Imbibition of pretreated velvetleaf (Abutilon theophrasti Medic.) seed was most rapid when the chalazal region was wetted, but submergence of the chalaza in water delayed seed germination. Respiratory measurements indicated that the seed coat did not inhibit gas exchange under our experimental conditions. Effects of 2,4-dinitrophenol on germination indicated that radicle emergence and radicle growth were distinct processes, and that radicle emergence was more sensitive to low concentrations of the uncoupler.

Aerial applications of pelleted tebuthiuron (N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N, N′-dimethylurea} at 1 kg/ha to rangeland reduced canopy cover of the subshrub false broomweed (Ericameria austrotexana M.C. Johnst.) by 69 to 78%. Complete control was obtained with 2 kg/ha of tebuthiuron. Picloram (4-amino-3,5,6-trichloropicolinic acid) pellets applied at 2 kg/ha controlled false broomweed in two of four experiments; effectiveness of soil-applied picloram appeared to depend upon sufficient rainfall to move the herbicide into the soil soon after treatment. After the first growing season following application, density of herbaceous species tended to be higher in plots receiving either herbicide than in untreated plots. Picloram suppressed curlymesquite [Hilaria berlangeri (Steud.) Nash], but had little effect on other components of the herbaceous vegetation. Abundance of curlymesquite increased substantially following applications of tebuthiuron, at the expense of annual and short-lived perennial grasses and herbaceous broadleaf species.

Two applications of benefin (N-butyl-N-ethyl-α,α,α,-trifluoro-2,6-dinitro-p-toluidine), profluralin [N-(cyclopropylmethyl)-α,α,α-trifluoro-2,6-dinitro-N-propyl-p-toluidine], prosulfalin N-[[4-(dipropylamino)-3,5-dinitrophenyl]sulfonyl]-S,S-dimethylsulfilimine, and napropamide [2-(α-naphthoxy)-N,N-diethylpropionamide] provided full-season large crabgrass [Digitaria sanguinalis (L.) Scop.] control in turfgrass. Benefin was applied at 3.3 kg/ha in March and 2.2 kg/ha in May while other herbicides were applied at 2.2 kg/ha in March and May. Satisfactory goosegrass [Eleusine indica (L.) Gaertn.] control was obtained with a single March treatment with prosulfalin, but two applications of napropamide in March and May were required for similar control. None of the herbicide treatments severely injured common bermudagrass [Cynodon dactylon (L.) Pers.] or Kentucky bluegrass [Poa pratensis (L.) ‘Common’] in these studies. Prosulfalin at 6.6 kg/ha applied as single treatment or 3.3 kg/ha in each of two applications resulted in moderate injury to Kentucky bluegrass in 1 of 2 yr.

At least two weedings within the first 5 weeks after sowing are necessary to minimize yield reduction caused by weeds in cowpea [Vigna unguiculata (L.) Walp.]. During a 3-yr study, a preemergence application of metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] at 2.0 to 3.0 kg/ha, DCPA (dimethyl tetrachloroterephthalate) at 10.0 kg/ha, or pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine] at 2.0 or 2.5 kg/ha controlled most annual grasses effectively. Itchgrass (Rottboellia exaltata L.) was controlled with pendimethalin at 2.5 kg/ha, but metolachlor had no effect on this weed. These herbicides caused no crop injury and cowpea grain yield was as good as in the handweeded and weed-free plots. Alachlor [2-chloro-2′, 6′-diethyl-N-(methoxymethyl) acetanilide] applied preemergence at 2.0 kg/ha was effective against annual weeds, but was phytotoxic to cowpea.

Three grass selective herbicides, sethoxydim, fluazifop, and haloxyfop, applied to wild oat plants at the five-leaf stage inhibited growth and induced chlorosis in leaves. Young and actively growing tissues were affected first. Stem elongation in wild oat was inhibited within 2 days of treatment with sethoxydim and within 5 days of treatment with fluazifop or haloxyfop. At these same observation times, internodes that were elongating rapidly at the time of treatment were constricted at the base. These symptoms were followed by necrosis of the internode tissue. Histological examination of the affected internodes indicated that the herbicides inhibited cell division in very young internodes and inhibited both cell division and cell elongation in slightly older internodes. Initial injury occurred in the epidermal, cortical, and procambium cells of the peripheral regions of the stems located at the base of the affected internodes. Necrosis then progressed to the center of the stem tissue and all cells in the internodes were killed within 14 days of treatment. All three herbicides caused similar morphological and histological effects on developing wild oat stems.

