Medusahead [Taeniatherum caput-medusae (L.) Nevski] is one of the most detrimental invasive annual grasses impacting the sustainability and function of rangeland in the western United States. This annual grass possesses high concentrations of tissue silicon (Si) that may facilitate invasion through key plant characteristics such as increased plant fitness, structure, and antinutritive qualities. These characteristics may affect known invasive processes such as increased plant productivity, slow litter decomposition, and decreased herbivory, facilitating a positive feedback cycle of invasion. However, Si is not considered an essential element and is often overlooked as a factor of T. caput-medusae invasion. Thus, this article provides a synthesis of plant Si, T. caput-medusae, and the self-reinforcing feedback cycle of invasion. We also discuss how current control strategies address plant characteristics determined by Si and suggest research avenues that may aid in novel or improved control strategies that target the T. caput-medusae–silica relationship.

In greenhouse studies, the butyl ester of fluazifop {(±)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]propanoic acid} applied preemergence at 0.035 kg ai/ha gave 91% control of goosegrass (Eleusine indica Gaertn. ♯ ELEIN), 79% control of large crabgrass [Digitaria sanguinalis (L.) Scop. ♯ DIGSA], and 73% control of giant foxtail (Setaria faberi Herrm. ♯ SETFA). In field studies, the butyl ester of fluazifop at 0.56 kg ai/ha applied preemergence gave 79% control of goosegrass, 76% control of large crabgrass, and 53% control of giant foxtail. The butyl ester of fluazifop at 0.07 kg/ha gave 79 to 85% control of each grass at the pretillering stage in field studies. The same rate applied at the early tillering stage gave 84% control of goosegrass but only 53 and 58% control of large crabgrass and giant foxtail, respectively. Relative species response was similar from spray and leaf-spot applications of the herbicide. The butyl ester of fluazifop at 4 μg/plant when spot-applied to leaves of the three grasses at the tillering stage resulted in 65% control of goosegrass but only 20 and 25% control of large crabgrass and giant foxtail, respectively.

Sweet potato clones were evaluated for metribuzin tolerance in field and greenhouse studies. ‘Tinian’ was the most tolerant and ‘SC-1149-19’ and ‘J-2’ were the most susceptible clones. ‘Jewel’, a clone widely grown commercially, was sensitive to metribuzin in the field and greenhouse. ‘Sumor’ and ‘White Triumph’ exhibited intermediate metribuzin tolerance. All metribuzin application-by-rate treatments reduced plant stands, caused season-long crop injury, and reduced yields. Metribuzin applied 3 wk posttransplant at 1.1 kg ha−1 did not reduce total yield of Tinian. Total yield of all the clones except Tinian were reduced by metribuzin regardless of timing or rate.

Alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] at 3.4 and 6.7 kg/ha controlled weeds present in sweet potatoes (Ipomoea batatas L. ‘Jewel’, ‘Centennial’, ‘Caromex’) at two locations in North Carolina. Yield was not reduced from slight early season injury observed with the herbicide treatments. Most sweet potato root samples contained less than the detectable limit of 0.05 ppmw of alachlor. At one location, samples of the cultivars Jewel and Centennial that received 6.7 kg/ha contained 0.06 and 0.05 ppmw, respectively. Averaged over locations, residue levels in soil samples from plots treated at 6.7 kg/ha decreased from 1.55 ppmw initially to less than 0.05 ppmw by 16 weeks after application.

Fluazifop {(±)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl] oxy] phenoxy] propanoic acid} controlled annual grasses in cucumbers (Cucumis sativus L. ‘Calypso’ and ‘Poinsett’), cantaloupe (Cucumis melo L. ‘Edisto 47’), zucchini squash (Cucurbita pepo L. ‘Elite’), and sweet potato (Ipomoea batatas L. ‘Jewel’) when applied as single or repeated applications of 0.3 or 0.6 kg ai/ha during 1982 and 1983 in North Carolina. All crops were generally tolerant to fluazifop and yields were equal to cultivated controls in all but one experiment. Residues detected in the various crops by high-performance liquid chromatography (HPLC) decreased as time elapsed between application and harvest increased. Approximately 1 ppm (w/w) of fluazifop was detected in cucumbers at 8 days after application, but residues were below the least detectable limit (0.05 ppm) when cucumbers, squash, and cantaloupe were harvested at 18 to 29 days after application. Fluazifop concentrations were 0.06 ppm or less in sweet potatoes at 55 days after application, and no fluazifop was detected at 84 days after application. Small cucumbers (<12 cm long) had greater fluazifop residues than larger fruit (>12 cm long) 20 days following application.

