The objective of this 2-yr study was to determine the optimal length of time between stale-seedbed preparation and planting that maximized weed control along with growth, development, and yield of cucumbers, compared with conventional seedbeds. Stale-seedbeds were prepared 40, 30, 20, and 10 d before planting (DBP), with an additional treatment of 40-DBP seedbed that received an application of glyphosate at 0.9 kg ae/ha, 20 DBP (40 and 20 DBP). The control (0 DBP) was prepared at planting. Glyphosate plus glufosinate ammonium at 1.26 and 0.042 kg ae/ha were applied after cucumber seeding to kill any emerged weeds. The experiment was a split-plot design in which one half of the main plots were treated with a preemergence application of clomazone at 0.42 kg ai/ha after cucumber seeding. Management of the stale-seedbed influenced the level of weed control and final crop yield. Generally, the 40-DBP seedbed had the highest weed biomass at planting and the lowest at harvest. Cucumber density, leaf number, and vine length were reduced in this treatment, and flowering was delayed because of the high weed biomass present during seedling emergence. All stale-seedbeds, with the exception of the 40-DBP stale-seedbed, had greater yields compared with the control (0 DBP) seedbed. The optimal timing of stale-seedbed preparation was 20 to 30 DBP. Seedbed preparation could be expanded to 40 DBP; however, an application of glyphosate at 20 DBP would be required to optimize yield. The stale-seedbed in combination with herbicides was a superior integrated weed management tool compared with conventional weed management practices.

Field experiments were conducted in 2000 and 2001 in Virginia to evaluate the incidence and severity of maize chlorotic dwarf virus and maize dwarf mosaic virus in response to postemergence (POST) johnsongrass control in two corn hybrids. Previous research demonstrated the increased disease severity in virus-susceptible corn hybrids as an indirect effect of POST johnsongrass control. The increased disease severity resulted from greater transmission by insect vectors, which moved from dying johnsongrass to the crop. Recent observations have indicated a lack of virus tolerance in glyphosate-tolerant corn hybrids commercially available in Virginia. A transgenic glyphosate-tolerant hybrid and a nontransgenic virus-tolerant hybrid, similar in growth characteristics and maturity, were subjected to POST treatments of nicosulfuron, whereas the glyphosate-tolerant hybrid was also treated with glyphosate. Both nicosulfuron and glyphosate, broadcast or directed, provided essentially complete johnsongrass control, although initial johnsongrass control was greater with glyphosate treatments. Little or no disease incidence occurred in the virus-tolerant hybrid. With the virus-susceptible hybrid, significant increases in disease incidence were observed in response to any herbicide treatment applied to johnsongrass-containing plots relative to the same treatment applied to weed-free plots. Johnsongrass control with nicosulfuron or glyphosate caused similar disease incidence and severity in this hybrid, regardless of application method. Results of these experiments indicated that growers' choice of hybrid genetics should focus primarily on disease resistance rather than on herbicide resistance in fields that are infested with johnsongrass.

Field experiments were conducted in 1996, 1999, and 2000 to evaluate weed control and snap bean response to postemergence applications of fomesafen at registered and reduced rates. S-Metolachlor was applied preemergence to all plots to suppress annual grasses. Snap bean injury generally increased as fomesafen rate increased, but at rates up to 0.28 kg ai/ha, injury by fomesafen was similar to or less than that from bentazon. Fomesafen at rates as low as 0.07 kg/ha provided near-complete control of common ragweed, and rates of 0.14 kg/ha or more of fomesafen controlled ivyleaf and pitted morningglories and 5-cm or smaller common lambsquarters as effectively as did bentazon. Control of all weed species from fomesafen alone at 0.21 kg/ha did not improve with the addition of bentazon at 0.28 kg/ha. Although snap bean injury from fomesafen was as high as 43% 1 wk after treatment, snap bean yield and net returns were similar to those from S-metolachlor alone. In a rate and application timing study, fomesafen at 0.14 kg/ha applied to three-trifoliolate snap bean was the least injurious to the crop, whereas applications at 0.28 kg/ha to one- or two-trifoliolate snap bean provided the best weed control.

