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Experiments were conducted on the use of granular fenuron, 25% active, for white pine release and establishment. The effect of fenuron on the plant community was also investigated. Studies included in the white pine release experiment were the effect of season of treatment on the kill of hardwoods and percentage survival of indigenous white pine, and the effectiveness of fenuron on various species of hardwoods. All hardwood stems over six feet tall were treated with four grams of active fenuron placed on the soil at the base of the tree. Treatments were made in May 1960, August 1960, and March 1961. The May treatment was most effective in the removal of the hardwoods. There was a reduction of 59% of pines over six feet tall and an increase of 45% of pines less than six feet tall. Red maple and the oaks were the species most susceptible to the treatment; whereas, chestnut, sassafras, and black locust were the least susceptible. In the white pine establishment experiment, the residual effect of fenuron on white pine seedlings, and its effectiveness at a lower rate on various hardwoods were lnvestigated. Each hardwood stem over six feet tall was treated with one gram active fenuron in the spring of 1960. In March 1961, 100 white pine seedlings were planted in each of three plots previously treated. Five months after planting there was 99% survival of the pine seedlings. The one-gram rate was effective against red maple and the oaks but had little effect on sassafras, cucumber tree, and chestnut. Fenuron in some manner changed the composition of the plant community. Fireweed, poke, and several species of Panicum were present in the treated plots but were not present in the untreated buffer strips. / Master of Science

The activity and the selectivity of cycloxydim {2-[1-(ethoxyimino)butylJ-3-hydroxy- 5-(2H-tetrahydrothiopyran-3-yl)-2-cyclohexen-l-one}, code designation BAS 517, were examined flIst with etiolated seedlings of com (Zea mays L.) and soybean [Glycine max (L.) Merr.]. Etiolated soybean seedlings were not affected by cycloxydim. The degree of growth inhibition of com varied with concentration of cycloxydim and incubation time. Compared to mesocotyls and coleoptiles, radicles of corn were the most sensitive to cycloxydim. Meristematic tissues appeared to be the site of action of cycloxydim as root meristems were the first to show symptoms. A band of reddening tissue developed at meristematic tips followed by the complete cessation of root growth. In a study comparing activities of technical grade and formulated cycloxydim and sethoxydim, {2-[ l-(ethoxyimino )butyl}- 5-[2-(ethylthio )propy11-3-hydroxy-2-cyclohexen-l-one}, formulated compounds were more potent than the technical grade chemicals without formulation additives. Technical sethoxydim was more potent than technical cycloxydim. Root tips excised from com and soybean seedlings were used subsequently for cycloxydim treatments. The activity and selectivity of cycloxydim expressed at the isolated root tip level were similar to those of cycloxydim bioassayed with whole seedlings. However, root tips appeared to be more sensitive than the whole seedlings. Injury at the tissue and cell levels of the 2-mm root tips that were treated with various concentrations of cycloxydim was examined after 24 hours incubation. Concentrations of 0.1, 1, and 10 μM cycloxydim caused severe cell vacuolization. A gradient of decreasing injury from epidermal cells toward the center of roots was observed. This pattern of injury appeared to reflect the penetration of cycloxydim into roots along a concentration gradient. / Ph. D.

