Experiments were conducted in 1990 and 1991 to determine bioavailability of PRE herbicides at three soil profile depths and two soil types in established common bermudagrass sods. Prodiamine and oryzalin reduced ‘Tifgreen’ bermudagrass root growth in the 5- to 7.5-cm layer of a Bosket very fine sandy loam soil at 2 and 4 wk after treatment in 1991. These herbicides decreased bermudagrass root weight in both the Bosket and Marietta sandy clay loam both years in samples taken from the 2.5- to 5-cm depth layer. In the 0- to 2.5-cm soil layer, all herbicides reduced root weight in 1990. In a bioassay, prodiamine caused decreased Tifgreen bermudagrass root growth at concentrations as low as 4 ppb by wt in the very fine sandy loam soil, while 8 ppb was necessary in the sandy clay loam soil. Prodiamine was detected in the very fine sandy loam at 4 wk after treatment at all depths in 1991 (65, 45, and 39 ppb in the 0- to 2.5-, 2.5- to 5-, and 5- to 7.5-cm soil layers, respectively). Oryzalin was also detected at all depths in 1991 when sampled at 2 and 4 wk after treatment in the very fine sandy loam. Pendimethalin was present in concentrations of 38, 39, and 37 ppb in the sandy clay loam at 2 wk after treatment in the 0- to 2.5-, 2.5- to 5-, and the 5- to 7.5-cm soil layers, respectively. Pendimethalin was also detected in the very fine sandy loam at 2 wk after treatment at concentrations of 55, 69, and 36 ppb in the 0- to 2.5-, 2.5- to 5-, and 5- to 7.5-cm soil layers, respectively.

Effects of nonchemical weed control on weed and seedbank demography in ridge-tilled fields were simulated to examine long-term effects on weed population dynamics. Ridge tillage entails active seedbank management, including removal of much of the seedbank from the crop growth zone (ridge) to the interridge zone (furrow) at planting, and other seedbank manipulations. Dynamics of simulated yellow foxtail and velvetleaf populations were sharply sensitive to the rate of seed removal from the ridge. Variation in winter seed survival, ridge plant demography (emergence, survival, and seed production), and seed survival in the furrow after removal from the ridge also had strong effects on simulated dynamics. To prevent rapid population growth in a simulated corn-soybean rotation, very effective control of seedlings emerging with each crop was necessary to offset seed production from seedlings emerging after ridge rebuilding in corn years. Dynamics of both species were highly sensitive to cultivation efficiency. Effective “rescue” weed control must be available to prevent sharp increases in weed seedbanks during years when cultivation is ineffective.

A mixture of bermudagrass with creeping bentgrass on golf greens results in a poor putting surface. An experiment was conducted to determine the frequency of herbicides applied alone or with flurprimidol on growth suppression of three bermudagrass cultivars and on tolerance of creeping bentgrass. Ethofumesate plus flurprimidol applied initially in mid-March or early April at 1.7 + 0.8 kg ha−1 and followed by four applications of 0.4 + 0.2 kg ha−1 beginning 6 wk later and then at 3-wk intervals, effectively suppressed bermudagrass growth throughout the growing season regardless of cultivar. Creeping bentgrass treated with ethofumesate plus flurprimidol at 1.7 + 0.8 kg ai ha−1 caused > 30% injury for 3 to 4 wk following application in 2 of 3 yr, but the turfgrass fully recovered. Injury was < 30% when ethofumesate plus flurprimidol was applied as repeated applications at 0.4 + 0.2 kg ha−1. Siduron plus flurprimidol applied initially in March or April at 54 + 0.8 kg ai ha−1 and followed by four additional applications at 14 + 0.2 kg ha−1 did not effectively suppress bermudagrass growth. Results with siduron alone at 54 kg ha−1 in March or April and repeated at monthly intervals at the same rate throughout the summer were similar to that obtained with siduron plus flurprimidol. Fenoxaprop applied at 0.07 kg ai ha−1 in early April and repeated at monthly intervals through August at the same rate effectively suppressed growth of common bermudagrass but did not suppress growth of Tifway or Tifgreen bermudagrasses. Creeping bentgrass was severely injured following fenoxaprop treatments.

This research presents a modified floatation method to extract weed seed from soil samples and an automated imaging technique to enumerate seed following extraction. Rapid floatation of weed seed from soil in a K2CO3 solution was facilitated by centrifugation. High recovery of seed from soil samples was accomplished in a relatively short time. Recovery of seed of giant foxtail and velvetleaf from a silt loam soil was near 100% and recovery of redroot pigweed was 86%. Germination of common lambsquarters, giant foxtail, and velvetleaf seed was reduced by exposure to K2CO3 under certain conditions. However, these reductions do not appear to reduce the utility of this extraction technique. The automated image capture technique for counting seed showed similar limitations as with manual counting. Seed count accuracy generally decreased with increased number of seed and smaller seed sizes. In general, the image capture technique compared favorably with manual seed counting and has the potential to further improve seedbank assessment methods.