Some herbicides applied to corn or soybean can carry over and injure vegetable crops grown the following year. Injury and yield effects on vegetable crops from carryover of nicosulfuron and flumetsulam 1 yr after application to corn and from carryover of imazethapyr and imazamox 1 yr after application to soybean were evaluated. Experiments were conducted at Oakes, ND, on sandy loam soils with pH 7.7 in 1994 and pH 7.3 in 1996. Nicosulfuron and flumetsulam were applied to corn, and imazethapyr was applied to soybean at a standard herbicide rate (1×) (35, 56, and 70 g ai/ha, respectively) or 2× rates in 1994 and at 1×, 2×, or 4× rates in 1996. Imazamox was also applied to soybean at 1× (35 g ai/ha), 2×, or 4× rates in 1996. Cabbage, carrot, potato, onion, acorn squash, and tomato were planted in 1995, 1997, and 1998. In 1995, residual imazethapyr delayed tomato maturity but did not reduce tomato yield. Other vegetable crops were not injured by herbicide residues. In 1997, flumetsulam carryover injured plants and reduced yields of cabbage and squash; nicosulfuron carryover injured cabbage and onion plants and reduced onion yield; and imazethapyr carryover injured cabbage, onion, and tomato plants and reduced tomato yield. Most crop injury occurred only at the 2× or 4× herbicide rates. Potato and carrot were not injured by herbicide carryover. Herbicides applied to corn or soybean in 1996 did not carry over to injure any vegetable planted in 1998. The low rate of injury to vegetable crops in this study was mostly due to adequate soil moisture and warm summer soil temperatures, which enhanced microbial degradation of these herbicides and hastened their dissipation. Low clay and moderately low soil organic matter, and slightly alkaline soil pH, also promoted dissipation of some herbicides.

The potential weediness of hybrid bermudagrass cultivars in nontarget areas is an important factor when considering the development of herbicide-resistant cultivars. Field studies evaluated the response of common bermudagrass, hexaploid hybrid ‘Tifton-10’, and two triploid hybrid bermudagrass cultivars (‘TifEagle’ and ‘TifSport’) to clethodim, fluazifop-p, glufosinate, glyphosate, and quizalofop-p. Glyphosate was more consistent than clethodim and clethodim plus glyphosate in controlling common bermudagrass. The triploid cultivars were equally sensitive to each of these treatments, whereas Tifton-10 control was highest with treatments that included glyphosate. Variability between years in control of common bermudagrass was attributed to differences in plant size at application, with greater control of smaller plants. All herbicides reduced common bermudagrass plant diameters ≥ 93% in 1999 when grown without a crop. However, in 2001, only herbicide treatments that included two applications of 1.1 kg ai/ha glyphosate reduced plant diameters 6 to 59%. None of the other treatments reduced common bermudagrass plant diameters compared with pretreatment values. When grown with cotton, fluazifop-p and 4.5 kg/ha glyphosate were the only treatments consistent across cultivars and years. All herbicide treatments reduced triploid hybrid bermudagrass plant diameters ≥ 90%, whereas Tifton-10 plant diameters were reduced > 86% by all treatments, with the exception of clethodim. As in the non-cropland study, common bermudagrass plant diameters were reduced ≥ 97% by herbicides in 1999, whereas in 2001, only fluazifop-p and glyphosate treatments reduced plant diameters compared with the nontreated control. Both the lack of aggressiveness and susceptibility to common herbicides of the triploid hybrid cultivars relative to common bermudagrass indicates that these non–pollen-producing or -receiving cultivars are reasonable candidates for the introduction of herbicide resistance.

