Witchweed, [Striga asiatica (L.) Kuntze5 STRLU], an important parasitic weed throughout much of the old world, occurs as a recent (30 yr) introduction in a limited region in North and South Carolina. Reproduction in this population has been shown to occur primarily through self-pollination of flowers. Genetic diversity in two populations from this region was assessed, utilizing starch gel electrophoresis and staining of enzymes. An average of 64 individuals was observed to be monomorphic at an estimated minimum of 32 genetic loci coding 18 enzyme systems. This lack of polymorphism is attributable to the probable founding of the population by one or a few individuals coupled with the predominantly autogamous breeding system.

Twenty-nine wild oat (Avena fatua L. ♯3 AVEFA) and Avena sterilis L. ♯ AVEST accessions having various areas of origin and/or growth characteristics were grown to maturity in controlled environmental chambers. The four environments consisted of a 16-h photoperiod until 3 weeks after emergence, when the photoperiod was decreased (DP) 1 h per week for 8 weeks at a constant 14, 20, or 26 C (DP 14, DP 20, or DP 26); and an 8-h photoperiod until 3 weeks after emergence, when the photoperiod was increased (IP) 1 h per week for 8 weeks at a constant 20 C (IP 20). The relative growth rate of the accessions was similar in each environment. The length of the second leaf was up to 15 cm greater under 8- than 16-h day length at 20 C for some accessions, but was similar under both day lengths for other accessions. The width of the second leaf was greater under 14 than 20 C and was narrower with a 16-h than an 8-h photoperiod. Tiller initiation was slower in the DP 14 and IP 20 than in the DP 20 and DP 26 environments. Days to panicle emergence for individual accessions ranged from 10 to 57 days higher in the DP 14 than in the DP 20 environment. Seed produced on plants grown at 14 C had lower germination in water and 1500 ppm gibberellic acid than seed from plants grown at 20 C. Wild oat accessions varied in morphological characteristics, days to panicle emergence, and dormancy, and responded differently to changes in photoperiod and temperature. Wild oat morphological characteristics, days to panicle emergence, and dormancy did not consistently relate to species or area of origin.

The culture of ovules excised from velvetleaf (Abutilon theophrasti Medic., ♯4 ABUTH) capsules 5 days after anthesis was used to measure the effects of abscisic acid (ABA) and sucrose on embryo development and prevention of precocious germination. ABA at 1 × 10-7 M combined with 6% sucrose in the medium for the first 14 days of culture increased embryo development but prevented precocious germination. Higher concentrations of ABA inhibited embryo development. Without ABA, precocious germination increased directly with the concentration of sucrose in the medium, and embryos died. In vivo, ABA reached its highest concentration in ovules 5 days after anthesis but was undetectable after 16 days. Parental control of embryo development may involve ABA and an increasing concentration of osmoticum as seeds dehydrate during maturation.

Postemergence applications of BAS 9052 OH {2-[1-(ethoxyimino) butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexene-1-one} and Ro-13-8895 {acetone-O-[d-2-[p-[(α,α,α-trifluoro-p-tolyl-oxy] phenoxy] propionyl]oxime} at 0.3 kg/ha selectively controlled four- to five-leaf-stage quackgrass [Agropyron repens (L.) Beauv. ♯3 AGRRE] 75 and 80%, respectively, 105 days after treatment (DAT). KK-80 {4-[4-[4-(trifluoromethyl)phenoxy) phenoxy]-2-pentenoic} at the same rate and stage of growth controlled quackgrass only 45%. None of the herbicides had preemergence activity on quackgrass. All herbicides applied at 0.8 kg/ha controlled two- to three- or four- to five-leaf-stage quackgrass an average of 88%, whereas applications at the six- to seven-leaf stage controlled quackgrass only 27%, 105 DAT. Cultivation alone 1 DAT provided quackgrass control that gave soybean [Glycine max (L.) Merr.] yields equal to any herbicide or herbicide plus cultivation treatment In a greenhouse experiment, BAS 9052 OH killed 50-bud rhizome systems in 1 day, whereas KK-80 and Ro-13-8895 required 14 and 7 days of translocation, respectively, to provide similar control.

