 5.8 and > 1,000 μM for the S and R biotypes, respectively. Gene sequencing of the R biotype revealed a Pro-197-Ser substitution that confers resistance to ALS inhibitors. This is the first report of ALS-inhibitor resistance in annual sedge and herbicide resistance in a sedge species from a turfgrass system.

Invasive watermilfoils, specifically Eurasian watermilfoil and the interspecific hybrid of Eurasian watermilfoil × northern watermilfoil, continue to be problematic for water resource managers. Herbicides are often used to control these nuisance weeds and have been historically successful in controlling Eurasian watermilfoil. A population of hybrid watermilfoil from Townline Lake in Michigan has shown increased tolerance to the herbicide fluridone. The objective of this work is to determine if cross- and multiple tolerance have also developed in this population. Eurasian watermilfoil plants collected from multiple sites and plants from Townline Lake were treated with 0, 5, 10, 20, 40, or 80 µg L−1 of fluridone, norflurazon, or topramezone. Fluridone and norflurazon inhibit phytoene desaturase, whereas topramezone is a 4-hydroxyphenylpyruvate dioxygenase-inhibiting herbicide. Chlorophyll fluorescence (Fv/Fm) and pigment content was measured at 10 d after treatment. Townline Lake plants responded differently from susceptible plants when treated with fluridone, norflurazon, and topramezone at 40 µg L−1. These results indicate that the Townline population of hybrid watermilfoil has inherent tolerance to multiple herbicide modes of action. These results are especially significant as topramezone has recently been labeled for aquatic use. Screening of additional herbicides to determine potential herbicide tolerance of the Townline Lake population is recommended.

In recent years, interest has increased in late-winter sowing of corn in northern Italy because of many agronomic advantages. However, cold and rainy weather slows initial crop growth, which can favor weed infestation. There is, therefore, a need for appropriate timing of weed control tactics based on an understanding of the competitive relationship and dynamics between crop and weeds. Five experiments were conducted over 4 yr, with a series of treatments increasing either duration of interference or length of weed-free period. Yield data were fitted with sigmoidal equations to find the critical point (CP) and critical period of weed control (CPWC). Although the CP is determined only by the competition between weed and crop, the CPWC is also market dependent. To quantify the effect of weed flora on the CP, a multiple regression model was tested, taking into account weed density, inflection point, and slope parameter of the Gompertz model of the cumulated infestation. The results confirmed that the late-winter sowing date increases the importance of late winter– and early spring–emerging weeds. In general, the precompetitive period was longer in the late winter–sown corn than in traditional midspring-sown corn. The delayed start of the CPWC makes control more difficult with a preemergence herbicide, which raises questions on the utility of this agronomic technique. Multiple regression analysis showed that the position of CP can be estimated with the density, earliness, and competitiveness of the infestation. Furthermore, the slope/inflection point ratio of the Gompertz model appears to be independent of sowing date. Results suggest that the weed–crop competition mechanism can be represented with simply the weed flora dynamic parameters and that a combination of crop–weed competition studies and emergence prediction models can predict the position of CP and give useful information about the CPWC and weed management.

Phomopsis convolvulus Ormeno, a fungus, reduced growth and regeneration of field bindweed under greenhouse environments. Field bindweed seedlings at the cotyledon stage were severely injured and killed (95% mortality) with 108 conidia/m2. Three- to five-leaf seedlings (2 weeks old) were controlled when inoculated with 109 conidia/m2 (70% mortality; 98 and 89% reduction in dry weight of aboveground biomass and roots, respectively). This inoculum density reduced aboveground and root biomass, and adversely affected regeneration of 4-week-old seedlings and established plants, but few plants were killed. In controlled-environment studies, two inoculations were superior (P = 0.02) to one inoculation in reducing foliage aboveground of well-established seedlings (4 weeks old). However, new shoots produced between the first and second inoculation treatments were less diseased than expected.