Conidia of Myrothecium verrucaria, sprayed in an aqueous phase–paraffinic crop oil emulsion (1:1 v/v) at 470 L/ha, controlled red, ivyleaf, smallflower, and tall morningglory plants (three- to five-leaf stage) by causing severe necrotic injury to leaves and stems. Conidia were not efficacious if applied in an aqueous carrier without oil. When applied in the field as directed postemergence treatments to sugarcane, a concentration of 4 × 108 conidia/ml generally provided > 90% death of morningglory, comparable with the atrazine standard at 2.2 kg ai/ha, and did not cause significant crop injury. Conidia produced on potato dextrose agar or rice flour slurry were about equally effective. When killed by autoclaving, conidia continued to be efficacious, indicating that the symptoms produced by the fungus were not primarily caused by infection. A high performance liquid chromatography analysis of filtrates from the fungal growth media or of harvested conidia showed the presence of several macrocyclic trichothecenes (MT), some known to be phytotoxins. These included verrucarin A and H, roridin A and H, and isororidin E for filtrates and verrucarin A and roridin A for conidia. However, only trace amounts of MT were detected in leaves of treated morningglory plants at 24 h after treatment and none at 48 and 96 h even though the fungus was isolated from leaves up to 14 d after treatment. Further study is needed to identify the causal agents responsible for the phytotoxicity produced by M. verrucaria and to assess potential of this organism as a mycoherbicide.

Five herbicides were tested in green pea, and their residual effects on several rotational crops were investigated in northwestern Washington from 1998 through 2000. In both years, imazamox applied postemergence caused 21 and 28% early-season injury at 0.036 and 0.045 kg/ha, respectively, but only in 1999 did early-season injury result in yield loss compared with nontreated, weedy pea. Trifluralin, clomazone, and sulfentrazone caused 15 to 19% injury to pea in 1998 but not in 1999. Although pea treated with sulfentrazone produced more harvestable pods than nontreated pea (5.0 and 4.1 pods/plant, respectively), pod numbers were similar to peas treated with clomazone, pendimethalin, pendimethalin plus imazamox, or trifluralin. All rotationally grown crops were tolerant to herbicides used in green pea, except for strawberry in 1999, in which leaf area was reduced 23% in plots treated with 0.045 kg/ha imazamox compared with nontreated plots. Intensive tillage combined with favorable soil and climatic conditions in this study indicate that western Washington green pea producers may have greater flexibility in their choice of herbicides and rotational crop alternatives than previously believed.

Growth and growth rate characteristics of diffuse knapweed and bluebunch wheatgrass seedlings were evaluated under two temperature regimes, 10 and 16 C, and two moisture regimes, − 0.01 and − 0.03 MPa, in an environmental chamber. Intra- and interspecific interference were evaluated at 10 C and at either − 0.01 or − 0.03 MPa. Root–leaf ratios for bluebunch wheatgrass were greater than for diffuse knapweed at − 0.01 MPa. Under moist conditions, root penetration was greater for diffuse knapweed than for bluebunch wheatgrass roots; penetration was equal in warm or dry conditions. Wheatgrass root length was greater than knapweed length under dry conditions. Intraspecific interference was greater than interspecific interference for both species. Leaf area data and root–leaf ratios suggest that diffuse knapweed is the faster-growing species under moist conditions.

Field studies were conducted in 1999, 2000, and 2001 to investigate weed control and crop safety with preemergence (PRE) and postemergence (POST) applications of mesotrione alone and in tank mixtures with acetochlor and atrazine. Corn injury was less than 4% with all mesotrione treatments in 1999 and 2001, but it was 8 to 20% in 2000, when rainfall was 3.1 cm 7 d after PRE applications. Mesotrione PRE at 0.16 and 0.24 kg ai/ha did not adequately control most broadleaf weeds or giant foxtail. Tank mixtures of mesotrione plus acetochlor controlled smooth pigweed and giant foxtail but did not adequately control common ragweed, common lambsquarters, or morningglory species. Control by tank mixtures of mesotrione plus atrazine at 0.56 kg ai/ha was frequently low and varied with rainfall after PRE applications. All weed species were controlled 80% or more by mesotrione plus acetochlor PRE or atrazine plus acetochlor PRE followed by mesotrione POST at 0.11 kg/ha.

