Two or more pesticides together may produce a growth response in plants that is not predictable by their individual or independent toxicities. This unpredicted (dependent) response results from an interaction, a concept that usually is not easily interpreted. Dependent responses are further complicated by the fact that they can be either synergistic or antagonistic. Several methods exist for identifying and measuring phytotoxic interactions. Nearly all methods have certain shortcomings, however. Additive and multiplicative models (mathematical expressions) are the two basic approaches to determining pesticide interactions. The two-parameter, isobole, and calculus methods axe additive; whereas, Colby and regression estimate are multiplicative models. Regression estimate analysis considers deviations due to experimental errors, and a statistical significance can be attached to the interaction magnitude, thereby overcoming the deficiencies of the Colby method, but both methods seem to be limited to response data in which the combined pesticide concentration is the sum of the individual pesticide concentrations. The two-parameter method seems to be limited to response data in which the combined concentration is equal to the individual pesticide concentration and to response data in which a pesticide concentration necessary to produce a 50% of control value is interpolated rather than extrapolated. The calculus method is a mathematical expression of the growth response, and interaction is measured by derivation of the equation obtained. The calculus method is difficult to interpret and has a major weakness because it depends upon the multiple regression equation of the observed data. The regression estimate method is recommended as a reasonable approach to interpretation of interaction type data, with a SAS language computer program available from the author.

The effect of the bleaching and non-bleaching phenylpyridazinones, norflurazon [4-chloro-5-(methylamino)-2-(α,α,α-trifluoro-m-tolyl)-3(2H)-pyridazinone] and BAS 29095 [4,5-dimethoxy-2-phenyl-3(2H)-pyridazinone], on chlorophyll fluorescence of the green alga, Scenedesmus acutus, was investigated and compared to other bleaching herbicides, difunon3 [EMD-IT 5914, 5(dimethylamino-methylene)-2-oxo-4-phenyl-2,5-dihydrofuranecarbonitrile-(3)] and the diphenyl ether, oxyfluorfen [2-chloro-1-(3-ethoxy-4-nitrophenoxy)-4-(trifluoromethyl)benzene]. Following chlorophyll fluorescence during short-term cultivation in the presence of herbicides and comparing it to physiological parameters such as chlorophyll content and packed cell volume allowed for rapid screening to detect different primary herbicidal modes of action. Two primary bleaching effects caused by either inhibition of carotene biosynthesis (certain substituted pyridazinones and difunon) or peroxidative degradation of membrane lipids (certain diphenyl ethers) led to completely different fluorescence signals. Growth of algae in the presence of a carotene biosynthesis inhibitor resulted in a rapid rise to maximum fluorescence, followed by a single decay phase, whereas bleaching diphenyl ethers led to a rapid loss of total fluorescence. Non-bleaching phenylpyridazinones, which act as weak electron transport inhibitors, inhibited variable fluorescence. Detoxication during algal growth became evident by recovery of the fluorescence induction.

The effect of johnsongrass [Sorghum halepense (L.) Pers.] on soybean [Glycine max (L.) Merr.] yields, grades, and net returns were determined by regression analysis of data from 252 field research plots from 1971 through 1976. Johnsongrass control from 0 to 100% was achieved by various combinations of summer fallow, dalapon (2,2-dichloropropionic acid) as a preplanting application at 8.3 kg/ha, or trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) at 0.8 to 1.7 kg/ha incorporated into the soil before planting. Repeated usage of trifluralin at 1.7 kg/ha, either with or without use of summer fallow, provided the most effective and most profitable soybean production. Foreign material in soybean seed samples was about 0.8% with 100% johnsongrass control and nearly 6% with no control. At least 70% johnsongrass control was required to avoid deductions from gross harvested weights caused by seed moisture levels exceeding 13%. Levels of johnsongrass control in excess of 60% tended to increase soybean test weights. With 100% johnsongrass control, about 1.2% damaged soybean seed occurred; whereas, without johnsongrass control, about 1.7% seed damage occurred. The level of johnsongrass control had little effect on the level of split soybean seed at harvest. Failure to control johnsongrass resulted in predicted soybean grades of 4.1; whereas, nearly 100% johnsongrass control was necessary to provide U.S. No. 1 grade soybeans. Soybean yields were increased 4.8 to 6.2% for each 10% increase in johnsongrass control when plots were not fallowed or after fallowing, respectively. Net returns in soybean production were nearly twice as great with 100% johnsongrass control as when johnsongrass was not controlled.