Field studies were conducted in 1998 and 1999 to evaluate crop response, weed control, Glycine max yield, and economic returns with sulfentrazone alone and tank-mixed with glyphosate, cloransulam, or chlorimuron at two preplant application timings in no-till, narrow-row, glyphosate-resistant G. max. No significant crop injury was observed. Setaria faberi and Polygonum pensylvanicum control 5 wk after planting (WAP) was generally greater with sulfentrazone applied early preplant (EPP) than with sulfentrazone applied at planting (AP). When applied AP, glyphosate plus sulfentrazone provided greater S. faberi control than sulfentrazone alone. Control of Amaranthus rudis, Ambrosia artemisiifolia, and Ipomoea hederacea was greater in 1998 than in 1999 because of more timely early-season precipitation. Sulfentrazone-based programs provided 80 to 100% control of A. rudis in 1998, but control in 1999 ranged from 72 to 95% at Columbia and 46 to 83% at Novelty. Cloransulam alone, at either application timing, was the only treatment that provided greater than 80% control of A. artemisiifolia at each site in each year. All sulfentrazone-based treatments provided greater than 80% control of I. hederacea in 1998, but control was less in 1999 and ranged from 54 to 91%. Xanthium strumarium control ranged from 5 to 94% with sulfentrazone alone; however, the addition of cloransulam or chlorimuron provided 75 to 99% control regardless of application timing. A blanket application of glyphosate was made 6 WAP over all preplant herbicide treatments, and weed control 5 wk after this treatment was greater than 79% with all sulfentrazone-based treatments. Sulfentrazone plus cloransulam or chlorimuron plus glyphosate EPP or AP followed by (fb) glyphosate postemergence (POST) generally provided the greatest weed control. Overall weed control was generally greater with the use of residual herbicides vs. glyphosate alone, although yield and net returns were not always greater. A greenhouse study was conducted to determine if altering the preplant application timing reduced sulfentrazone injury to G. max. Treatment variables included herbicide rate, temperature during a preplant incubation period, and application timing. Glycine max, Zea mays, and Sorghum bicolor were used as indicator species. Sulfentrazone caused less injury to G. max, Z. mays, and S. bicolor in soils incubated at 30 C when applied 20 d before planting compared to 0 d before planting. Equivalent amounts of crop injury were noted with sulfentrazone applied 20 or 0 d before planting in soils incubated at 5 C with all indicator species.

As a result of applying arsenicals as herbicides, desiccants, or soil sterilants, soil As levels can increase under some conditions. Three arsenicals, sodium arsenite, (NaAsO2), cacodylic acid, (hydroxydimethylarsine oxide), and MSMA (monosodium methanearsonate) were applied to bare soil at 1, 2, and 10 times the recommended maximum application rates. After incorporation, radishes (Raphanus sativus L.) and soybeans [Glycine max (L.) Merr.] were planted as indicator crops to assess the phytotoxicity of accumulated soil As residues. Soil As residues increased at all levels of application. However, only 50% of the As applied at 10 times the recommended rate was found after seven annual applications, regardless of the source. Total losses of As from the soil averaged 14 to 15% of that applied each year. At 10 times the recommended rate, As generally reduced radish and soybean yields, but seldom reduced yields at the two lower rates. Results indicated little danger of As residue accumulating to phytotoxic levels under normal application rates.

Tropical soda apple is an exotic perennial pasture weed currently reported in the U.S. only from Florida, Georgia, Alabama, Mississippi, and South Carolina. In experiments to determine its environmental requirements and potential range, tropical soda apple was grown in growth chambers in 16 day/night temperature regimes ranging from 18/8 to 36/26 C. After 100 d of growth, maximum height, leaf area, and shoot biomass occurred at day/night temperatures ranging from 24/26 to 36/26 C. The plants achieved 50% or more of maximum leaf area, biomass, and height at 18/26, 24/20, 24/26, 30/26, and 36/26 C. Plants survived in 8 C nights with day temperatures of 18 to 36 C, but biomass and leaf area were only 3 to 10% of maximum. Leaf appearance rate was linearly related to mean temperature over the range of 11.3 to 29.3 C, with a slope coefficient of 0.018 leaves/day/degree C. Flowering was delayed by photoperiods shorter than 10 h or temperatures lower than 24/20 C. In favorable temperatures, plants flowered within 60 d after emergence in photoperiods ranging from 8 to 16 h. Artificially pollinated flowers produced fruit with germinable seeds within 6 to 7 wk, at temperatures ranging from 23/17 to 32/26 C. A regression equation relating vegetative growth to day and night temperatures indicated that tropical soda apple could achieve 30% or more of its maximum growth rate during 7 mo of the year in southern Alabama, Louisiana, and Texas and during 4 to 5 mo of the year at sites in Georgia, South Carolina, North Carolina, Virginia, Tennessee, Kentucky, Illinois, Arkansas, Missouri, Kansas, and Oklahoma. Neither temperature nor photoperiod will limit its further spread in the southern U.S. and adjacent regions.

