Greenhouse and laboratory studies were conducted to determine the degree of dominance of the monogenic sulfonylurea herbicide resistance trait in diploid sugarbeet by comparing the response of homozygous and heterozygous resistant sugarbeet to primisulfuron, thifensulfuron, and chlorimuron on the whole plant and acetolactate synthase (ALS) enzyme level. Progeny tests suggested that the monogenic sulfonylurea herbicide resistance was semidominant. Subsequently, heterozygous resistant (R-1) and homozygous resistant (R-2) sugarbeet lines were sprayed with increasing rates of primisulfuron, thifensulfuron, and chlorimuron, and herbicide rates required for 50% growth reduction (GR50) were determined. GR50 values were also determined for homozygous susceptible sugarbeet lines (S-1 and S-2). The GR50 values indicated that the R-2 sugarbeet was 377, 269, and 144 times more resistant to primisulfuron, thifensulfuron, and chlorimuron, respectively, than susceptible S-2 sugarbeet. In contrast, R-1 sugarbeet was only 107, 76, and 57 times more resistant to primisulfuron, thifensulfuron, and chlorimuron, respectively, than S-1 sugarbeet, indicating at least a twofold difference in the magnitude of resistance between homozygous resistant and heterozygous resistant sugarbeet lines. ALS enzyme activity analysis were consistent with whole plant results. Thus, based on these two, maximum crop resistance can be obtained by developing homozygous resistant cultivars.

Field experiments were conducted to evaluate the hypothesis that johnsongrass control in corn causes increased maize dwarf mosaic virus (MDMV) and maize chlorotic dwarf virus (MCDV) disease severity because of increased movement of insect vectors from dying johnsongrass to the corn crop. Johnsongrass control treatments included 1) broadcast POST nicosulfuron, 2) directed POST imazethapyr, 3) mechanical control, and 4) no treatment. Disease severity in both a virus-susceptible and a virus-tolerant corn hybrid was evaluated. With the virus-susceptible hybrid, greater disease severity was observed where johnsongrass was controlled in the experimental area than where johnsongrass was not controlled. Increases in disease severity were independent of the method of johnsongrass control. Corollary studies conducted on the same site verified a double infection of corn with MDMV and MCDV and documented movement of blackfaced leafhoppers, the insect vector of MCDV, subsequent to treatment of johnsongrass.

A Bromus tectorum L. (downy brome) biotype with cross-resistance to sulfosulfuron has been identified. The resistant biotype was selected with primisulfuron in Poa pratensis L. (Kentucky bluegrass) plots near Madras, OR. The plots received two treatments of 20 g ai ha−1 primisulfuron in the fall of 1993, 1994, and 1995. In 1995, control of B. tectorum decreased and greenhouse studies confirmed that the biotype was resistant to primisulfuron. Cross-resistance to sulfosulfuron also was confirmed in greenhouse studies. Metabolism of sulfosulfuron rather than an insensitive site of action is the likely cause of the cross-resistance.

The effects of chlorimuron, imazaquin, imazethapyr, nicosulfuron, primisulfuron, and thifensulfuron were evaluated on a population of smooth pigweed in Painter, VA with no history of treatment with acetolactate synthase (ALS)-inhibitor herbicides. Imazethapyr and nicosulfuron gave the greatest smooth pigweed control, and subsequently were used in field and greenhouse studies to investigate susceptibility of smooth pigweed and livid amaranth populations to ALS-inhibitor herbicides. Approximately 5 million smooth pigweed plants from Painter were treated with imazethapyr or nicosulfuron from 1992 to 1994 and no ALS-inhibitor-resistant plants were identified. In the greenhouse, the response of smooth pigweed from Painter, VA, Marion, MD, and Oak Hall, VA and livid amaranth from Warren County, NJ to imazaquin or imazethapyr and nicosulfuron was investigated. Smooth pigweed from Marion and Oak Hall and livid amaranth from NJ had histories of treatment with ALS-inhibitors. Painter smooth pigweed control was 81 to 97% by imazethapyr and nicosulfuron while control of the Marion and Oak Hall populations was 3 and 18% by imazaquin at 560 and 1120 g ai/ha, respectively, and control by nicosulfuron at 35 g ai/ha was 50 to 73%. Control of livid amaranth from Warren County, NJ was 8 to 15% by imazethapyr at 560 g ai/ha, and was 30 to 58% by nicosulfuron at 35 g/ha.