Field studies were conducted during the 2000 to 2001 growing seasons to evaluate winter annual weed control and crop tolerance with fall-applied herbicides in corn at Belleville, IL. Atrazine, simazine, and rimsulfuron plus thifensulfuron applied in the fall controlled mouseear chickweed, henbit, and Carolina foxtail 93% or greater at planting the following spring. Flumetsulam controlled mouseear chickweed and henbit 98 and 93%, respectively, at planting. Metribuzin controlled mouseear chickweed and henbit 100 and 97%, respectively. CGA-152005 controlled mouseear chickweed, henbit, and wild garlic 93 to 100%. CGA-152005 provided the greatest control of wild garlic, with control ranging from 94 to 100% at planting. CGA-152005 plus simazine controlled 99 to 100% of all winter annual weeds evaluated. Reducing winter annual weed vegetation did not increase soil temperatures at 5-cm depth in May. CGA-152005 caused discoloration and height reduction of corn. CGA-152005 at the highest rate (60 g ai/ha) reduced corn plant height by 7% and grain yield by 8%.

Common cocklebur is an adaptable and competitive weed with variable morphology. To learn how the rate of net photosynthesis (Pn) by common cocklebur relates to traits that may influence competitiveness, we compared Pn, accumulation of biomass (shoot mass, root mass, and total plant mass), and total leaf area among six accessions of common cocklebur grown in a greenhouse. There were highly significant (P ≤ 0.01) differences among accessions for all measured traits. Correlations of each measure of biomass with total leaf area were positive and highly significant, but correlations of Pn with each biomass measure and with total leaf area were negative. The negative correlations were largely a result of relatively low biomass accumulation by the two accessions with the highest Pn. This contrasted with results from a previous experiment on field-grown common cocklebur plants, in which the correlation of Pn with shoot mass was positive (r = 0.64). Despite the negative Pn-to-biomass correlations in the greenhouse study, the rank among accessions for Pn in the greenhouse was nearly the same as with the field-grown plants (Spearman rank correlation r = 0.89). We conclude that the relative Pn rates among common cocklebur accessions grown in the greenhouse may be used to predict their relative Pn rates in the field, but relationships of Pn with biomass or leaf area observed in the greenhouse may not be a reliable indicator of relationships in the field.

Field studies were conducted in Arizona and California to evaluate the performance of glyphosate-tolerant lettuce and to determine the critical time of weed removal. Glyphosate was applied as a single or as a sequential application at 840 g ae/ha. Single glyphosate applications were made to lettuce at the two-, four-, six-, and eight-leaf stages. Sequential applications were made to lettuce at the two- or four-leaf stage followed by (fb) a second application 14 d after the first. Weed control efficacy, weeding times, and lettuce yield were all measured. Overall, glyphosate applied postemergence (POST) provided better weed control than the commercial standards bensulide or pronamide applied preemergence. Single glyphosate applications at the four-leaf stage and sequential applications at the two-leaf stage fb a second application 14 d later provided excellent control of most weeds, including redroot pigweed. Estimates of the critical time of weed removal were 26 to 29 d after emergence. Glyphosate treatments caused no adverse effects on lettuce. Lettuce head fresh weights in the glyphosate treatments were equal to or higher than those in bensulide or pronamide treatments. For crops such as lettuce, with few effective herbicides, the development of glyphosate-tolerant lettuce offers the opportunity to develop effective POST weed control programs.

Metolachlor, dimethenamid, acetochlor, and a commercial premixture of flufenacet plus metribuzin were applied 60, 45, 30, and 15 d before planting (DBP) and at planting (preemergence [PRE]) at Dekalb and Urbana, IL in 1995 and 1996. The soil types were a Drummer silty clay loam (fine-silty, mixed, superactive, mesic Typic Endoaquoll) with 2.8% organic carbon and a pH of 6.5 at Dekalb and a Flanagan silt loam (Fine, smectitic, mesic Aquic Argiudoll) with 2.0% organic carbon and a pH of 6.2 at Urbana. Herbicide and application timing affected giant foxtail control and densities. Neither herbicide nor application timing affected crop injury or grain yield. Test of the main effects showed that metolachlor and flufenacet plus metribuzin were more effective than acetochlor or dimethenamid in controlling giant foxtail at 30 and 60 d after planting (DAP) and in reducing foxtail density at 60 DAP. Giant foxtail control 60 DAP was greater than 80% for both metolachlor and flufenacet plus metribuzin for all application timings. Dimethenamid and acetochlor applied at 60 and 45 DBP provided less giant foxtail control when compared with metolachlor and flufenacet plus metribuzin applied at the same time. All herbicides provided greater than 90% giant foxtail control at the PRE application timing. Giant foxtail control provided by metolachlor and flufenacet plus metribuzin was insensitive to application timing from 60 DBP to PRE, whereas both dimethenamid and acetochlor efficacy was lowered when applied between 30 and 60 DBP.