Puccinia obtegens (Link) Tul., an autoecious rust pathogen, is a potential biological control agent of Canada thistle [Cirsium arvense (L.) Scop.]. Ten ecotypes of Canada thistle were inoculated with uredospores of P. obtegens and sporulation was observed on all ecotypes. Infection types varied among and within ecotypes, indicating that host-resistance is one factor limiting rust infection. No correlation was found between Canada thistle susceptibility to the rust and host plant ecotype classification, stomatal density, amount of leaf pubescence, or spore germinability on leaf surfaces.

Control of littleseed canarygrass (Phalaris minor Retz.), wild oat (Avena fatua L.), common lambsquarters (Chenopodium album L.), and field bindweed (Convolvulus arvensis L.) in dwarf wheat (Triticum aestivum L.) fields was studied with five herbicides and hand weeding. Terbutryn [2-(tert-butylamino)-4-(ethylamino)-6-(methylthio)-s-triazine] and 1-benzthiazol-2-yl-1,3-dimethylurea controlled the weeds more effectively than did two hand weedings. Diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea], nitrofen (2,4-dichlorophenyl p-nitrophenyl ether), and dichlormate (3,4-dichlorobenzyl methylcarbamate), proved only partially effective against weeds, and were therefore significantly inferior to two hand weedings. Spikes per meter of row length and test weight of wheat were increased significantly by 1-benzthiazol-2-yl-1,3-dimethylurea (1.5 to 2 kg/ha), terbutryn (0.5 kg/ha), and two hand weedings over the yields of the weedy check. The 1-benzthiazol-2-yl-1,3-dimethylurea showed a high degree of selectivity in the wheat crop at 2 kg/ha and increased yields to those of weed-free plots. Terbutryn was almost as effective as 1-benzthiazol-2-yl-1,3-dimethylurea, which increased the wheat yields over those of the weedy check by 19.8 and 42.4% during the 1974/75 and 1975/76 seasons, respectively. Tolerance of dwarf wheat to terbutryn at 0.75 kg/ha was variable, resulting in significant decreases in yield components and grain yield. Dry-matter yield of wheat was negatively correlated with the dry matter production of weeds.

Young orange [Citrus sinensis (L.) Osbeck ‘Pineapple sweet orange’] trees are more sensitive to bromacil (5-bromo-3-sec-butyl-6-methyluracil) than young mandarin (Citrus reticulata Blanco ‘Cleopatra mandarin’) trees. Pineapple sweet orange roots absorbed twice as much 14C from bromacil, and accumulated three times as much in the leaves, as did Cleopatra mandarin. The amount of conjugated metabolites formed was the same in the roots of the two cultivars, but twice as much formed in the leaves of Cleopatra mandarin as in the leaves of Pineapple sweet orange. The principle metabolite was 5-bromo-3-sec-butyl-6-hydroxymethyluracil; a minor metabolite was tentatively identified as 5-bromo-3-(3-hydroxyl-1-methylpropyl)-6-methyluracil. No 5-bromouracil was detected. Citrus cultivars differ in their ability to accumulate and metabolize bromacil into conjugated nonphytotoxic compounds.

Experiments were conducted from 1973 through 1975 on Lucedale sandy loam to determine the influence of in-row cotton (Gossypium hirsutum L. ‘Stoneville 213’) densities on the competitiveness of low-level infestations of sicklepod (Cassia obtusifolia L.) and pigweed (Amaranthus spp.). Weeds were established at densities of 0, 4, 12, and 32 weeds per 15 m of row and allowed to compete the entire season with cotton grown at densities of 5, 10, or 20 plants/m of row corresponding to 47000, 94000 and 187000 cotton plants/ha. Conventional cultural practices were employed in these experiments. Cotton yields were inversely related to weed density; however, the density of cotton did not influence the competitive effect of sicklepod or pigweed. Pigweed or sicklepod dry weed weight was reduced when competing with 187000 cotton plants/ha.

A phytotoxin that inhibited seedling root growth was isolated and purified from rhizomes of quackgrass [Agropyron repens (L.) Beauv.]. Aqueous solutions of the phytotoxin (0.1%, w/v) significantly inhibited the seedling root growth of corn (Zea mays L. ‘New Jersey 8’), oat (Avena sativa L. ‘Clintland 60’), cucumber (Cucumis sativus L. ‘Straight Eight’), and alfalfa (Medicago sativa L. ‘Vernal’). Growth inhibition was not due to extremes of pH or osmotic potential of the inhibitor solutions. The phytotoxin was isolated and purified from a methanol:water extract of dried and ground rhizomes by adsorption and elution from charcoal. A golden-brown, crystalline substance was obtained that was soluble in water, partly soluble in semipolar solvents, and insoluble in nonpolar solvents. Thin layer chromatography (TLC) and high pressure liquid chromatography (HPLC) suggested that a single compound was present. Elemental analysis showed that it contained 42.17% carbon, 5.06% hydrogen, 38.28% oxygen, 4.02% nitrogen, 2.03% sulfur, 0.03% chlorine, and 0.03% phosphorus. The compound was tentatively identified as a glycoside with a molecular weight of 460. Its ultraviolet (UV) absorption spectrum showed an intense maximum at 307 nm.