Populations of water starwort, a winter annual or biennial weed in the pink family (Caryophyllaceae), can no longer be controlled by tribenuron following successive use of this herbicide over several years. Whole-plant bioassays have established that the resistant water starwort populations JS17, JS08, JS16, and JS07 showed high-level (from 203-fold to 565-fold) resistance to tribenuron. In vitro acetolactate synthase (ALS) assays revealed that resistance was due to reduced sensitivity of the ALS enzyme to tribenuron. The half-maximal inhibitory concentration (I50) values for JS17, JS08, JS16, and JS07 were 72, 71, 70, and 76 times greater, respectively, than were those of the susceptible population JS24. This altered ALS sensitivity in the resistant populations was due to a mutation in the ALS gene resulting in a Pro197 to Ser substitution (JS17, JS08, and JS16) and a Pro197 to Leu substitution (JS07). This study established the first documented case, to our knowledge, of evolved tribenuron resistance in water starwort and concluded that the molecular basis of resistance is due, at least in part, to a target-site modification at Pro197 in the ALS gene.

Sunflower broomrape and Egyptian broomrape are two holoparasites of sunflower that cause severe yield loss. The aim of the current study was to clarify the relationship between temperature and sunflower broomrape and Egyptian broomrape parasitism of the resistant sunflower variety ‘Ambar’ (R) and the susceptible variety ‘Adi’ (S). Experiments were conducted in pots under controlled conditions in a multiclimate greenhouse at five temperature (day–night) regimes: 17:9 C, 20:12 C, 23:15 C, 26:18 C, and 29:21 C. The resistant confectionery sunflower variety (R) was susceptible to sunflower broomrape only at low temperature regimes of 17:9 C, whereas the S variety was highly susceptible from 29 to 9 C. Temperature correlated negatively with the number of sunflower broomrape and Egyptian broomrape shoots that emerged above the soil in the R variety, and positively in the S variety. The number of degenerated tubercles found on the roots of the resistant variety correlated positively with temperature in both sunflower broomrape and Egyptian broomrape. No such correlation could be found in the susceptible sunflower variety grown in the presence of these two parasites. The resistance response of the R variety to sunflower broomrape and Egyptian broomrape was similar regardless of temperature regime. Parasite seeds were successfully germinated and attached to the sunflower roots but the parasite at the infection site degenerated and died. On the other hand, the susceptible variety (S) was equally susceptible to Egyptian broomrape and sunflower broomrape in all the temperature regimes tested, and susceptibility increased as temperature increased.

The effect of sequential hexane, ethyl acetate, and aqueous methanol extracts of ‘Regal’ sweetpotato periderm on seed germination of sweetpotato, proso millet, and seven weed species was studied. The hexane extract, which contained the nonpolar components of the periderm tissue, was least inhibitory. It inhibited velvetleaf, proso millet, black nightshade, and redroot pigweed germination, and maximum inhibition was 56% for black nightshade at 200 mg of periderm extracted ml–1. The ethyl acetate fraction was inhibitory to proso millet, velvetleaf, black nightshade, goosegrass, tall morningglory, coffee senna, and redroot pigweed. The estimated I50 for ethyl acetate ranged from 17 mg periderm extracted ml–1 for black nightshade to 201 mg ml–1 for coffee senna. Sweetpotato, tall morningglory, and eclipta germination was not inhibited by this extract at the concentrations tested. The aqueous methanol extract was much more inhibitory than the hexane or ethyl acetate extracts, and there was considerable variation between species in response to this extract The I50 estimates for the aqueous methanol extract were 0.5, 0.6, 2.8, 4.4, 5.1, 9.6, 15.7, 21.0, and 25.8 mg ml–1 for velvetleaf, proso millet, black nightshade, goosegrass, sweetpotato, tall morningglory, eclipta, coffee senna, and pigweed, respectively.

Greenhouse studies were conducted to assess the intensity of smooth pigweed and common purslane aboveground interference (AI) and belowground interference (BI) with lettuce and to determine primary mechanisms of interference of each species as affected by P fertility rates. Lettuce was transplanted in mixtures with either smooth pigweed or common purslane according to four partitioning regimes: no interference, full interference, BI, and AI. Soil used was low in P for optimum lettuce yields, therefore P was added at rates of 0, 0.4, and 0.8 grams of P per liter of soil. Shoot and root biomass and plant height were measured for each species, as well as P tissue content. The data obtained indicated that smooth pigweed interfered with lettuce primarily through light interception by its taller canopy. A secondary mechanism of interference was the absorption of P from the soil through luxury consumption, increasing the P tissue content without enhancing smooth pigweed biomass accumulation. In contrast, common purslane competed aggressively with lettuce for P. Because the weed grew taller than lettuce, light interception was a secondary interference factor.

Field experiments were conducted from 1967 to 1971 to determine the influence of incorporation and time of application on the performance and persistence of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine]. Both grass and broadleaf weeds were effectively controlled by atrazine on all dates of application. Both the 2.8 and 5.6 kg/ha rates were effective as preplant incorporated or preemergence treatments. Persistence of atrazine did not differ with respect to method of application. Applications of atrazine made in April and May were degraded more rapidly than were applications made in February and March. Loss curves generally corresponded to a first-order reaction with atrazine showing an average half-life of 20 days from February to July.