Field studies evaluated the effect of brown patch control on preemergence herbicide efficacy in tall fescue. Pendimethalin (1.7 followed by [fb] 1.7; 3.4 kg ai/ha), prodiamine (0.7 fb 0.6; 1.3 kg ai/ha), and oxadiazon (2.2 fb 2.2; 4.5 kg/ha), applied sequentially and as a single application, were evaluated for smooth crabgrass control with and without the use of azoxystrobin, a fungicide that controls brown patch. Azoxystrobin suppressed brown patch and increased smooth crabgrass control with pendimethalin in both years. This enhanced efficacy with azoxystrobin was attributed to improved tall fescue turf density and thus increased competition between this turf species and smooth crabgrass. Longer soil-residual herbicides such as oxadiazon and prodiamine provided high levels of smooth crabgrass control (often >90%). With the exception of oxadiazon at 4.5 kg ai/ha in 2000, smooth crabgrass control with oxadiazon and prodiamine was unaffected by the use of azoxystrobin.

Studies examined the effect of herbicides on the quality and root strength of winter-installed tall fescue and hybrid bermudagrass sod. Sod was installed in February and treated approximately 30 to 45 d after installation with a variety of preemergence (PRE) and postemergence (POST) herbicides. Tall fescue quality was not affected by any herbicide treatment in 2000 or 2001. Root strength reduction occurred only in 2001 by dithiopyr applied PRE at 2.5 kg ai/ha, which is 4.5 times the maximum use rate. No other herbicides decreased tall fescue root strength in either year. Bermudagrass quality was decreased by triclopyr plus clopyralid applied POST, which resulted in 22 and 6% injury 42 d after initial treatment (DAIT) in 2000 and 2001, respectively. However, POST herbicides did not affect bermudagrass root strength in either year. PRE herbicide treatments did not reduce root development in bermudagrass in 2000. However, dithiopyr applied at 0.84 and 2.5 kg/ha reduced bermudagrass rooting by 59 and 70%, 60 DAIT, in 2001. Pendimethalin reduced root development of bermudagrass by 62% when applied at 10.2 kg/ha, three times the maximum use rate, in 2001. However, by 120 DAIT, only dithiopyr applied at 2.5 kg/ha decreased bermudagrass root development.

Field studies were conducted in 1999 and 2000 at Marianna and Gainesville, FL, to evaluate the response of three runner-type peanut cultivars, ‘Georgia Green’, ‘C-99R’, and ‘MDR-98’, to flumioxazin applied preemergence at 0, 71, 105, and 211 g/ha in a weed-free environment. Peanut exhibited excellent tolerance to flumioxazin, regardless of flumioxazin rate or peanut cultivar, at Gainesville in 1999 and both locations in 2000. In 1999, at Marianna, flumioxazin caused early-season stunting and season-long reduction in peanut canopy width. Peanut response was independent of cultivar and did not exceed 25%, with an increase in stunting with higher flumioxazin rates. Peanut stunting was associated with cool and extremely wet growing conditions during the first 2 mo after planting in 1999 at Marianna. Peanut yield and grade parameters, in both years, were not affected by flumioxazin treatment.