The unicellular green alga, Chlorella sorokiniana Shihira and Krauss, was cultured in vitro using an inorganic growth medium with various concentrations of the herbicide glyphosate [N-(phosphonomethyl)glycine]. Growth rates, in terms of ceil doublings per day, were determined for cells inoculated into media containing from 5.91 × 10−6 M to 591 × 10−6M glyphosate. Comparisons with a control, which supported 10.4 doublings/day, showed growth only slightly reduced at 5.91 × 10−6M and 11.8 × 10−6M, with averages of 9.7 and 9.5 doublings/day; reduced by more than half at 17.7 × 10−6M with an average of 4.4 doublings/day; and prevented at all higher concentrations. In addition to completely preventing growth at 23.7 × 10−6M and above, glyphosate seemed to cause a deterioration of the inoculum cells. Filtering the media containing 5.91 × 10−6M, 11.8 × 10−6M, and 17.7 × 10−6M through kaolinite prior to introduction of the alga reduced the inhibition previously noted at 17.7 × 10−6M and resulted in growth rates from 9.1 to 9.5 doublings/day.

The herbicide 3,6-dichloropicolinic acid was more effective than 2,4,5-T [(2,4,5-trichlorophenoxy)acetic acid] or equal-ratio combinations of 2,4,5-T with picloram (4-amino-3,5,6-trichloropicolinic acid) or dicamba (3,6-dichloro-o-anisic acid) when aerially applied as broadcast sprays for control of honey mesquite [Prosopis juliflora (Swartz) DC. var. glandulosa (Torr.) Cockerell]. Applied at 0.56 kg/ha, 3,6-dichloropicolinic acid controlled 60 and 68% of the honey mesquite at two locations; whereas, the mixture of 2,4,5-T and picloram controlled only 21 and 30% of the honey mesquite at the same rate of application. The herbicide 3,6-dichloropicolinic acid was equally effective when applied alone or in 1:1 mixtures with 2,4,5-T, picloram, or triclopyr {[(3,5,6-trichloro-2-pyridinyl)oxy] acetic acid} at equivalent rates. Applying 3,6-dichloropicolinic acid at 1.12 kg/ha resulted in an absolute increase in mortality of 26% over the 0.56-kg/ha rate.

Two experiments were conducted to determine the most effective herbicide treatments for fringed sagebrush (Artemisia frigida Willd.) control in rangelands. In the first study the potassium salt of picloram (4-amino-3,5,6-trichloropicolinic acid) at 0.1, 0.6, and 0.8 kg/ha; picloram + the propylene glycol butyl ether ester of 2,4-D [(2,4-dichlorophenoxy)acetic acid] at 0.1 + 1.1 kg/ha; the monoethanolamine salt of 3,6-dichloropicolinic acid at 1.1 kg/ha; 3,6-dichloropicolinic acid + 2,4-D at 0.6 + 1.1 kg/ha; and the dimethylamine salt of dicamba (3,6-dichloro-o-anisic acid) + 2,4-D at 0.6 + 1.1 kg/ha applied in late summer at flowering, showed the greatest potential for fringed sagebrush control. In the second experiment, the potassium salt of picloram at 0.3 kg/ha applied either in the spring or late summer gave 94 to 100% fringed sagebrush control, respectively, 1 yr after application. Control of fringed sagebrush had declined to 50% 3 yr after application. Production of perennial grasses from plots receiving picloram at 0.3 kg/ha in 1976 was increased by 410 kg/ha in 1977, 450 kg/ha in 1978, and 430 kg/ha in 1979 over that of the untreated plots.

The ability of honeyvine milkweed [Ampelamus albidus (Nutt.) Britt.] to establish and develop from seed was studied in laboratory and field experiments. Optimum germination temperature was 30 C. Seed incubated at cooler temperatures and then transferred to 30 C germinated equally as well as seed incubated continually at 30 C. Germination was unaffected at moisture stress levels between 0 and −4.6 bars, but was decreased at levels between −4.6 and −12.8 bars. The optimum pH range for germination was between 5 and 7. The greatest emergence of seedlings occurred at a depth of 0.5 cm with no emergence occurring at depths greater than 5 cm. Greater emergence was obtained with a loam than sandy loam at all planting depths except 0.5 cm. Field plots seeded on May 1 gave maximum seed production. Seed production decreased with each 2-week delay in planting. Planting dates of July 10 or later produced no seed. Seed germination was affected little by delayed planting. The following spring, regrowth from plants seeded on June 12 or later was less vigorous than regrowth from plants seeded earlier.