Experiments were conducted in the field and laboratory from 1978 to 1981 to determine the effect of crop residues, manure, and inorganic nitrogen on wild oat (Avena fatua L. ♯3 AVEFA) seed (seed as used here represents an intact floret) dormancy. Turkey manure usually stimulated wild oat seed germination more than crop residue, cow manure, or inorganic nitrogen. Seed germination of different wild oat accessions varied in response to turkey manure. Germination of dormant seed in water was stimulated after 8 weeks or after overwintering in the presence of turkey manure in the field and after 10 days in the presence of turkey manure in petri dishes in the laboratory. Turkey manure caused growth of wild oat shoots in petri dishes, but usually roots were absent. A nonmicrobial, water-soluble component of turkey manure was responsible for stimulating wild oat seed germination. Turkey manure treatment of dormant wild oat seed increased the percentage of seed that would not germinate in gibberellic acid. Germination of nondormant wild oat seed of Accession 77 was reduced, but Accession 64 was unaffected by turkey manure. Germination of wild oat was low when the seeds overwintered on the soil surface, regardless of seed age or length of overwintering.

Glyphosate [N-(phosphonomethyl)glycine] phytotoxicity to greenhouse-grown wheat (Triticum aestivum L. ‘Centurk’) was greatly reduced by Ca2+ in the carrier solution. Addition of ethylenediaminetetraacetic acid (EDTA) restored glyphosate phytotoxicity in hard water and increased phytotoxicity in deionized water. The phytotoxicity of 0.1 kg ae/ha glyphosate plus 1.0% (w/v) EDTA was equal to that observed for 0.2 kg/ha of the herbicide alone in deionized water or 0.4 kg/ha in water containing 200 ppm Ca2+. EDTA was more compatible with glyphosate in combination with X-77 or MON-0011 nonionic surfactant than with cationic surfactant (contained in a commercial formulation). Phytotoxicity could be increased by adding EDTA and nonionic surfactant to the latter formulation. EDTA increased glyphosate phytotoxicity to wheat when applied in water that contained high levels of calcium and bicarbonate. Glyphosate activity was greater at pH 4 than at pH 6 or 10, but EDTA was equally effective at pH 4, 6, or 10 for all levels of calcium tested up to 800 ppm. NaHCO3 probably reduced phytotoxicity by increasing the pH of the glyphosate solution.

Detrimental effects on growth and yield of soybeans [Glycine max (L.) Merr. ‘Amsoy 77′] from density and duration of competition by ivyleaf morningglory [Ipomea hederacea (L.) Jacq. ♯3 IPOHE] was evaluated in 1981 and 1982 near West Lafayette, IN. Ivyleaf morningglory was planted at densities of 1 plant per 90, 60, 30, and 15 cm of row in 1981 and 1 plant per 60, 30, 15, and 7.5 cm of row in 1982. Each density of ivyleaf morningglory competed for 22 to 46 days after emergence and the full season in 1981, and for 29 to 60 days after emergence and the full season in 1982. The best indicators of competition effects were leaf area index, plant dry weight, and yield of soybeans. Ivyleaf morningglory was more competitive during the reproductive stage of soybean growth. Photosynthetic irradiance and soil moisture measurements indicated that ivyleaf morningglory does not effectively compete for light or soil moisture. All densities of ivyleaf morningglory could compete with soybeans for 46 and 60 days after emergence in 1981 and 1982, respectively, without reducing soybean yield. Full-season competition from densities of 1 ivyleaf morningglory plant per 15 cm of row significantly reduced soybean yield by 36% in 1981 and 13% in 1982. The magnitude of soybean growth and yield reduction caused by a given density of ivyleaf morningglory was greater when warm, early season temperatures favored rapid weed development.