Greenhouse and laboratory experiments were conducted to evaluate the tolerance of four commercial sugarbeet varieties to s-metolachlor and dimethenamid-P, determine the principle site of absorption of these herbicides, and determine the physiological basis for differential tolerance among varieties to these herbicides. ‘Beta 5833R’ was the most tolerant sugarbeet variety, and ‘Hilleshog 7172RZ’ was the most susceptible sugarbeet variety to injury from s-metolachlor and dimethenamid-P. The primary site of s-metolachlor and dimethenamid-P absorption was through the sugarbeet roots; however, some absorption occurred through the sugarbeet hypocotyl. Sugarbeet injury was greater from dimethenamid-P than s-metolachlor when sugarbeet was grown in soil. However, when sugarbeet was grown hydroponically, injury from the herbicides was similar, indicating that the relative availability of these herbicides in the soil greatly influenced sugarbeet injury. Reduced translocation and slower metabolism of 14C-dimethenamid-P in both the roots and shoots of the sugarbeet plants most likely contributed to the greater susceptibility of sugarbeet to dimethenamid-P compared with s-metolachlor. Metabolism of 14C-herbicides in sugarbeet shoots was 0.7 to 2.1 h slower in the more susceptible sugarbeet varieties compared with the more tolerant variety, Beta 5833R. This was the most significant factor contributing to differences in sugarbeet variety tolerance to both s-metolachlor and dimethenamid-P.

Spreading dogbane is an important weed of wild blueberry fields that decreases yields and hinders harvest operations. A range of experiments was conducted to evaluate the impact of abiotic factors on dogbane seed germination. Freshly harvested seeds were largely nondormant with viability ranging between 67 and 84%. Prolonged exposure to light neither promoted nor inhibited germination. Germination rates and total seed germination varied with temperature and osmotic potential. Significantly fewer seeds germinated at 5 C compared with 10, 15, and 20 C. There was a significant quadratic relationship between dogbane germination and osmotic potential, with significant numbers of seeds germinating at levels as low as −0.5 MPa. Emergence rates declined exponentially with depth in the soil and as many as 9% of seeds germinated but were unable to reach the soil surface. Results indicate that substantial seed germination in blueberry fields is possible and primary dispersal without wind occurs over a very short distance.

A study was undertaken to determine whether the relative leaf stages of common annual weeds and crops could serve as a reliable indicator of the time of weed emergence. Ten annual broadleaved and grass weeds were sown at successive intervals in field corn and soybean at Harrow, ON, Canada, in 1997, 1998, and 1999. All weeds emerging at a particular crop leaf stage were assigned to a cohort. Leaf numbers of the crop and different weed cohorts were recorded at 2- to 3-d intervals up to the eight-leaf stage of corn and the fourth trifoliate of soybean. For each weed species, categorical data analysis revealed a high degree of association between the leaf stage of a crop and the leaf number expected for an individual weed of a given cohort. For example, by the third trifoliate of soybean, most of the weeds emerging with the crop (VE cohort) had 8 to 10 leaves, whereas weeds in the V1, V2, and V3 cohorts averaged about seven, four, and two leaves, respectively. Year to year variation in the correspondence between crop and weed leaf numbers generally was small once variation due to time of weed emergence was removed, with one exception. Dry surface soil conditions during emergence of the VE cohort in corn in 1999 resulted in delayed leaf appearance of many weeds with respect to the crop. The relationship between weed and crop leaf stages can provide information for management decisions in two ways: (1) it indicates the relative time of weed emergence in assessing the need for control, and (2) it indicates the crop stage at which scouting for particular weed leaf stages should occur.