Carbon-14-labeled glyphosate [N-(phosphonomethyl)glycine] was foliarly applied to ligustrum (Ligustrum japonicum Thunb.) and blue pacific juniper (Juniperus conferta Parl. ‘Blue Pacific’) at the following growth stages: cold acclimation, winter dormancy, budbreak, shoot elongation, and termination of the first flush of growth (shoot termination). At shoot elongation juniper plants absorbed 2% of applied 14C by 14 days after treatment (DAT). Applications at other growth stages resulted in no significant absorption of 14C by junipers. Within 7 DAT the amounts of radioactivity absorbed by ligustrum were significant and depended upon growth stage in the following order: budbreak < shoot termination < shoot elongation. Absorption by overwintered leaves occurred at budbreak but not at elongation or termination. Transport of absorbed 14C in ligustrum was primarily acropetal and occurred only in budbreak and flowering treatments. Differences in tolerance to glyphosate between juniper and ligustrum appear to be related to differential absorption. Seasonal differences in ligustrum tolerance also appear to be associated with differences in absorption. Although growth stage affected transport in ligustrum, differential transport does not appear to play a major role in seasonal influences on long-term glyphosate phytotoxicity.

Field violet germination occurred during May to early July and again during September to November in Nova Scotia (N.S.), Canada. Germination occurred from October until early April in the North Carolina (NC) piedmont. Germination was stimulated by tillage. Seed collected in N.S. and in NC germinated optimally in the dark under a temperature regime of 15/5 C, on an 8/16 h thermoperiod. Germination at constant temperatures decreased with increasing temperatures, from 5 to 25 C. Seed collected in NC was capable of germinating at a higher constant temperature of 25 C than seed collected in N.S. Germination of seed collected in NC was partially inhibited by constant irradiance with a photosynthetic photon flux density of 100 μM m−2 s−1 (400 to 700 nm). Seed did not germinate when exposed to constant red or far-red irradiance.

Chlorimuron antagonized the activity of quizalofop on broadleaf signalgrass when applied in greenhouse studies as a postemergence tank mix. In vitro leaf disc assays utilizing 14C-acetate or 14C-pyruvate as substrates were conducted to ascertain the effect of clorimuron and quizalofop on fatty acid biosynthesis and to determine if antagonism between the two herbicides occurs at the biochemical sites of action. Incorporation of 14C-acetate in control treatments was linear with time to 120 min. Acetate incorporation in the presence of quizalofop (1.1 μM) was also linear but was inhibited 30 min after initialization of the reaction. The concentration of quizalofop that inhibited 14C-acetate incorporation 50% (I50) was 0.54 μM. Chlorimuron, up to 155 μM, had no effect on 14C-acetate incorporation. A mixture of quizalofop (1.1 μM) and chlorimuron (4.8 μM) inhibited 14C-acetate incorporation similar to that of quizalofop alone at 1.1 μM. Quizalofop I50 for incorporation of 14C-pyruvate was 1.1 μM, and clorimuron at 4.8 μM decreased incorporation 15%. Excess unlabeled pyruvate (5 μM) had no effect on either 14C-acetate or 14C-pyruvate incorporation in the presence of both herbicides. It is believed that antagonism of quizalofop by clorimuron is not due to an excess pool of pyruvate resulting from inhibition of acetolactate synthase by chlorimuron.