Tolerance of barley to chlorsulfuron at various stages of crop development was studied in weed-free field and pot experiments. Five barley crops were sown between mid-May and mid-July in the field in 1988 and sprayed simultaneously with four rates of chlorsulfuron when the corresponding stages of crop development ranged from growth stage 7 (GS 7) to GS 31. Early-sown barley yielded more than late-sown barley, but percent grain yield reduction from chlorsulfuron was similar for all planting dates. In a field experiment in 1989, barley was sown at three dates (May, June, and July) and each was treated with chlorsulfuron at 0, 15, 30, and 60 g ai ha−1 at either GS 12 or GS 15. Grain yield of barley sown in May and treated with 15 g ha−1 chlorsulfuron at GS 12 was reduced 35%, compared with no effect when applied at GS 15; this yield reduction was related to spike size and number. Application of chlorsulfuron at GS 12 and GS 15 to barley sown in June caused yield reductions of 20 and 11%, respectively. In contrast, barley sown in mid-July was not affected by chlorsulfuron. Applications of chlorsulfuron at GS 12,13,14, and 15 led to reductions of 75,35,15, and 5% shoot dry matter (DM) of plants grown in pots and measured at GS 33. Root DM was more sensitive to chlorsulfuron than shoot DM. Tolerance of barley to chlorsulfuron increased with age of the plant at the time of application.

Irrigated sandy soils have a high potential for leaching of agricultural chemicals to ground water. Objectives of this study were to determine the movement and persistence of metolachlor applied annually to a Hubbard loamy sand soil (Udorthentic Haploboroll) under irrigated potato production. Metolachlor was applied to the plots at rates of 1.7 and 3.4 kg ai ha-1. A KBr tracer was applied to the same plots at the rate of 130 kg ha-1. Intact soil cores were collected before and after herbicide application during the cropping season. Water samples were collected weekly from suction samplers, 90 and 150 cm deep in each plot, during the growing season. Potato yields of 60 and 59 Mg ha-1 in 1989 and 1990, respectively, were above Minnesota state averages. Bromide tracer data confirmed the movement of water through the soil profile to at least a 150-cm depth. No detectable quantities of metolachlor were found below 30 cm in soil during either year. Time for dissipation of 50% of metolachlor ranged from 85 to > 106 d. Dissipation in 1990 appeared slower than in 1989 and residues remaining at the end of the season, averaged across application rates, were 44 and 64% in 1989 and 1990, respectively. No water samples contained metolachlor in 1989. Metolachlor was detected in 18 of 409 water samples collected during and after the growing season in 1990. Median detectable concentration of metolachlor was 0.4 μg L-1 in water samples from suction samplers at 90- and 150-cm depths in eight of 16 plots. Metolachlor Kd and Koc values, 3 and 220, respectively, were similar to reported values. Metolachlor sorption in the soil profile was correlated, in order of contribution, to organic carbon content, cation exchange capacity, percent sand, and percent clay.

Laboratory, greenhouse, and field studies were conducted to determine the allelopathic potential of lantana and Siam weed on the germination and growth of five crops. Emergence and dry weight of bioassay species were affected when lantana or Siam weed debris was present on the soil surface or incorporated into the soil. However, presence of Siam weed on the soil surface did not affect dry weight of rape. Emergence and dry weight of crops (except emergence of spinach) were not affected when crops were grown in soil samples collected from beneath field-grown lantana. Dry weight of Chinese cabbage and chilli was reduced when these crops were grown in Siam weed-contaminated soil. Germination of Chinese cabbage, chilli, and rape decreased progressively when exposed to increasing concentrations of aqueous lantana extract. However, the lantana extract at full strength (66.7 g L−1) did not reduce germination of spinach and cucumber seed. Siam weed extract, when applied at full strength to seed of spinach, Chinese cabbage, rape, and chilli, reduced germination by 10, 12, 21, and 19% of control, respectively. Full-strength extract of lantana and Siam weed decreased seedling length and fresh weight of all crops.