Interactions of nine dosage rates each of chloramben (3-amino-2,5-dichlorobenzoic acid) and diphenamid (N, N-dimethyl-2,2-diphenylacetamide) were studied using Italian ryegrass (Lolium multiflorum Lam.) as a test species. Injury score, estimated percent stand reduction, height, and fresh weight data were taken 1 week after seeding. Techniques for developing linear regression models based on these data and evaluating them graphically in terms of completeness and statistical validity are discussed. Three-dimensional graphs of these models indicate areas of possible herbicide interaction.

Triclopyr {[(3,5,6-trichloro-2-pyridinyl)oxy]acetic acid}, 2,4,5-T [(2,4,5-trichlorophenoxy)acetic acid], and 3,6-dichloropicolinic acid were applied to the foliage of juvenile crop plants at 0.002, 0.009, 0.03, 0.14, and 0.56 kg/ha. Corn (Zea mays L.), oat (Avena sativa L.), wheat (Triticum aestivum L.), grain sorghum [Sorghum bicolor (L.) Moench], and kleingrass (Panicum coloratum L.) were generally more tolerant to the herbicides than were peanuts (Arachis hypogaea L.), cotton (Gossypium hirsutum L.), cucumber (Cucumis sativus L.), and soybean (Glycine max [L.] Merr.). Triclopyr was usually more phytotoxic to corn, oat, grain sorghum, and kleingrass than either 2,4,5-T or 3,6-dichloropicolinic acid at 0.14 and 0.56 kg/ha, but few differences occurred among herbicides at lower rates. Kleingrass was not affected at any rate of 3,6-dichloropicolinic acid. Wheat tolerated most rates of all three herbicides. At 0.56 kg/ha, triclopyr and 3,6-dichloropicolinic acid caused greater injury to peanuts than did 2,4,5-T; whereas, 2,4,5-T and triclopyr were more damaging to cotton and cucumber than 3,6-dichloropicolinic acid. The three herbicides at 0.14 and 0.56 kg/ha killed soybeans. Soybean injury varied from none to severe at 0.002 to 0.03 kg/ha, depending upon species investigated, but many plants showed morphological symptoms typical of the auxin-type herbicides.

Field and greenhouse experiments were established to determine the phytotoxicity of Canada thistle [Cirsium arvense (L.) Scop.] residue on crop growth. Field examination of infestations showed that as Canada thistle shoots increased in an area, the number of kochia [Kochia scoparia (L.) Schrad.], marshelder (Iva xanthifolia Nutt.) and foxtail barley (Hordeum jubatum L.) plants decreased. Conversely, as Canada thistle shoots decreased, the aforementioned annual and perennial plants increased. In greenhouse studies, roots and shoots of Canada thistle that were mixed with soil reduced the growth of sugarbeet (Beta vulgaris L. ‘Mono Hy D2’), wheat (Triticum aestivum L. ‘Centurk’), alfalfa (Medicago sativa L. ‘Dawson’), and Canada thistle seedlings. Corn (Zea mays L. ‘Jacques No. 1004’) and dry edible beans (Phaseolus vulgaris L. ‘Great Northern No. 59’) grown in soil treated with Canada thistle residue were affected to a lesser extent. When Canada thistle residue was mixed with soil, crop growth was inversely proportional to the amount of Canada thistle residue added to the soil. Both roots and shoots of Canada thistle were toxic to crops when mixed with the soil. The effects of Canada thistle residue on crop growth lasted for about 60 days. Neither autoclaving residue and soil nor fertilization of soil had any effect on residue toxicity. Canada thistle leaf leachate inhibited the growth of sugarbeets watered daily with the leachate.

Greenhouse pot experiments were conducted to study the effect of various levels of nitrogen, applied as ammonium nitrate (NH4NO3) and ammonium sulfate [(NH4)2SO4], on hemp broomrape (Orobanche ramosa L.) infestation on tobacco (Nicotiana tabacum L. ‘Jordan 1’) and tomato (Lycopersicon esculentum Mill. ‘Caludia Raf’). Infestation of tobacco and tomato by hemp broomrape was drastically reduced as higher levels of both fertilizers were applied. However, the dry weights of roots and shoots of both crop plants, as well as tomato yield, were reduced when NH4NO3 was applied at 50 g/pot, and when (NH4)2SO4 was applied at higher than 60 g/pot. In a separate pot experiment in the field, hemp broomrape infestation on tomato was drastically reduced, and tomato shoot dry weight, as well as tomato yield, were increased when (NH4)2SO4 was applied at higher than 40 g/pot as 20-g increments repeated at 10-day intervals compared to those of the control.