To assess the risk of gene movement between winter wheat and jointed goatgrass, information about the reproductive development of jointed goatgrass, winter wheat, and related hybrid plants is required. Seedlings from five jointed goatgrass populations, winter wheat, spring wheat, and jointed goatgrass by wheat reciprocal hybrid plants were exposed to 4, 7, or 10 C temperatures for 0, 2, 4, 5, 6, 6.5, 7, or 8 wk. Vernalized seedlings were transferred to a greenhouse set to 30/18 C day/night temperatures and 16-h photoperiod. Growth stages on all plants were recorded twice a week. All spring wheat and spring wheat related hybrid plants reproduced (as measured by the first reproductive node) in the absence of vernalization. Plants of jointed goatgrass population A-R, winter wheat, and winter wheat related hybrids were unlikely to reproduce in the absence of vernalization. Plants of jointed goatgrass populations B-W, G-S, E-S, and F-W reproduced in the absence of vernalization, and the likelihood that these plants would reproduce was different from all other plants. Plants that entered their reproductive phases together were not in synchronous development at anthesis. Plants in these studies differentially passed through the reproductive phases between the first reproductive node and anthesis. Our results demonstrate that variation in vernalization response exists among several jointed goatgrass populations, and reveal that the reproductive behavior of vernalized jointed goatgrass plants at anthesis is delayed compared to vernalized winter wheat and related hybrid plants. Hybrid plants produced between spring wheat and jointed goatgrass were vernalization insensitive. We hypothesize that hybridization between wheat and jointed goatgrass occurs as a result of cross-pollination between the younger reproductive tillers of jointed goatgrass and older reproductive tillers of wheat. The use of an early maturing wheat cultivar may exploit the difference in reproductive development and reduce the risk of hybrid production.

Soil samples with and without a past history of dimethyl-2,3,5,6-tetrachloroterephthalate (DCPA) treatment were obtained from New York. A third sample was obtained from a field in Colorado where DCPA had been ineffective as a herbicide. The populations of bacteria and actinomycetes in these soils were determined and the actinomycetes were more prevalent in the samples with previous DCPA treatment than in untreated samples. Subsequent treatment resulted in an increase in actinomycete population and little change in bacterial population. In pure culture and in soils, the addition of DCPA had no adverse effect upon microbial growth, and several microorganisms appeared to utilize the herbicide as a carbon source. Two degradation products were methyl-2,3,5,6-tetrachloroterephthalate and 2,3,5,6-tetrachloroterethalic acid.

Nematodes are the most damaging pathogens of cotton and one of the most important pathogens of peanut. Weeds can support nematode reproduction and reduce the effectiveness of crop rotation as a management tool. This study documents the relative host status of tropical spiderwort for (1) the reniform nematode and the southern and peanut root-knot nematodes and (2) the fungal pathogen southern stem rot. A reproductive factor (RF) was calculated for each nematode (final number divided by initial number). Galling was estimated (0–10 scale) for the root-knot species. The southern root-knot nematode reproduced well on tropical spiderwort, with a gall rating of 3.1 and an RF of 15.5. The peanut root-knot nematode also reproduced well on tropical spiderwort, with a gall rating of 2.1 and an RF of 7.2. Trials with the reniform nematode were analyzed independently. In the first trial with the reniform nematode, the RF was 2.4 on tropical spiderwort and 1.4 on cotton. In the second trial, the RF was 3.6 on tropical spiderwort and 13.5 on cotton. The severity of symptoms caused by southern stem rot was estimated on a scale of 0 to 10. In the first trial, peanut had a disease severity rating of 4.0 and tropical spiderwort had a rating of 1.4, and the fungus could be seen growing on 40% of the tropical spiderwort plants. In the second trial, peanut had a disease severity rating of 10.0 and tropical spiderwort had a rating of 5.0, but the fungus could be seen growing on all of the tropical spiderwort plants. It appears that tropical spiderwort is a sufficiently good host for some of the primary nematode and fungal pathogens of major crops in the southeastern United States that its presence at typical plant population densities will greatly reduce the pathogen-suppressive effects of crop rotation.

Research was conducted to formulate a seedling johnsongrass emergence model as influenced by temperature and burial depth using the poikilotherm rate equation. A series of constant-temperature growth chamber experiments with johnsongrass seed buried at various depths in fritted clay was conducted to develop a temperature/burial emergence database. The poikilotherm rate equation was fit to the emergence data from burial depths of 0 to 2.5 cm at constant temperatures between 20 and 44 C. These data were then combined to formulate a single poikilotherm rate equation to model the emergence of seedling johnsongrass from 0 and 2.5 cm deep and 20 to 44 C. This combined model was validated against two independent emergence data sets with good results.