Extensive field and greenhouse studies were done to characterize varietal response of three recently commercialized sulfonylurea corn herbicides: nicosulfuron, primisulfuron, and thifensulfuron. Most of the 94 varieties tested were highly tolerant to these herbicides. The 37 inbreds represented all major inbred families now used in hybrid seed production as well as several sensitive experimentals. Twenty-one defined hybrids from these inbreds as well as 36 commercially coded hybrids were also tested. Sensitive inbreds produced tolerant hybrids when crossed with tolerant inbreds. Sensitive hybrids occurred when both parents were sensitive. Genetic analysis of sensitive by tolerant crosses showed that sensitivity is controlled by a single recessive gene. Nicosulfuron had the widest corn safety margin and fewest sensitive varieties. Dose response analysis showed varieties can vary more than 40 000-fold in sensitivity. Only corn varieties with the AHAS-modified XA-17 gene showed any change in enzyme sensitivity. This gene overcame sensitivity to sulfonylureas, even when the organophosphate insecticide terbufos was present. Thus, breeders have three options to eliminate sulfonylurea sensitivity: backcross sensitive inbreds with tolerant, always use at least one tolerant hybrid parent, or use the XA-17 gene.

Greenhouse and laboratory studies were conducted to determine the extent of cross-resistance of chlorsulfuron-resistant sugarbeet (CR1-B) to other herbicides that inhibit acetolactate synthase (ALS) and to determine the physiological basis of resistance. Cross-resistance to metsulfuron, imazaquin, and imazethapyr was not evident, while only marginal cross-resistance was observed to triasulfuron, DPX-L5300, and nicosulfuron. CR1-B was moderately resistant to chlorsulfuron and chlorimuron and was highly cross-resistant to thifensulfuron and primisulfuron. Further greenhouse studies demonstrated that CR1-B was not significantly injured by thifensulfuron and primisulfuron applied at or exceeding the field use rate. Studies with 14C-primisulfuron showed that differential absorption or metabolism of primisulfuron could not account for the observed resistance. ALS enzyme assays showed that the CR1-B ALS enzyme activity was 66, 26, and 13 times less sensitive to chlorsulfuron, thifensulfuron, and primisulfuron inhibition, respectively, compared to ALS enzyme extracted from sensitive sugarbeets. An altered ALS enzyme, which is less sensitive to sulfonylurea herbicide inhibition, appears to be the physiological basis of resistance.

Wild carrot is becoming a serious weed problem in Michigan continuous no-tillage crop production. Greenhouse and field research was conducted to identify effective management strategies for wild carrot control in no-tillage cropping systems. In the greenhouse, PRE applications of acetochlor plus dichlormid (5.8:1 w:w), cyanazine, linuron plus chlorimuron (18:1), and metribuzin plus chlorimuron (10:1) and POST applications of bentazon, CGA-152005, clopyralid, cyanazine, and MON 12000 provided the greatest control of wild carrot seedlings. In the field, PRE and POST treatments containing chlorimuron consistently controlled overwintered wild carrot greater than 71% at 30 d after the POST application in no-tillage soybean. Atrazine, MON 12000, nicosulfuron, and primisulfuron applied POST consistently controlled overwintered wild carrot greater than 78% at 30 DAT in no-tillage corn. Glyphosate at 0.84 or 1.68 kg ae/ha applied in October to established wild carrot provided greater than 74% control the following spring. Early preplant (EPP) applications of glyphosate at 0.84 kg/ha in no-tillage soybean gave 95 and 24% control of overwintered wild carrot in St. Clair and Lenawee Counties, MI, respectively, at 70 DAT. PRE applications of glyphosate at 0.84 kg/ha controlled overwintered wild carrot less than 69% at 58 DAT in no-tillage corn and soybean. Fall applications of 2,4-D ester at 1.12 kg ae/ha provided 18 and 88% control of wild carrot the following spring in Clinton and Lenawee Counties, respectively. EPP applications of 2,4-D ester at 1.12 kg/ha provided 7 and 72% control of overwintered wild carrot in St. Clair and Lenawee Counties, respectively.