An isolate of Fusarium lateritium (Nees) emend. Snyder and Hansen was associated with naturally occurring disease of spurred anoda [Anoda cristata (L.) Schlecht.] and prickly sida (Sida spinosa L.) plants. In greenhouse inoculation studies, the Fusarium isolate was pathogenic to healthy spurred anoda and prickly sida plants. The disease symptoms were characterized by leaf, stem, and root lesions. The stem lesions enlarged with time and formed cankers that often girdled the stems and killed inoculated plants. In host-range studies, velvetleaf (Abutilon theophrasti Medic.), Venice mallow (Hibiscus trionum L.), okra [Abelmoschus esculentus (L.) Moench], and hollyhock [Althaea rosea (L.) Cav.] were also susceptible to the pathogen. This is the first report of these six species as hosts for this pathogen. Corn (Zea mays L.), cotton (Gossypium hirsutum L. and G. barbadense L.), soybean [Glycine max (L.) Merr.], and 18 other representative crop and weed species in eight families were resistant to the pathogen. F. lateritium may be a useful biological herbicide for susceptible malvaceous weeds.

Most seed of broom snakeweed [Gutierrezia sarothrae (Pursh) Britt. & Rusby] and threadleaf snakeweed [G. microcephala (DC.) Gray] were dormant when collected soon after maturity, requiring 4 to 6 months of storage at room conditions to break dormancy. An alternating temperature regime of 10 C (16 h) and 20 C (8 h) favored snakeweed germination during the afterripening period, but germination was greatest under alternating temperatures of 20 C (16 h) and 30 C (8 h) after 24 months of storage. After 6 months of storage, maximum germination (70% or more) of threadleaf snakeweed seed occurred at continuous temperatures of 15 to 25 C; whereas, broom snakeweed seed germinated best at 15 and 30 C. Germination of both species in darkness was half or less of germination with daily 8-h periods of light. Germination of seed lots containing a high proportion of dormant achenes was enhanced by preimbibition storage at 50 C; after dormancy was naturally broken, exposure to 50 C prior to germination decreased early seedling vigor and growth but not germination.

Combined effects of herbicides, herbicide rates, and cultivation for weed control in soybeans [Glycine max (L.) Merr. ‘Williams’] were studied using full and one-half rates of either chloramben (3-amino-2,5-dichlorobenzoic acid) or alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] + linuron [3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea] with no, one, and two cultivations. The full rate was 2.2 kg/ha for alachlor and chloramben and 0.70 kg/ha for linuron. Alachlor + linuron was better than chloramben, and the full rate was better than the one-half rate. One or two cultivations were better than no cultivation. Cultivations were more effective when used with alachlor + linuron than when used with chloramben or when used alone. Alachlor + linuron at the full rate with one or two cultivations produced the best soybean yields. A one-half rate of alachlor + linuron with one or two cultivations yielded the same as a full rate of alachlor + linuron alone or with one cultivation, chloramben at one-half rate with two cultivations, and chloramben at a full rate with two cultivations. Cultivations can be effective by increasing weed control and yields when herbicide rate or effectiveness has been reduced. Use of a cultivation in addition to the preemergence herbicides used in this study is necessary for improved weed control and yields.

Aerial applications of 20% tebuthiuron JA/-[5-(1,1- dimethylethyl) - 1, 3, 4 - thiadiazol-2 -yl] -N, N' dimethylurea pellets at 2.2 or 4.4 kg/ha in the spring to heavy brush cover in Texas Post Oak Savannah did not increase the amount of grass, compared to that of untreated pastures, until the growing season after application. By the second growing season after tebuthiuron application, however, native grass stands were composed of a higher proportion of perennial species of good-to-excellent grazing value than were stands on adjacent, untreated rangeland. Tebuthiuron at 4.4 kg/ha did not improve botanical composition of grass stands, but increased the amount of grass during the second and third growing season after application compared to those where 2.2 kg/ha were applied. Daily steer gains were increased in one experiment in the fall after application of 2.2 kg/ha of tebuthiuron in the spring, but were unchanged after two growing seasons in another. Days of available grazing were increased, however, by the second or third growing season after treatment with 2.2 kg/ha of the herbicide in both experiments.