Studies were conducted to investigate the uniformity of epicuticular wax deposition on leaf blades of johnsongrass. Johnsongrass leaves grown under drought stress had greatly increased epicuticular wax weights compared to leaves from plants with adequate moisture, but relative humidity (95% vs. 40 ± 5%) had little effect on wax deposition. Wax weights decreased as leaves matured. Sections of lower leaf surfaces of young johnsongrass leaves tended to have more wax than sections of upper leaf surfaces, but weights were nearly equal on upper vs. lower leaf surfaces of older leaves. The narrow side of asymmetrical johnsongrass leaf blades often had more wax per unit area than the wide side. The area over the midvein contained more wax per unit area than either the narrow or wide side of the leaf blade. Greatest wax concentrations on individual leaves were over the midvein area near the leaf apex. Leaf blades of johnsongrass had more wax per unit area than leaves of corn or grain sorghum.

Effects of a crude preparation of a phytotoxin from Pseudomonas fluorescens strain D7 (D7) on various physiological processes were evaluated in roots of downy brome seedlings. Cell division, respiration, and synthesis of protein, RNA, and DNA were not inhibited or only slightly inhibited under treatment conditions that caused substantial inhibition of root elongation. Significant inhibition of each of these processes was detected by known inhibitors included in each study to verify sensitivity of the procedures used. Disruption of lipid synthesis and membrane integrity by the crude phytotoxin preparation was significant and might account for inhibition of root elongation. Additional studies on these two possible target sites were conducted with partially purified phytotoxin. In intact roots, incorporation of [14C]acetate and [14C]malonic acid into lipophilic fractions was reduced by 50% or greater during a 1-h treatment with concentrations between 10 and 100 μg ml−1 of the partially purified phytotoxin preparation. In a membrane integrity study, a similar treatment increased radioactivity efflux 250% from seedlings preloaded with scyllo-[R-3H]inositol. Additionally, disruption of lipid synthesis and membrane integrity by the partially purified phytotoxin was dose dependent. Collectively, these findings indicate that the D7 phytotoxin may inhibit downy brome root elongation through its effects on lipid synthesis and membrane integrity.

Experiments were conducted on a Cedarbluff silt loam and a Cecil sandy loam to determine dissipation of imazaquin and imazethapyr as influenced by burning small-grain straw and tillage prior to soybean planting. Corn bioassay detection limits for imazaquin and imazethapyr in the Cedarbluff silt loam were 2.5 to 30 and 5 to 40 ppbw, respectively. Bioassay detection limits for imazaquin and imazethapyr in the Cecil sandy loam were 2.5 to 20 and 10 to 40 ppbw, respectively. Imazaquin and imazethapyr activity was not detectable in soil by 110 to 152 d after treatment. Imazethapyr dissipation was not affected by burning or tillage in the Cedarbluff silt loam and dissipated more slowly in 1989 than imazaquin. Imazaquin dissipation in the Cedarbluff silt loam in 1988 was slower in burned plots than in nonburned plots but was not affected by burning in 1989. No differences were observed between imazaquin and imazethapyr dissipation in the Cecil sandy loam and neither burning or tillage influenced their rate of dissipation in either year. No-till-planted cotton was injured at both locations by imazaquin and imazethapyr that had been applied the previous year.

This study was conducted to determine if field evaluations could be used to select insects for biological control of musk thistle. Host specificity and larval development of a weevil, Hadroplonthus trimaculatus, and a fleabeetle, Psylliodes chalcomera, were studied in field trials near Rome, Italy, in which insects were allowed free choice of several hosts. Natural populations of these two insects, which do not occur in North America, were exposed to North American species of Cirsium, Carduus, and selected crops. Adult insects and larvae on host plants were identified and counted on test plants from North America and native attraction plants. In addition to infesting musk thistle, weevil adults and larvae were recorded on flodman thistle, wavyleaf thistle, and spinosissimum thistle. Consequently, this insect was not suitable for introduction into North America. The fleabeetle would be satisfactory for biological control since no adults or larvae were recorded on Cirsium spp. or economic plants. These studies show that field trials are a valid method for identifying specific and nonspecific candidate insects for biological control of weeds.

Knowledge of timing and extent of weed emergence before and immediately after crop seedbed preparation is needed to decrease need for preplant herbicides and increase efficacy of postemergence weed control in crops with either mechanical or chemical methods. Such knowledge is important for weeds that infest most crops over a wide area. For these reasons a mechanistic seedling emergence model based solely on soil temperature was developed for common lambsquarters. The model was validated using four sets of field data collected in 1988, 1990, and 1991 near Morris, MN. Agreement of predicted and observed emergence values across all site-years was 0.95 and the coefficient of determination (R2) was 0.98 (P < 0.001). Agreement for individual site-years was 0.96, 1.08, 1.08, and 0.98 and associated R2 values were 0.99, 0.99, 0.99, and 0.98 (P < 0.001 for each site-year), indicating close agreement between predicted and actual emergence values.