The susceptibility of sweet pepper (Capsicum annuum L. ‘Keystone Resistant Giant’) and tolerance of hot pepper (Capsicum chinense L. ‘Bohemian Chili’) to bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] was demonstrated under greenhouse and field conditions. Sweet pepper growth and fruit production were inhibited by foliar applications of bentazon at rates of 0.6 to 6.7 kg ai/ha. Severity of injury increased with application rate. Injury symptoms in sweet pepper were leaf chlorosis, with necrosis and death of shoot apices. Foliar applications of bentazon to hot pepper resulted in little or no injury. Bentazon inhibited Hill reaction activity of isolated chloroplasts from both species to a similar extent, suggesting that selectivity of this compound in hot pepper is not due to resistance at the chloroplast level.

Seedlings from golden delicious apples (Malus domestica Borkh. ‘Smoothee’) were grown in the greenhouse with orchardgrass (Dactylis glomerata L. # DACGL), Kentucky bluegrass (Poa pratensis L. # POAPR), red sorrel (Rumex acetosella L. # RUMAA), and Korean lespedeza (Lespedeza stipulacea L. # LESSL) at four densities and four fertility levels. Orchardgrass and red sorrel inhibited tree growth more than Kentucky bluegrass on a per plant basis. Korean lespedeza inhibition of apple growth was associated with the amount of root nodulation on the Korean lespedeza. Apple shoot dry weight over all fertility levels, ground cover species, and densities was predicted by a regression model based on increase with fertility and decrease with ground cover dry weight. These results indicate that the competitiveness between equal number of ground cover species is due to differences in biomass production. Addition of nitrogen to the system increased nitrogen levels in apples but did not completely overcome apple growth inhibition.

Five perennial cool-season grasses: Kentucky bluegrass, creeping red fescue, tall fescue, orchardgrass, and perennial ryegrass were treated at the 2- to 3-leaf, 3- to 4-leaf, 4- to 6-leaf, and 6- to 7-leaf growth stages with four rates (0, 0.14, 0.28, and 0.56 kg ai/ha) of the butyl ester of fluazifop. Creeping red fescue was not injured by any rate at any growth stage. Reduction in shoot dry weight increased linearly with increasing fluazifop rate for the remaining four species, except for the 6- to 7-leaf stage in Kentucky bluegrass and the 4- to 6-leaf stage in tall fescue and perennial ryegrass. Significant differences between growth stages within each rate varied with species. In general, tolerance increased at the 4- to 6-leaf and 6- to 7-leaf growth stages.

In greenhouse studies, soil organic matter reduced the herbicidal activity of ethalfluralin (N-ethyl-N-(2-methyl-2-propenyl)-2,6-dinitro-4-(trifluoromethyl)benzenamine). Fifty percent inhibition (I5.0) values for barnyardgrass [Echinochloa crus-galli (L.) Beauv.] stand, injury, and shoot fresh weight increased as the soil organic-matter level increased. No difference in ethalfluralin tolerance was found among 16 cucumber (Cucumis sativus L.) cultivars. When grouped according to market type, fresh market cultivars tended to be injured more than pickling cultivars by excess ethalfluralin. Both shoots and roots of cucumber absorbed the herbicide, but exposure of roots to ethalfluralin was more toxic than exposure of shoots. Field studies indicated that with certain edaphic and environmental conditions, cucumbers can be injured by preemergence applications of ethalfluralin. Injury was greatest in a low organic-matter soil following a heavy rain. Ethalfluralin at 1.3 kg/ha gave adequate weed control in 1 and 3% organic-matter soils, but not in a 9% organic-matter soil.

Terbacil (3-tert-butyl-5-chloro-6-methyluracil) metabolism was evaluated as a possible basis of tolerance in two species, strawberry (Fragaria × ananassa Duchesne) and goldenrod (Solidago fistulosa Miller). Reported cultivar variation in strawberry tolerance to terbacil was examined by comparing herbicide metabolism patterns in an established tolerant cultivar, 'Sunrise’, with those of the reported susceptible cultivar, ‘Guardian’. A terbacil-sensitive plant, cucumber (Cucumis sativus L. ‘Chipper’), was also used as a basis for comparison. Using gradient elution high performance liquid chromatography (HPLC), two terbacil metabolites were separated and quantitated from methanol extracts of the three species treated with 14C-terbacil via roots in solution culture. The minor metabolite was identified as the non-phytotoxic derivative, 3-tert-butyl-5-chloro-6-hydroxymethyluracil, based on its co-migration with authentic 3-tert-butyl-5-chloro-6-hydroxymethyluracil in two chromatographic systems. The major metabolite was a glycoside, which yielded the hydroxylated derivative upon β-glucosidase hydrolysis. In all species, metabolites accumulated more rapidly and extensively in roots than in leaves. Metabolism was greater in the two tolerant species than in cucumber. However, the greater tolerance of goldenrod to terbacil compared to that of strawberry was apparently unrelated to differences in herbicide metabolism.