A time course of Egyptian broomrape development on tomato roots grown in pots in the greenhouse revealed an increase in the number of parasitic attachments from 2.6 at 4 weeks after planting (WAP) to 139 when rated 2 wk later. Rimsulfuron sprayed on tomato foliage as single and split applications (12.5 g ai/ha) 3, 4, 5, 6, and 7 WAP did not decrease Egyptian broomrape damage to tomato plants, as expressed by reduced tomato plant height and fresh weight, and the total number of parasitic attachments on the roots was not significantly different compared to the nontreated control. A single foliar application of rimsulfuron (25 g ai/ha) to tomato at 10 days after planting (DAP) and split applications at 10 and 20 and 10, 20, and 30 DAP significantly reduced the number of Egyptian broomrape inflorescences to 2.2, 0.2, and 0, respectively, 9 WAP compared with 19.6 inflorescences in the nontreated control. Application of the herbicide under the same conditions to tomato plants in pots covered with activated charcoal did not significantly influence the number of Egyptian broomrape inflorescences. Chlorsulfuron and triasulfuron applied directly to the soil at rates ≥ 3: 3.75 g ai/ha completely controlled Egyptian broomrape. Primisulfuron and chlorimuron, each at 22.5 g ai/ha, reduced the number of inflorescences to 0.6 and 0.4, respectively. Chlorimuron did not significantly reduce the total number of parasitic attachments on the roots, but primisulfuron at the same concentration significantly increased the total number of parasitic attachments on the root system to 50 as compared to 23 in the nontreated control. Tribenuron had no effect on the parasitism process, as reflected in the number of inflorescences and parasitic attachments on the roots.

Control and regrowth of hemp dogbane, wild blackberry, and triazine-resistant common lambsquarters (TR-CHEAL) were studied in no-till corn from 1992 to 1994. Hemp dogbane, wild blackberry, and TR-CHEAL population increased 10, 123, and 177%, respectively, between 1992 and 1994 in plots treated with PRE applications of paraquat, atrazine, and metolachlor (weedy checks). POST applications of tank mixtures of 35 g ai/ha nicosulfuron or 20 g/ha primisulfuron with 280 g/ha 2,4-D or 140 g/ha dicamba, and 560 g/ha dicamba applied alone controlled hemp dogbane, wild blackberry, and TR-CHEAL 67 to 98%. These treatments reduced the population or prevented expansion of these weeds the year following treatment. In 1992, corn yield response to weed control was inconsistent. In 1993 and 1994, all plots treated with POST herbicides yielded higher than the weedy check. Corn yield of plots treated with combinations of nicosulfuron or primisulfuron with 2,4-D or dicamba and 560 g/ha dicamba applied alone were 102 to 149% and 124 to 153% higher than the weedy check in 1993 and 1994, respectively.

In vitro acetolactate synthase (ALS) activity from three commercial imidazolinone-resistant corn hybrids (ICI 8692 IT, Pioneer 3751 IR, and Ciba 4393 IMR) was compared to imidazolinone-sensitive isogenic hybrid controls for sensitivity to 11 herbicides representing four classes of ALS-inhibiting herbicide chemistry. Acetolactate synthase activity from Pioneer IR and Ciba IMR was cross-resistant to all four classes of ALS inhibitors, ranging from 48- to 5,000-fold. The ICI IT hybrid displayed only four- to eightfold resistance to the six imidazolinone herbicides and the pyrimidinylthiobenzoate herbicide, pyrithiobac, but no cross-resistance to the sulfonylurea and triazolopyrimidine sulfonanilide herbicides. The four- to eightfold enzyme resistance to imidazolinone herbicides provides whole-plant resistance; however, the sevenfold enzyme resistance to pyrithiobac was insufficient to afford whole-plant protection to a field application rate of the herbicide. A second imidazolinone-specific resistance allele, XI-12, currently under commercial development, was examined for the level of dominance at the enzyme level. In the heterozygous state, imazethapyr resistance was fivefold, compared to 250-fold in the homozygous condition, indicating XI-12 is a semidominant trait. No cross-resistance to nicosulfuron or primisulfuron was observed in the heterozygous XI-12 hybrid extracts nor to nicosulfuron in the XI-12 homozygote; however, a fivefold resistance to primisulfuron was detected in the XI-12 homozygote.