An indigenous soil-borne fungus was isolated from infected seeds and seedlings of Texas gourd [Cucurbita texana (A.) Gray] and evaluated as a mycoherbicide. The pathogen was identified as Fusarium solani App. & Wr. f. sp. cucurbitae Snyd. & Hans. Only Cucurbita species were susceptible in host range tests. Ample quantities of microconidia for greenhouse and field plot inoculum were produced in shake culture on Richards' solution. Granular inoculum for field plot tests was produced in 5% (w/w) cornmeal/sand medium. In controlled-environment experiments, inoculated Texas gourd seedlings were killed over a range of air temperatures from 16 to 40 C. Optimum air temperature for disease development was 26 to 30 C. Complete kill of 4- and 8-cm-tall seedlings was achieved after 3 weeks when sprayed until runoff with a microconidial suspension containing 2.0 × 106 spores/ml. Texas gourd seedlings were effectively controlled in field plots (99% maximum) with liquid or granular inoculum. Soil samples from infested plots indicated that the fungus persists up to 12 months in a fine sandy loam, but after 12 months inoculum levels were insufficient to cause disease on seedlings grown under optimum conditions for disease development.

The decline in the natural soil population of seeds of wild mustard [Brassica kaber (DC.) L.C. Wheeler var. pinnatifida (Stokes) L.C. Wheeler ♯ SINAR] under nine cultural and chemical practice treatments was studied under field conditions. Wild mustard plants were not allowed to produce seeds in any treatment. After seven growing seasons, treatments in which the soil was not disturbed (continuous grass sod or continuous chemical fallow) had approximately 50% of the original population of wild mustard seeds remaining. In contrast, less than 3% of the original population of wild mustard seeds remained in a treatment that involved plowing three times a year with additional tillage throughout the growing season. This 3%, however, was equivalent to 2.4 million seeds/ha in the plow layer. Eradication of wild mustard from an infested field may be impractical with currently available techniques.

The impact of two weed management systems on the weed seed reserves of the soil, on the yearly weed problem, and on corn (Zea mays L.) production was assessed where corn was grown under furrow irrigation for 6 consecutive years. In one system, 2.2 kg/ha of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] was applied annually to the same plots as a preemergence treatment. In the other system, a mixture of 1.7 kg/ha of atrazine plus 2.2 kg/ha of alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] was applied preemergence, followed by a postemergence application of 0.6 kg/ha of the alkanolamine salts of 2,4-D [(2,4-dichlorophenoxy)acetic acid]. The response of weeds and corn is presented only where atrazine was applied annually because the results were similar between both weed management systems. Weed seeds from eight annual species were identified, with redroot pigweed (Amaranthus retroflexus L. ♯ AMARE) and common lambsquarters (Chenopodium album ♯ CHEAL) comprising 82 and 12%, respectively, of the initial 1.3 billion weed seeds/ha that were present in the upper 25 cm of the soil profile. After the sixth cropping year, the overall decline in the total number of redroot pigweed and common lambsquarters seeds was 99 and 94%, respectively. Very few weeds produced seeds during the first 5 yr, and no weed seeds were produced during the sixth year where atrazine was applied annually. When the use of atrazine was discontinued on one-half of each plot at the beginning of the fourth year, the weed seed reserve in soil began to increase due to an increase in the weed population. After 3 yr of not using atrazine, the weed seed reserve in soil had built up to over 648 million seeds/ha, and was then within 50% of the initial weed seed population. In the fifth and sixth years, grain yields were reduced 39 and 14%, respectively, where atrazine had been discontinued after 3 yr.

Hydrilla [Hydrilla verticillata (L. f.) Royle ♯ HYLLI] was cultured in flowing water in outside aquaria on rooting media differing in fertility and texture. Nutrient content was determined for rooting media at the beginning of the experiment and for plant tissues at harvest. Growth, as measured by rates of stem elongation, areal coverage of aquaria by plants, and biomass accrual, was highly related to N-, P-, and K-fertility of rooting media. Rooting media were the most important source of phosphorous since water supplies were not adequate to support plant growth. There was evidence of P-deficiencies in plants grown on infertile rooting media. Both N and K were adequately supplied from either water, rooting media, or both. Production of plant biomass was most closely related to P-levels in tissues which were closely related to supplies in rooting media. Pretest soil N (comparable to preestablishment conditions) was best related to biomass production, indicating that growth and establishment of hydrilla may be predictable through assessment of hydrosoil fertility. The need for sampling and analytical procedures to assess fertility is emphasized.