Uptake, translocation, and metabolism of 14C-terbacil was investigated in 12-leaf (tolerant) and 3-leaf (susceptible) field violet plants. Field violets with 12 leaves absorbed less 14C-terbacil g−1 of fresh weight from solution culture than did plants with three leaves. Plants with three leaves translocated twice as much radioactivity to foliage than did plants with 12 leaves. Most 14C in roots (77%) and foliage (57%) of field violet plants with 12 leaves was in polar metabolites. Metabolism studies indicated that most 14C (79%) in foliage extracts from field violet plants with three leaves was 14C-terbacil. Polar metabolites were not detected in roots of field violet plants with three leaves.

Herbicides were evaluated for control of field violet in strawberries in North Carolina and in Nova Scotia, Canada. DCPA at 11.0 kg ai ha−1 and terbacil at 0.5 kg ai ha−1 PRE controlled field violet 80 to 95%. Control with simazine PRE at 1.0 kg ai ha−1 was 70 to 75%. Oxyfluorfen at 0.6 to 1.1 kg ai ha−1 controlled 95 to 100% of the established field violet in dormant strawberries following a mid-winter application. Control with acifluorfen and lactofen applied during mid-winter was 75 to 90%. Oxyfluorfen applied to dormant plantings at rates in excess of 0.60 kg ai ha−1 caused foliar damage; however, yield was not reduced at rates up to 4.5 kg ha−1. Oxyfluorfen at 0.25 kg ha−1 caused severe injury to newly-transplanted strawberries.

Tomatoes (Lycopersicon esculentum ‘Roma VF’) and weeds were seeded at two locations in 1973 and 1974. In separate plots, jimsonweed (Datura stramonium L.), tall morningglory [Ipomoea purpurea (L.) Roth.], and common cocklebur (Xanthium pensylvanicum Wallr.) seedlings were thinned to densities ranging from 2.7 to 86 plants/m2. Densities of large crabgrass [Digitaria sanguinalis (L.) Scop.] ranged from 11 to 430 plants/m2. There were a few instances of difference in nutrient concentrations in tomato and weed leaf tissue, but no clear relationship was evident between concentration of N, P, K, Ca, Mg, and S and weed density. Considerably more fruit weight was produced per kilogram of total assimilated N, P, and K in the weed-free than in the weed-infested tomato plots.

Field studies were conducted to determine response of ‘Atlantic’ and ‘Fontenot’ potato varieties and eight weed species to rimsulfuron and metribuzin. Rimsulfuron was applied at 9, 18, 27, 35 and 70 g ai/ha and metribuzin at 69, 140, 210, 280 and 560 g ai/ha. Each herbicide was applied PRE or POST alone or in 1:8 combinations of rimsulfuron to metribuzin. Yield of either variety was not reduced by any treatment. Rimsulfuron applied PRE or POST at the suggested field use rate of 35 g/ha controlled smooth pigweed, redroot pigweed, ivyleaf morningglory, and common cocklebur 100%, common lambsquarters 90%, and jimsonweed 60%. Large crabgrass and fall panicum were controlled 92% by rimsulfuron at 35 g/ha. Reduced rates of 9 g/ha rimsulfuron plus 69 g/ha metribuzin, applied PRE or POST, controlled all weeds 100% except jimsonweed, fall panicum, and large crabgrass. For complete control of these weeds, rimsulfuron at 35 g/ha plus metribuzin at 280 g/ha was required.