We compared eight weed control systems in soybeans [Glycine max (L.) Merr.] using several combinations of the herbicides trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine), alachlor [2-chloro-2′,6′-diethyl-N-methoxymethyl)acetanilide], dinoseb (2-sec-butyl-,4, 6-dinitrophenol), chloroxuron [3-[p-(p-chlorophenoxy)phenyl]-1,1- dimethylurea], and linuron [3-(3,4-dichlorophenyl)-1- methoxy-1-methylurea] with or without cultivation to cultivation only and cultivation plus handhoeing. Cultivation plus handhoeing controlled the most weeds. Eight weed control systems using herbicides or herbicides plus cultivation controlled more weeds in soybeans than the system using only three cultivations. There was no significant difference in soybean yield between the system using three cultivations and six other systems using herbicides or herbicides plus cultivation. Trifluralin or alachlor applied as a preemergence treatment followed by postemergence treatments of chloroxuron, dinoseb, and linuron controlled weeds nearly as well as cultivation plus handhoeing. When dinoseb was applied repeatedly to leaves early after emergence, it severely injured soybeans, reduced soybean yield, and did not control sicklepod (Cassia obtusifolia L.). Sicklepod was the dominant weed species remaining at harvest.

A higher population of common purslane (Portulaca oleracea L.) developed on a smooth seedbed than a rough seedbed. Purslane germinated best when the seed were placed on the soil surface, and the percentage germination decreased progressively as depth of seed placement increased. Growth of common purslane was greater at elevated nutrient levels and was dependent on adequate phosphorus in the nutrient media. High soil temperature requirements for optimum germination make common purslane a middle to late-growing season weed in the central United States. Often preemergence herbicides which are effective on common purslane have become ineffective by the time the common purslane population has developed, and postemergence herbicides which do not injure the individual crop species must be relied on for common purslane control.

Plants produced by yellow nutsedge (Cyperus esculentus L. # CYPES) tubers planted in a field at the U.S. Cotton Research Station, Shafter, CA, at monthly intervals were harvested 2, 4, or 8 weeks after planting or at the end of a full season that varied from 12 to 37 weeks. In the 2 yr of the study, 1982 and 1983, tubers were slow to sprout in March and April, taking up to 8 weeks. From June through October about 60% of the plants emerged within 2 weeks of the planting date. Parent plants produced daughter plants by the 4-week harvest except for March, April, and October 1982, and March 1983 planting periods. All planting periods had daughter plants by the 8-week harvest. In 1982, plants of the June full-season planting had significantly more daughter plants than plants of other full-season treatments even though the June full season was 12 weeks shorter than the March full season. In 1983, the number of daughter plants produced by plants for the full-season harvest was equal for all monthly plantings regardless of the length of the full season. In 1982, for the May through September plantings, more tubers were produced by plants at the 8-week harvest than the number of tubers originally planted. In 1983, only plants of the June and August plantings produced more tubers by the 8-week harvest than had been planted. In both years, plants that grew for a full season produced more tubers than the number planted, up to 200-fold more.

Ethofumesate [(±)-2-ethoxy-2,3-dihydro-3,3-dimethyl-5-benzofuranyl methanesulfonate] was applied to dry soils (2%, w/w moisture content) that were either wetted immediately or remained dry for 1 to 8 days prior to wetting. Two degradation products were isolated with thin-layer and column chromatography from soils that were kept dry for 4 days before wetting. The major product was identified as 2,3-dihydro-3,3-dimethyl-2-oxo-5-benzofuranyl methanesulfonate which accounted for more than 80% of the degradation products. The products were detected 1 day after application to dry soil (between 1 and 3% moisture content), while at moisture contents greater than 3% (w/w) very little ethofumesate was degraded. Radiolabeled ethofumesate and breakdown products were extracted with methanol from dry and wetted soils, and the soil samples were oxidized to quantify nonextractable radioactivity. The percentage of nonextracted ethofumesate was at least 5% greater in dry soil than in wet soil. Activity loss of ethofumesate applied to dry soil probably is due to both chemical degradation and strong adsorption.