Atrazine metabolism was studied in four warm-season forage grasses to determine if metabolism was the basis for differential atrazine tolerance among the grasses. Big bluestem and switchgrass are atrazine tolerant while indiangrass and sideoats grama are atrazine susceptible in the seedling stage. Metabolism of atrazine in big bluestem and switchgrass occurred primarily by glutathione conjugation. The major metabolic product isolated from indiangrass and sideoats grama was the N-deethylated metabolite of atrazine. Glutathione conjugation by big bluestem and switchgrass occurred at a faster rate than N-dealkylation of atrazine in indiangrass and sideoats grama. Differential tolerance to atrazine among the grasses studied was probably due to the metabolic route by which they detoxify atrazine and the rate of metabolism for that specific route. Intraspecific differences in atrazine tolerance in indiangrass were due to the amount of metabolite produced in relationship to the amount of parent atrazine remaining in the shoot tissue. The more tolerant indiangrass lines had a higher metabolite to parent atrazine ratio than susceptible lines. This study confirmed differences in seedling atrazine tolerance of four indiangrass lines observed in previous greenhouse studies.

The absorption and translocation of 14C-labeled α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) and 2-(3,4-dichlorophenyl)-4-methyl-1,2,4-oxadiazolidine-3,5-dione methazole from nutrient solutions of various temperatures by (Arachis hypogaea L. ‘Starr’) seedlings were determined. The accumulation of trifluralin in roots at 24 hr after exposure to trifluralin was greatest at 21 C and decreased at higher temperatures up to 38 C. The amounts of trifluralin translocated and accumulated in hypocotyls, tops, and cotyledons were small but generally increased with temperature. The initial rate of absorption of trifluralin was greater in excised lateral root tips than in tap root tips, but there was a greater accumulation in excised tap roots at 24 hr. The initial rates of absorption were higher for excised lateral roots at high temperatures. Total absorption of trifluralin at equilibrium was not proportional to the initial rates of absorption but was highest at low (21 C) and high (38 C) temperatures for excised lateral roots. The absorption of methazole by roots and translocation to other plant parts increased linearly with temperature, and it tended to accumulate in the mature leaf tissue.

Among the numerous research reports on the fate of herbicides in soils, the literature dealing with the degradation of herbicides under field conditions is probably most difficult to interpret. Few have attempted to evaluate critically the purposes of these studies. This review will not be a comprehensive recitation of previous studies on herbicide degradation under field conditions, but is aimed at pointing out certain problem areas in the approach to research on this subject. The problems relate both to a lack of clear understanding of the degradation process and a lack of appropriate methodology for studying degradation under field conditions.

Weed control and sugarbeet (Beta vulgaris L.) injury from applications of methyl m-hydroxycarbanilate m-methyl-carbanilate (phenmedipham) were influenced by additives, volume of additive, and species in both field and greenhouse experiments. Oils were more effective than the surfactant as additives to phenmedipham on green foxtail (Setaria virdis (L.) Beauv.), yellow foxtail (Setaria glauca (L.) Beauv.), redroot pigweed (Amaranthus retroflexus L.), or common lambsquarters (Chenopodium album L.). Herbicidal activity of phenmedipham on kochia (Kochia scoparia (L.) Schrad.) or wild mustard (Brassica kaber (D.C.) L.C. Wheeler var. pinnatifida (Stokes) L.C. Wheeler) was not enhanced by any additive. Linseed oil (2.34 L/ha) enhanced the herbicidal activity of phenmedipham on green foxtail, yellow foxtail, and redroot pigweed more than petroleum (2.34 L/ha) or sunflower (Helianthus annus L.) oil (2.34 or 9.35 L/ha). However, linseed oil reduced the herbicidal activity of phenmedipham on kochia.

Two species of annual nightshades were commonly found as agronomic weeds in Minnesota. Eastern black nightshade (Solanum ptycanthum Dun.), the most common species, was found throughout the southern half of the state. Hairy nightshade (Solanum sarrachoides Sendt. ♯3 SOLSA) was found only in a few scattered locations. Eastern black nightshade seedling emergence began in mid-April or early May, and more than 80% of the total yearly emergence occurred before June. Hairy nightshade emergence began in May, but less than 70% of its total yearly emergence occurred before June. Eastern black nightshade berries first contained viable seeds 4 to 5 weeks after flowering of the plants and a week or more before the berries began to turn black. Hairy nightshade generally required a week longer than eastern black nightshade for viable seed production after flowering.