Radiolabeled (14C) 2-[4-(3-chloro-5-trifluoromethyl-2-pyridinyloxy)phenoxy] propionic acid (Compound C); its methyl-,n-butyl-, and ethoxyethyl-esters; and fluazifop-butyl {(±)-butyl 2-[4-[(5-(trifluoromethyl)-2-pyridinyl)oxy] phenoxy] propionate} were applied to the primary shoots of young rhizomatous quackgrass [Agropyron repens (L.) Beauv. # AGRRE]. Plants were sampled from 0.5 to 24 days after treatment (DAT) and analyzed for radiochemical. All treatments caused phytotoxic symptoms in primary shoots, rhizomes, and tillers and significantly reduced growth of primary shoots and rhizomes. Treatment with compound C or its methyl and butyl esters eliminated regrowth from all rhizomes excised from treated plants. The ethoxyethyl-ester and fluazifop-butyl controlled regrowth from rhizomes from 50% of the plants and substantially reduced shoot regrowth from the remainder. Rhizomes that did produce shoots contained significantly less radiochemical than those from which no regrowth occurred. At 24 DAT, a maximum of only 1% of each radiochemical applied was translocated to the first tiller and rhizomes and had been sufficient to cause marked phytotoxic symptoms. Soon after application (1 DAT), the major metabolites in the treated leaves and remainder of the plant for all compounds were the free acid and substantial amounts of polar conjugates that were hydrolyzable to the free acids. Radiochromatography of extracts from rhizomes and first tillers from all treatments gave similar chemical profiles, with the free acids and their conjugates as the predominant components present. These results provide evidence that esters of the pyridinyloxyphenoxypropionic acids are rapidly hydrolyzed after absorption by quackgrass; the free acids are then translocated and are the active form of these herbicides.

Hexazinone [3-cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4(1H,3H)-dione] toxicity, absorption, translocation, metabolism, and effect on photosynthesis were investigated with rooted cuttings of highbush blueberry (Vaccinium corymbosum L.), rabbiteye blueberry (V. ashei Reade), and hollow goldenrod (Solidago fistulosa Miller # SOOFI). Highbush and rabbiteye blueberry plants were three times more tolerant to root applications of hexazinone than hollow goldenrod. Blueberry plants absorbed an average of 7.9% of root-applied 14C-hexazinone and hollow goldenrod absorbed an average of 10.1%. An average of 6.8% of root-absorbed hexazinone (14C-label) was translocated from the roots of blueberry to stem and leaves. Radioactivity in hollow goldenrod was distributed equally between roots and shoots. The majority of radioactivity in both species was recovered as hexazinone. Root-absorbed hexazinone caused a rapid inhibition of photosynthesis in intact hollow goldenrod leaves. Root-absorbed hexazinone was capable of inhibiting photosynthesis in intact blueberry leaves; however, this occurred only when roots were exposed to high concentrations of hexazinone.

The butyl ester of fluazifop {[(±)-2-[4-[[5-trifluoromethyl)-2-pyridinyl] oxy] phenoxy)propanoic acid} at 0.26 μM in nutrient solution inhibited root growth of hydroponically grown goosegrass (Eleusine indica Gaertn. ♯ ELEIN), large crabgrass [Digitaria sanguinalis (L.) Scop. ♯ DIGSA], and giant foxtail (Setaria faberi Herrm. ♯ SETFA). Treating the soil and plant foliage at 0.035 or 0.07 kg ai/ha did not result in greater phytotoxicity than exposing only the foliage of each grass to the herbicide. Foliar-applied fluazifop was retained on the foliage in similar amounts by each of the species. Translocation of 14C to all plant parts was detected 6 h after foliar application of the butyl ester of 14C-fluazifop to the grasses in the pretillering or tillering stage. The majority (90%) of 14C absorbed by each of the species remained in the treated leaf. In hydroponic studies, each species exuded 14C into nutrient solution following foliar application of the 14C-labeled herbicide. The exuded material was predominantly fluazifop with small amounts of compounds more polar than the butyl ester of fluazifop. Uptake and translocation studies suggest that the greater sensitivity of goosegrass to fluazifop may be related to higher concentrations of the herbicide present in plant tissue.