Weed management programs in "Superior" potato with PRE and POST rimsulfuron treatments were investigated during 1992, 1993, and 1994. Common ragweed control by PRE combinations of metolachlor with linuron or metribuzin was higher when treatments included PRE or POST rimsulfuron. Common lambsquarters control was 93 to 96% by treatments that included POST rimsulfuron. Applications of 35 g ai/ha rimsulfuron plus 280 g ai/ha metribuzin POST controlled weeds comparable to sequential applications. Potato recovered from occasional injury caused by rimsulfuron, rimsulfuron plus metribuzin, and organophosphate insecticides combined POST with rimsulfuron plus metribuzin. Several acetolactate synthase (ALS)-inhibiting herbicides were evaluated for yellow nutsedge control in the greenhouse. Herbicides were applied POST to yellow nutsedge at actual or anticipated commercial rates. Yellow nutsedge control was 92 and 71% from halosulfuron and chlorimuron, respectively. Control ranged from 48 to 69% from primisulfuron, pyrithiobac, and rimsulfuron. Control from nicosulfuron and imazethapyr was 45 and 68%, respectively, while thifensulfuron and CGA-152005 had almost no activity on yellow nutsedge. Chlorimuron, imazethapyr, and halosulfuron were the only herbicides which reduced yellow nutsedge regrowth. Rrimsulfuron was evaluated in tomato at 26 and 35 g ai/ha, sequentially at 26 g/ha, at 26 g/ha plus metribuzin at 280 g ai/ha, and metribuzin at 280 g/ha were evaluated POST for weed control in transplanted "Agriset" tomato. Common lambsquarters was controlled by rimsulfuron at 35 g/ha. Rimsulfuron plus metribuzin gave consistent control of common ragweed but jimsonweed and goosegrass control was generally low. Rimsulfuron treatments caused < 12% injury to tomato. Tomato yield was consistently high in the metribuzin, metribuzin plus rimsulfuron, and rimsulfuron sequential treatments. In greenhouse studies, giant foxtail and large crabgrass control by rimsulfuron was above 95 and 85% respectively, but goosegrass was not controlled. Height of four tomato cultivars was not reduced, but dry weight of "Floradade" and "Sunbeam" was reduced by rimsulfuron. In 1993, 1994 and 1995, PPI clomazone at 390 g ai/ha, POST rimsulfuron at 35 g ai/ha, and PPI trifluralin at 560 g ai/ha were evaluated for weed control in transplanted "Keystone RG3" bell pepper. Common lambsquarters and jimsonweed control was highest by clomazone treatments, while common ragweed control was low from all treatments. Keystone RG3 in the field and greenhouse and "Camelot," "Jupiter" and "Memphis" in the greenhouse were injured by POST rimsulfuron and had lower height and dry weight than untreated controls. In the greenhouse, black nightshade control was below 23% and jimsonweed control was below 49% by rimsulfuron POST. The absorption, translocation, and metabolism of rimsulfuron was investigated in three Solanaceous weed species. Rimsulfuron uptake did not differ between black nightshade and eastern black nightshade while less labeled herbicide was absorbed by hairy nightshade. Black and eastern black nightshade translocated up to 50% of the labeled herbicide out of the treated leaf with 40 to 50% of the herbicide being moved to the actively growing regions of the plant. In hairy nightshade, an average 40% of the labeled herbicide was moved out of the treated leaf and less than 30% of the translocated herbicide was moved basipetally. Most major metabolites were apparent at 24 and 48 hours however, there were no differences in metabolite composition. Rimsulfuron will be an effective herbicide for use in weed management programs in potato and tomato, however rimsulfuron causes too much injury in pepper to be used. / Ph. D.

Repeated use of acetolactate synthase (ALS)-inhibiting herbicides in recent years has resulted in the selection of 89 weed species resistant to these herbicides. One management strategy that can eliminate or slow the development of resistance is applying mixtures of herbicides with different modes of action. This research involved the characterization of ALS-inhibiting herbicide-resistant smooth pigweed (<i>Amaranthus hybridus</i> L.), as well as investigations on weed management programs in corn (<i>Zea mays</i> L.) utilizing mesotrione, a triketone, in mixtures with other herbicides. ALS-inhibiting herbicide-resistant smooth pigweed biotypes were collected from fields in Virginia, Delaware, Maryland, and Pennsylvania to evaluate response to ALS-inhibiting herbicides and to determine the molecular mechanisms of resistance. Sequencing of the ALS genes from these biotypes revealed two amino acid substitutions known to confer resistance, Ala₁₂₂ to Thr and Ser₆₅₃ to Asn, and one that has not been previously reported in plants, Asp₃₇₆ to Glu. The smooth pigweed biotype with an Asp₃₇₆ substitution displayed resistance to four classes of ALS-inhibiting herbicides that included imidazolinone (IMI), sulfonylurea (SU), pyrimidinylthiobenzoate (PTB), and triazolopyrimidine sulfonanilide (TP) chemistries. Transformation of this smooth pigweed ALS gene into <i>Arabidopsis thaliana</i> confirmed that the Asp₃₇₆ substitution is responsible for the resistance. Other biotypes that had a substitution at Ala₁₂₂ exhibited resistance to an IMI herbicide, little to no resistance to SU herbicides, and increased sensitivity to a PTB and a TP herbicide, whereas, biotypes that had a substitution at Ser₆₅₃ exhibited high-level resistance to an IMI herbicide and lower resistance to PTB and SU herbicides. Experiments were also conducted to investigate the effectiveness of mesotrione in preemergence (PRE) and postemergence (POST) corn weed management programs in Virginia. Mesotrione applied PRE in mixtures with <i>S</i>-metolachlor and atrazine controlled common lambsquarters (<i>Chenopodium album</i> L.), smooth pigweed, common ragweed (<i>Ambrosia artemisiifolia</i> L.), and morningglory (<i>Ipomoea</i> spp.) species when a timely rainfall followed application. POST applications of mesotrione controlled common lambsquarters and smooth pigweed, but common ragweed and morningglory species were not always controlled. Common ragweed and morningglory species were controlled by mesotrione in a mixture with atrazine POST. Large crabgrass [<i>Digitaria sanguinalis</i> (L.) Scop.] and giant foxtail (<i>Setaria faberi</i> Herrm.) control was generally better when the ALS-inhibiting herbicides nicosulfuron plus rimsulfuron or rimsulfuron plus thifensulfuron plus atrazine were applied in a mixture with mesotrione. Mixtures of mesotrione with other POST herbicides in a total POST program produced corn yields comparable to standard PRE followed by POST weed management programs. / Ph. D.