The impact of two weed management systems on the weed seed reserves of the soil, on the yearly weed problem, and on barley (Hordeum vulgare L. ‘Steptoe’), corn (Zea mays L. ‘Pioneer 3709′), and sugarbeet (Beta vulgaris L. ‘Mono Hy D2′) production was assessed where these crops were grown in rotation for 6 consecutive years. Weeds were controlled in each crop with a moderate (system I) or intensive (system II) level of herbicides, plus conventional tillage. Weed seeds from seven annual genera were identified, with redroot pigweed (Amaranthus retroflexus L. ♯3 AMARE) and Chenopodium comprising 56 and 30%, respectively, of the initial 1377 million weed seeds/ha that were present in the upper 25 cm of the soil profile. After the sixth cropping year, the overall decline in the total number of weed seeds in soil was 96% in system I and 97% in system II. Over the 6-yr period, about 1.3 times more weeds escaped control in system I than in system II; and within a crop, the fewest number of weeds escaped annually in sugarbeets, and the most in barley. Yields of barley grain, corn silage, and recoverable sucrose were similar each year in the two weed management systems.

Sicklepod (Cassia obtusifolia L. ♯6 CASOB), hemp sesbania [Sesbania exaltata (Raf.) Cory ♯ SEBEX], and showy crotalaria (Crotalaria spectabilis Roth. ♯ CVTSP) were grown in controlled-environment greenhouses with day/night temperatures of 23/17, 26/20, 29/23, and 32/26 C. Plant height, leaf area, and total dry weight at 53 days after planting increased significantly with temperatures between 23/17 and 29/23 C in sicklepod and showy crotalaria, and between 23/17 and 32/26 C in hemp sesbania. In all species, leaves were produced more rapidly as temperature increased. Axillary branch leaf production was greatest at 29/23 C in sicklepod, and at 29/23 and 26/20 C in showy crotalaria; hemp sesbania showed little axillary branch development. Total dry-matter production in all species was closely correlated with leaf area duration, but not with net assimilation rate. Differences in total main stem leaf area in all species were primarily due to differences in leaf number rather than to differences in leaf size at maturity.

The effect of repeated inoculations of the rust Puccinia cardui-pycnocephali Sydow (Uredinae) on growing rosettes of Mediterranean specimens of slenderflower thistle (Carduus tenuiflorus Curt. ♯3 CRUTE) and Italian thistle (Carduus pycnocephalus L. ♯ CRUPY), and Australian specimens of Italian thistle was investigated, using a Mediterranean strain of the rust collected mainly on Italian thistle. Growth of slenderflower thistle was not affected, but growth of both specimens of Italian thistle was decreased significantly when inoculated with the rust. This decrease was significantly higher on Australian specimens.

Field studies were conducted to document the response of itchgrass [Rottboellia exaltata (L.) L.f. ♯3 ROOEX] in soybean [Glycine max (L.) Merr. ‘Forrest’] to selected herbicides and postplanting cultivation. Early cultivation stimulated emergence of itchgrass seedlings; however, when plots were cultivated two or three times, itchgrass was effectively removed from the tilled area. Cultivation had no effect on the density, height, standing biomass, or seed production of itchgrass plants in the soybean row but did increase soybean yield. In response to a density reduction of 90%, the average weight, number of tillers and branches, and seed production of single itchgrass plants increased by a factor of 2.9, 3.1, 2.3 and 2.6, respectively, in trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine)-treated plots. Individual plants did not increase growth following density reduction by a postemergence application of diclofop {2-[4-(2,4-dichlorophenoxy)phenoxy] propanoic acid}. Itchgrass plants competing for the entire season in plots treated with trifluralin reduced soybean yield approximately 21 g per weed, while itchgrass recovering from diclofop treatment and competing the remainder of the season reduced yield approximately 5 g per weed.