Imazamethabenz, an acetohydroxyacid synthase (AHAS)-inhibiting herbicide, was applied at a sublethal rate (200 g ai ha–1) to foliage of greenhouse-grown wild oat plants at the 2-leaf stage without tillers. Main shoot tissues were sampled 1 to 2 wk after imazamethabenz treatment and prepared for light microscopic examination. Histological observations showed that imazamethabenz treatment did not produce drastic changes on leaf structures but did affect chloroplast integrity of fully expanded 2nd leaf and inhibited differentiation of the young 3rd leaf 1 wk after application. The internode length and the number of cells in internodes of the herbicide-treated main stem were greatly reduced, presumably due to the inhibition by imazamethabenz of intercalary meristem cell division in early stages of internode elongation. One week after imazamethabenz application, histochemical studies revealed an increased accumulation of starch granules in chloroplasts of the 2nd leaf but reduced starch levels in the main stem. This confirmed the hypothesis that AHAS inhibitors affected phloem transport of photosynthates.

Based on an herbarium survey, clover broomrape (Orobanche minor J. E. Smith) was found to have an 116 yr history in the United States. It has been collected 98 times in a total of 32 counties in 12 states and the District of Columbia, and it appears to have become established in seaports and along railroads. It most commonly parasitizes red and white clover, but also burley tobacco, carrots, and several species of ornamental shrubs and herbs. Distribution of clover broomrape indicates either a pattern of repeated introductions from foreign sources, or a persistence in areas where the parasite has become established. While not presently a significant threat, the potential for damage to crops of economic importance cannot be ruled out. The flowers are autogamous.

Infection of Hydrilla verticillata by Macrophomina phaseolina was investigated using scanning and transmission electron microscopy. Sprigs of plants in petri plates were inoculated with suspensions of fungal hyphae. Samples of inoculated and noninoculated plants were taken over time. Fungal cells attached to lower epidermal cell walls but not the upper epidermal cell walls of leaves. In less than 40 h, penetration through the cell wall was completed and colonization of host cells was observed. Penetration of upper epidermis was limited to the cell wall adjacent to a lower epidermal cell. No penetration was observed through the outer cell wall of upper epidermis. Inhibition of penetration through the outer cell wall of the upper epidermis may be attributable to an osmiophilic layer below the cell wall. Disruption of the host cell walls and subsequent host cell death was preceded by massive colonization of the host by this pathogen.

In a 3-yr field study, seven dinitroaniline herbicides controlled all weeds adequately, except for two species. None of them controlled shepherdspurse [Capsella bursa-pastoris (L.) Medic.], and only nitralin [4-(methylsulfonyl)-2,6-dinitro-N,N-dipropylaniline] controlled common ragweed (Ambrosia artemisiifolia L.). EPTC (S-ethyl dipropylthiocarbamate) was included as a standard of comparison and provided satisfactory control of all weed species. Three-year averages of alfalfa (Medicago sativa L.) injury ratings at herbicide rates estimated to be equivalent in biological activity to 0.8 kg/ha and 1.7 kg/ha trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) respectively were: butralin [4-(1,1-dimethylethyl)-N-(1-methylpropyl)-2,6-dinitrobenzenamine] 0% and 3%, benefin (N-butyl-N-ethyl-α,α,α-trifluoro-2,6-dinitro-p-toluidine) 2% and 9%, profluralin [N-(cyclopropylmethyl)-α,α,α-trifluoro-2,6-dinitro-N-propyl-p-toluidine] 5% and 19%, AC92390 (N-sec-butyl-2,6-dinitro-3,4-xylidine) 6% and 17%, trifluralin 6% and 26%, fluchloralin [N-(2-chloroethyl)-2,6-dinitro-N-propyl-4-(trifluoromethyl)aniline] 14% and 45%, and nitralin 18% and 49%. EPTC caused an average 4% injury at 3.4 kg/ha. At equivalent rates to 0.8 kg/ha trifluralin, most treatments caused an increase in first cutting alfalfa yields accompanied by a decrease in weed yields, except in 1974 when few treatments resulted in increased alfalfa yields. Alfalfa recovered from early season injury caused by higher rates of the herbicides, so that total alfalfa yields were usually not reduced compared to the control. Total herbage yields (alfalfa + weeds) were not increased by any treatment, and were sometimes decreased, especially by high rates of the herbicides.