Following six consecutive annual applications of imazaquin in combination with trifluralin or pendimethalin to several soybean fields on the Delmarva Peninsula, unacceptable smooth pigweed control was observed. Field and greenhouse studies were conducted to determine if this population of smooth pigweed was resistant to imazaquin and other herbicides. In field research, imazaquin and imazethapyr gave complete control of the susceptible (S) population while providing no control of the resistant (R) population; pyrithiobac controlled 99 and 90% of the R and S populations, respectively. Pendimethalin, metribuzin, MON-12000, and flumiclorac gave less than 75% control of both S and R populations. Chlorimuron, primisulfuron, CGA-152005, and lactofen gave above 75% control, and thifensulfuron and nicosulfuron gave above 90% control of both S and R populations. Seeds were collected from the R and S smooth pigweed populations for research in the greenhouse. Greenhouse studies confirmed high levels of resistance to imazaquin and imazethapyr and low levels of cross-resistance to rimsulfuron and chlorimuron in the R population. Susceptibility of the R population to nicosulfuron, thifensulfuron, pyrithiobac, and pendimethalin was comparable to that of the S population.

Field trials were conducted in New Jersey during 1991 and 1992 to evaluate the efficacy of nicosulfuron, primisulfuron, dicamba, and clopyralid on horsenettle control in field corn. Horsenettle biomass was reduced 74%, 68%, 64%, 61%, 48%, and 40% by primisulfuron + dicamba, primisulfuron, nicosulfuron + dicamba, dicamba, clopyralid, and nicosulfuron, respectively. No treatment was statistically more effective than dicamba. Corn yields were not influenced by the herbicide treatments. Horsenettle populations were not reduced by any treatment.

Broadleaf signalgrass is sensitive to nicosulfuron and resistant to primisulfuron, but corn is resistant to both. Research was conducted to determine the effect of varying light level and air temperature on absorption, translocation, and metabolism of nicosulfuron and primisulfuron in broadleaf signalgrass and corn. Corn absorbed between 60 and 85% of the applied nicosulfuron and primisulfuron within 72 h after treatment (HAT), depending on environmental treatment. Absorption, translocation, and metabolism all tended to be more rapid at higher temperature and light intensity. Nicosulfuron and primisulfuron translocation out of the treated leaf was < 4.5% of herbicide absorbed through 72 HAT. Corn rapidly metabolized both herbicides in both environments. However, primisulfuron was metabolized more rapidly (high = 99%, low = 92%) than nicosulfuron (high = 95%, low = 78%). Broadleaf signalgrass absorbed 20% more nicosulfuron than primisulfuron through 72 HAT. Nicosulfuron translocation out of the treated leaf in broadleaf signalgrass was ≤ 15% absorbed through 72 HAT, while primisulfuron translocation was ≤ 4% during the same time period. Primisulfuron metabolism was more rapid than nicosulfuron in broadleaf signalgrass. During the first 4 HAT, broadleaf signalgrass metabolized > 20 times more primisulfuron than nicosulfuron. By 72 HAT, broadleaf signalgrass under conditions of high light and temperature had metabolized nearly 90% of the primisulfuron absorbed but ≤ 7% of the nicosulfuron absorbed was metabolized during the same time. These results suggest that differential activity of nicosulfuron and primisulfuron on broadleaf signalgrass may be based on differential rates of metabolism to nonphytotoxic compounds; uptake and translocation differences agree with the differential broadleaf signalgrass activity. Additionally, environment has the potential to affect rates of sulfonylurea absorption, translocation, and metabolism.