Growth and competition of barnyardgrass [Echinochloa crus-galli (L.) Beauv. ♯ ECHCG] and redroot pigweed (Amaranthus retroflexus L. ♯ AMARE) were studied at different seed proportions, seed burial depths, and soil moisture levels. After 7 days, emergence from 1-, 2-, 4-, and 8-cm depths was 96, 90, 83, and 27% for barnyardgrass and 84, 73, 62, and 0% for redroot pigweed, respectively. Barnyardgrass was more competitive than redroot pigweed. Intraspecific competition of barnyardgrass was greater than interspecific competition from redroot pigweed. Increasing planting depth from 1 to 4 cm and increasing soil moisture from 30 to 50% (low) to 100% (high) of field capacity reduced the competitive ability of redroot pigweed.

Field experiments were established in 1978 and repeated in 1979 to monitor quackgrass [Agropyron repens (L.) Beauv. ♯3 AGRRE] rhizome growth through a growing season. Quackgrass shoots grew most vigorously when temperatures were moderate. They elongated and differentiated an inflorescence in June. Rhizome apical tips remained belowground in late spring, early summer, and early fall. Quackgrass rhizome tips grew vertically and emerged from the soil in midsummer and late fall. Experiments conducted in growth chambers revealed that photoperiod, temperature, and nitrogen level affected quackgrass shoot and rhizome growth. Shoots grew vigorously at 21C and elongated to differentiate an inflorescence when exposed to a photoperiod of 14 h or longer. Rhizome apical tips remained belowground when plants were grown at 21 C and received 16 or 12 h of light, but the tips curved upward and emerged from the soil when quackgrass was grown at 32 or 10 C, or when the plants received 14 or less than 10 h of light. The temperature and photoperiod responses coordinated to partition the growing season into primary rhizome and rhizome-tiller growth periods.

Freshly harvested Drummond rattlebox [Sesbania drummondii (Rydb.) Cory ♯3 SEBDR] seed germinated best after 2.5- to 4-h acid scarification in concentrated sulfuric acid. Optimum temperature for germination was 30 to 35 C. At 10 and 45 C no germination occurred for up to 7 days. Six days after planting in a silt loam soil there was 95 and 80% emergence of Drummond rattlebox from depths of 0.5 and 1 cm, respectively. No emergence was observed from depths of 11 cm or greater 12 days after planting.

Photosynthetic growth of 9 of the 11 known species of the family Rhodospirillaceae was strongly inhibited by atrazine. Growth of a wild-type strain of the bacterium Rhodopseudomonas sphaeroides van Niel was almost completely inhibited by 100 μM concentrations of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] and diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea]. These herbicides gave 50% inhibition of growth at concentrations of 35 and 25 μM, respectively. Through continuous culture of the wild-type R. sphaeroides in the presence of 100 μM atrazine, strains of the organism were selected in which photosynthetic growth was highly resistant to both atrazine and diuron. Members of the genus Rhodopseudomonas isolated from two natural ecosystems showed various levels of resistance to 100 μM atrazine, but Rhodopseudomonas showing a high level of resistance were more common among the isolates from a pond-mud sample which had been exposed to runoff from atrazine-treated cropland than from a sample of a lake mud never exposed to triazines.

Velvetleaf (Abutilon theophrasti Medic. ♯3 ABUTH) grown under monoculture consistently exceeded velvetleaf intercropped with soybeans [Glycine max (L.) Merr. var. ‘Amsoy 71′] in leaf area, nodes with fully developed leaves, canopy width, branches, and number of capsules as early as 3, 3, 4, 5, and 8 weeks, respectively, after simultaneous emergence. Velvetleaf plants without soybean competition eventually developed over nine times the dry matter of velvetleaf intercropped with soybean. The only components of velvetleaf plants sampled which sometimes increased significantly when soybean was defoliated in a manner simulating damage caused by the green cloverworm (GCW) (Plathypena scabra F.) were leaf area, number of leaves, and number of main-stem nodes. Soybeans in Iowa are attacked by the GCW late enough in the growing season that velvetleaf surviving previous weed management efforts benefited only slightly.