Terbufos and primisulfuron interactions were evaluated under growth chamber conditions using a sand culture system. Terbufos was applied to transplanted corn seedlings, followed in 5 d by foliar applications of primisulfuron plus nonionic surfactant. Primisulfuron and terbufos alone did not cause corn injury; however, shoot dry weight and shoot length were reduced 28 and 36% in terbufos treatments 96 h after primisulfuron application. Primisulfuron absorption and translocation were not affected by terbufos, but the half-life of primisulfuron increased from 2 to 3.5 h in terbufos treatments. Terbufos did not affect primisulfuron metabolite profiles. The basis for increased primisulfuron phytotoxicity in terbufos treatments appeared to result from reduced primisulfuron metabolism.

A large-scale continuous system for recovery of root exudates from millet and cowpea was developed and tested extensively. Large quantities of germination stimulants were exuded into the medium flowing through the plant trays, which could be demonstrated by germination of seeds of three species of angiospermous parasitic plants, Alectra vogelii, Striga gesneriodes, and witchweed. Recovery of root exudates was affected by adsorption on the macroreticular polymer resin XAD-4 followed by desorption with methanol. Furthermore, indications were found that germination inhibitors are released into the medium. Subsequent testing confirmed the quantitative release of the germination stimulants as well as a large number of other compounds.

Laboratory experiments were conducted to determine the selectivity and inhibitory activity of BAS 517 using whole seedlings or root tips of corn and soybean. Effects of BAS 517 on the morphology of corn root tips were examined as well. Etiolated corn seedlings showed high sensitivity to BAS 517; soybean seedlings were not affected. Growth inhibition of corn varied with concentrations of BAS 517 and incubation time. Radicles of corn were more sensitive than mesocotyls and coleoptiles. Root meristems were the first to show symptoms (reddening tissue followed by cessation of root growth). Results using root tips were similar to those using whole seedlings. However, root tips appeared to be more sensitive than whole seedlings. Concentrations of 0.1 and 10 μM of BAS 517 caused severe vacuolization of cells in the 0.2-cm root tip of corn. A pattern of decreasing injury from epidermal cells toward the centers of roots was observed.

In greenhouse experiments, conidia of Colletotrichum truncatum applied in an invert emulsion formulation controlled hemp sesbania 100% in the absence of a dew treatment. In field experiments, hemp sesbania control averaged 95 and 97% in 1989 and 1990, respectively, when this formulation was applied to hemp sesbania seedlings using tractor-mounted, air-assist nozzles. This level of weed control was comparable to that achieved from the herbicide acifluorfen. These results indicate that C. truncatum has excellent potential as a mycoherbicide for controlling hemp sesbania and that this potential can be augmented by formulating the pathogen as an invert emulsion.

A survey was conducted in Mississippi from 1976 and in Arkansas and Louisiana from 1977 through 1991 to determine annual variation of johnsongrass infestation in cotton and soybean fields. The survey route was 1534 km long. Levels of johnsongrass infestation were estimated in 752 ± 296 cotton fields and 884 ± 407 soybean fields in each year of the survey. The area of fields surveyed annually was 47 000 ± 21 000 ha of cotton and 52 000 ± 19 000 ha of soybeans. Johnsongrass was present in 55 to 90% of the cotton fields. The percent of cotton fields with johnsongrass was about the same in 1991 as in 1976–77. About 90% of the soybean fields in Mississippi and Louisiana had johnsongrass, but only 70 to 80% of soybeans fields in Arkansas had johnsongrass. Soybean fields with 1 to 5% levels of infestation increased slightly in Mississippi but remained about the same in Arkansas and Louisiana. Soybean fields with infestations of 6% or more slightly decreased in Arkansas and Louisiana but not in Mississippi. It was estimated that johnsongrass reduces the average annual value of harvested cotton $5.8 ± 1.9 million and soybeans $23.7 ± 0.6 million in the three states.

Studies were established in 1988, 1989, and 1990 at two locations in Missouri to study the influence of fall tillage and herbicides on carryover of several residual soybean herbicides to grass and legume forage crops. Chlorimuron, clomazone, imazaquin, imazethpyr, and metribuzin plus chlorimuron were applied at their registered and 2X-registered rates in soybeans. Forage crops were planted the following fall and spring after herbicide application and evaluated for carryover effects. Fall tillage did not influence the carryover potential of these herbicides. However, herbicides injured several of the rotational crops. This injury was crop species and herbicide specific.