Canada thistle control in no-tillage corn was studied in Western Maryland from 1990 to 1992. The best treatment at planting for controlling Canada thistle was a tank mixture of 2240 g/ha glyphosate plus 560 g/ha 2,4-D. Clopyralid applied at 210 and 280 g/ha alone and 106 g/ha tank mixed with 560 g/ha 2,4-D effectively controlled Canada thistle in no-tillage corn (85 to 96%). Nicosulfuron applied at 35 g/ha and 20 or 40 g/ha primisulfuron suppressed Canada thistle (59 to 75%). Tank mixtures of nicosulfuron or primisulfuron with 2,4-D or dicamba generally increased Canada thistle control (75 to 87%) compared with control by these herbicides applied alone. Treatments that controlled Canada thistle generally increased corn yields compared with the weedy controls. Yield increases were most dramatic in 1991 when precipitation was low.

A shattercane biotype resistant to primisulfuron was identified during greenhouse evaluations of seed obtained from southeastern and south-central Nebraska fields previously treated with primisulfuron or nicosulfuron a minimum of 3 consecutive yr. Absorption, translocation, metabolism, and acetolactate synthase (ALS) assay experiments were conducted to determine resistance mechanism(s) by comparing ALS-susceptible forage sorghum (ROX) to resistant shattercane (RS). The ROX had 10 and 12% greater 14C absorption than RS 24 and 96 hours after treatment (HAT), respectively. Absorption of 14C increased over time for both ROX and RS, while 14C translocation from the treated leaf to the shoots and roots was similar for ROX and RS. Unmetabolized primisulfuron accounted for > 80% of the radioactivity recovered 24 h after application for both ROX and RS. The ROX and RS had similar ALS-specific activities and total protein concentrations. K m values for RS and ROX were 10.4 and 5.8 mM pyruvate, respectively. The ALS isolated from RS was less sensitive than ROX to inhibition by primisulfuron. The I50 values for RS and ROX were 231 and 0.025 μM primisulfuron, respectively. The mechanism of primisulfuron resistance in this RS biotype is an altered ALS with decreased sensitivity to primisulfuron.

Field research was conducted to evaluate the response of soybean to various herbicides applied at rates to simulate drift damage. Dicamba, glyphosate, glufosinate, and the sulfonylurea herbicides CGA-152005, primisulfuron, nicosulfuron, rimsulfuron plus thifensulfuron, and CGA-152005 plus primisulfuron were applied to soybean at the two to three trifoliolate leaf stage in 1997 and 1998 at and ⅓ of the recommended use rates. The order of yield reduction after herbicide treatment was CGA-152005 > dicamba > CGA-152005 plus primisulfuron > rimsulfuron plus thifensulfuron > primisulfuron. Soybean yields were not reduced by glyphosate, glufosinate, and nicosulfuron. Applications of all herbicides at rates higher than of the use rate caused injury symptoms within 30 d after treatment. However, soybean plants had partially or fully recovered by the end of the growing season. Therefore, early-season injury symptoms from herbicide drift are not reliable indicators for soybean yield reduction.