Experiments were conducted at two locations in Texas during 2000 and 2001 to compare barnyardgrass, broadleaf signalgrass, and red rice control; rice tolerance; and grain yield after single and sequential applications of imazethapyr in imidazolinone-tolerant rice. Red rice and barnyardgrass control on a clay soil at Beaumont was at least 94% with imazethapyr at 70, 90, and 100 g ai/ha applied preplant incorporated (PPI) or preemergence (PRE) followed by (fb) at least 40 g/ha of imazethapyr applied early postemergence (EPOST). Broadleaf signalgrass control on a very fine sandy loam soil at Eagle Lake was at least 86% when imazethapyr was applied PPI or PRE fb EPOST applications of imazethapyr. Sequential postemergence applications at Beaumont resulted in at least 95% red rice and barnyardgrass control when 40 g/ha applied late postemergence followed any EPOST application. Sequential postemergence applications at Eagle Lake controlled broadleaf signalgrass 98% during both years. Imazethapyr applied postemergence injured rice 0 to 34% up to 12 d after treatment. Rice yield reductions were correlated to weed control and most likely were not associated with early-season rice injury.

Eight hard red spring wheat cultivars were tested for tolerance to five postemergence grass herbicides at two locations in Minnesota in 1999 and 2000 at the labeled, one and one-half, or twice the labeled rate. Fenoxaprop plus safener and ICIA 0604 caused the least injury and did not reduce grain yield for most cultivars. Flucarbazone caused intermediate injury and a slight decrease in grain yield for half the cultivars tested. Difenzoquat caused the most injury, regardless of whether the cultivar was genetically sensitive to difenzoquat. Tank-mixing difenzoquat with imazamethabenz reduced injury, even for cultivars that were not genetically sensitive to difenzoquat. Drought stress before application of the postemergence grass herbicides that contain difenzoquat resulted in more potential for crop injury. Excess precipitation combined with high temperatures after application resulted in more potential for crop injury for the other postemergence grass herbicides included in this experiment.

Yellow starthistle, spotted knapweed, and meadow hawkweed are three competitive noxious weeds that collectively infest about 640,000 ha of range and noncropland in Idaho. The purpose of this study was to determine the control of these three noxious weeds and undesirable annual grasses with imazapic applied at different rates and growth stages. Imazapic applied fall preemergence, spring postemergence (POST), or fall plus spring POST and picloram applied fall and spring POST reduced yellow starthistle plant density 75 to 100% compared with the untreated control, at about 100 d after spring treatment (DAST). However, by 440 DAST, yellow starthistle plant density did not differ from the untreated control in plots treated with imazapic, whereas picloram reduced plant density 82 to 99%. Imazapic did not control meadow hawkweed or spotted knapweed, whereas picloram reduced plant density of both species 91 to 100% 60 DAST compared with the untreated control and 71 to 100% 440 DAST. Imazapic did not affect weedy annual grass plant density compared with the untreated control at the yellow starthistle or the spotted knapweed sites. However, at the meadow hawkweed sites, imazapic applied spring POST at 210 g/ha and fall plus spring POST at 70 plus 140 g/ha reduced grass plant density 77 to 84% 60 DAST. All treatments that contained spring-applied imazapic reduced grass plant density 46 to 92%, 440 DAST at the meadow hawkweed site. At all locations, picloram increased weedy annual grass plant density two- to eightfold 440 DAST compared with the untreated control. Imazapic offers only moderate control of weedy annual grasses for a brief period after application and does not provide control of these three major noxious weed species in the West. Unfortunately, it eliminates a fairly new rangeland herbicide from the list of potential tank mix partners for control of auxinic herbicide–resistant weed species.

Field studies were conducted in the spring of 1997 and 1998 to quantify the effect of season-long yellow nutsedge interference on watermelon yield. The competitive ability of watermelon with yellow nutsedge was compared in two establishment methods (watermelon transplanted and direct seeded). Critical yellow nutsedge densities and the biological threshold (BT) were used to characterize the competitive ability of watermelon. The critical density in both direct-seeded and transplanted watermelons was 2 yellow nutsedge plants/m2. The BT of yellow nutsedge in seeded watermelons was 37 yellow nutsedge plants/m2, whereas the BT in transplanted watermelons was 25 plants/m2. Transplanting watermelons did not improve their competitive ability with yellow nutsedge. Percent yield loss was similar for both establishment methods at the respective yellow nutsedge densities. Over 40% yield loss was incurred with 12 yellow nutsedge plants/m2 for both establishment methods. Furthermore, it was concluded that watermelons are poor competitors with yellow nutsedge.