Fields studies conducted in 1987 and 1988 determined the weed control efficacy of ICIA-0051 and SC-0774 in conventional and no-till systems of corn (<i>Zea mays</i> L.) culture. Results of the preemergence and postemergence applications of ICIA-0051, across all treatments after 8 weeks, showed 85% control or better of triazine-resistant smooth pigweed (<i>Amaranthus hybridus</i> L.), while fall panicum (<i>Panicum dichotomiflorum</i> Michx.) control ranged from 43 to 87%. Giant ragweed (<i>Ambrosia trifida</i> L.) control ranged from 30 to 95%, while control of ivyleaf morningglory (<i>Ipomoea hederacea</i> (L.) Jacq.) was below 75% in the preemergence treatments and ranged from 89 to 99% in the postemergence treatments. In general, the addition of atrazine to the pre- and postemergence treatments of ICIA-0051 improved weed control. SC-0774 treatments gave 85% or better control of fall panicum, but inadequate broadleaf weed control (75% or worse). Soil mobility studies using soil thin-layer chromatography and soil leaching columns indicated that the movement of ICIA-0051 was highly negatively correlated with the organic/humic matter fraction. Although the mobility patterns were similar, ICIA-0051 was more mobile than SC-0774, which was more mobile than atrazine. A comparison of ICIA-0051 across soils indicated that the order of mobility was Appling loamy sand (Rf = 6.4) > Davidson clay (Rf = 5.6) > Bojac sandy loam (Rf = 5.0) = Frederick silt loam (Rf = 4.9) > Hyde silty clay loam (Rf = 1.1). Other soil properties such as the clay content and pH were not strongly correlated with ICIA-0051 movement. Results of the adsorption/desorption studies indicated that the organic/humic matter fraction was primarily responsible for the binding and retention of ICIA-0051 across the five soils investigated. Based on the K constants derived from the Freundlich equation, the order of adsorption was Hyde > Frederick > Davidson = Bojac > Appling. The desorption results indicated that ICIA-005l was not tightly bound to the soil particles, with losses between 20 and 50% of the amount adsorbed after two desorptions. Results of the greenhouse persistence study, using mustard (<i>Brassica kaber</i> L.) as a bioassay species, indicated that ICIA-0051 was more biologically available than atrazine. Similar to the adsorption and leaching results, the persistence of ICIA-0051 was highly positively correlated with the soils’ organic matter. Regardless of the rate used, crop injury decreased over time, although the highest rate (1 ppm of ICIA-0051) showed significant crop injury even after 6 months in several soils in the greenhouse studies. / Ph. D.

Thifensulfuron is a new herbicide of the sulfonylurea class under development by E. I. Dupont de Nemours Company Inc. for postemergence broadleaf weed control in soybeans [Glycine max (L.) Merr]. Field studies evaluated the influence of adjuvants and chlorimuron upon the efficacy of thifensulfuron. Thifensulfuron applied alone provided smooth pigweed (Amaranthus hybridus L. #AMACH) control at application rates 12% of those of the similar herbicide chlorimuron. Nonionic surfactant or crop oil concentrate increased soybean sensitivity to thifensulfuron, but an adjuvant was required to obtain consistent seedling common lambsquarters (Chenopodium album L. #CHEAL) control. Chlorimuron and thifensulfuron combinations did not control ivyleaf morningglory [Ipomoea hederacea (L.) Jacq. #IPOHE]. Greenhouse studies evaluated soybean cultivar sensitivity to thifensulfuron. Seven popular Virginia soybean varieties and one national variety (Williams 82) were screened for tolerance to thifensulfuron. Differences in varietal sensitivity was verified. Soybean varieties Vance, Essex, Hutcheson, and York proved to be more sensitive to 9.1 g ha⁻¹ thifensulfuron than FFR 561, Williams 82, or Deltapine 105. No relationship between sensitivity to thifensulfuron and Essex parentage could be drawn. The selectivity of the sulfonylurea class of herbicides is reportedly based on differential metabolism of the herbicide between sensitive and tolerant weed and crop species. Laboratory studies were conducted utilizing thifensulfuron-sensitive and tolerant weed species, velvetleaf (Abutilon theophrasti Medic. #ABUTH) and spurred anoda [Anoda cristata (L.) Schlecht #ANVCR], respectively, as well as the relatively tolerant Williams 82 and sensitive Vance soybean. Absorption and distribution studies indicated that all species absorbed and translocated similar amounts of ¹⁴C 1, 3, and 5 days after application of the methyl ester of [¹⁴C-thiophene] thifensulfuron. Metabolism studies indicated that both tolerant spurred anoda and sensitive velvetleaf metabolized thifensulfuron at similar rates 3 days after treatment. Metabolism appears to be the major mechanism for the selectivity of thifensulfuron to soybeans. The mechanism for spurred anoda tolerance to thifensulfuron has yet to be determined. This research indicates that broadcast foliar applications of 4.5 g ha⁻¹ thifensulfuron with 0.125% v/v nonionic surfactant or 1% v/v crop oil concentrate can provide selective postemergence smooth pigweed and common lambsquarters control for soybean production in Virginia. Caution should, however, be taken in prescribing greater than 4.5 g ha⁻¹ thifensulfuron due to the variability in cultivar sensitivity to thifensulfuron. / Ph. D.