Reports of unacceptable shattercane (Sorghum bicolor) control with acetolactate synthase (ALS)-inhibiting herbicides prompted the investigation of 29 fields in central and south-central Nebraska for ALS-resistant (ALSr) shattercane. These fields were located in three distinct geographical areas designated C, G, and P. Shattercane from 13 fields spanning all three areas was resistant to 80 g ai ha−1 (2 X field rate) primisulfuron. Accessions C and G were more resistant than accession P to primisulfuron and nicosulfuron. Accessions C and G were susceptible to imazethapyr, whereas accession P was resistant. The ALS resistance was associated with alterations in the ALS enzyme. Primisulfuron I50 values for ALS from ROX (forage sorghum), C, G, and P were 7, 8,510, 8,870, and 714 nM, respectively; nicosulfuron I50 values were 647, 4,110, 4,070, and 1,460 nM, respectively; and imazethapyr I50 values were 5,440, 13,100, 11,800, and 51,700 nM, respectively. Based on cross-resistance and enzyme sensitivities, at least two biotypes are represented in the three accessions of ALSr shattercane. Shattercane individuals from accessions C, G, and P were intercrossed to determine if the ALSr genes in each of the accessions were at independent loci. All the F2 populations were resistant to 80 g ai ha−1 primisulfuron, suggesting that the ALSr alleles in the three accessions are at the same locus or possibly linked loci. When the C, G, and P accessions were crossed with the wild type (WT), comparisons between the F1, susceptible, and resistant populations showed that primisulfuron resistance was expressed as a dominant, partially dominant, and additive trait for the C, G, and P accessions, respectively. The differences in ALSr allelic interactions indicate that primisulfuron resistance developed independently in each of the three accessions.

Seeds were collected from an imidazolinone-resistant (R) population of smooth pigweed near Marion, MD, and from an imidazolinone-susceptible (S) population near Painter, VA, and grown in the greenhouse. Acetolactate synthase (ALS) enzyme was extracted from both biotypes and assayed in the presence of CGA 152005, chlorimuron, halosulfuron, imazaquin, imazethapyr, nicosulfuron, primisulfuron, pyrithiobac, rimsulfuron, and thifensulfuron to determine if an altered ALS was the mechanism of resistance in the R biotype and to determine if this biotype was cross-resistant to other ALS inhibitor herbicides. The inhibitor concentration required to cause a 50% reduction in ALS activity (I50) was calculated for each herbicide. ALS from the R biotype was approximately 71-, 109,000-, and 9-fold more resistant to imazaquin, imazethapyr, and rimsulfuron, respectively, than that from the S biotype. ALS from the R biotype was approximately threefold more sensitive to pyrithiobac and thifensulfuron than that from the S biotype. R ALS was also slightly more tolerant to CGA 152005 and nicosulfuron and slightly more sensitive to primisulfuron and chlorimuron. ALS from both biotypes generally responded similarly to halosulfuron. Resistance in the R biotype was due to an altered form of ALS that is insensitive to the imidazolinone herbicides and rimsulfuron.

Efficacy of several acetolactate synthase (ALS)-inhibiting herbicides was evaluated for yellow nutsedge control over a 3-yr period on a State sandy loam and in the greenhouse. Herbicides were applied POST to yellow nutsedge at actual or anticipated commercial rates. Yellow nutsedge control was above 77% from MON 12000 and chlorimuron. Control ranged from 41 to 89% from primisulfuron, pyrithiobac, and rimsulfuron. Control from nicosulfuron and imazethapyr was 58% or less while thifensulfuron and CGA-152005 had almost no activity on yellow nutsedge. In greenhouse studies, MON 12000 controlled yellow nutsedge better than did chlorimuron, and imazethapyr gave better control than in the field.

Postemergence applications of nicosulfuron and primisulfuron may injure corn plants depending on the level of terbufos present in the young corn plants from application of terbufos for corn rootworm control. Field studies were conducted in 1992 and 1993 to evaluate the interaction of nicosulfuron and primisulfuron with terbufos and to relate terbufos levels, detected with a rapid detection kit, to injury. Terbufos was applied in-furrow at 0, 186, 375, and 750 g ai/100 m of row. Nicosulfuron was applied at 35 and 70 g ai/ha and primisulfuron at 40 and 80 g ai/ha when the corn was at the four-leaf stage. Prior to herbicide application, both fresh and frozen plant samples from each treatment were subjected to terbufos analysis. The correlation coefficient for terbufos detected in the shoot extract with observed herbicide injury to corn was r = 0.89 in 1992 and r = 0.94 in 1993. Injury ratings showed a greater correlation with terbufos levels than did corn shoot height. Thus, the rapid detection kit provided an efficient method to determine whether an injurious terbufos-herbicide interaction might occur.