The present importance of the discipline of weed control is outlined. More acres in the United States are treated today for weed control than for the control of insects or plant diseases, despite disparity in personnel engaged; e.g., a total in the North Central region of 44 men in weed control, 176 in entomology and 177 in plant pathology.

A review, particularly of the responses to herbicides of photosynthetic processes at the molecular level, with some discussion of the effects of herbicides on cell membranes.

A review of recent work, especially, on the multitude of factors affecting the germination of weed seeds.

Witchweed (Striga asiatica), a parasitic chlorophyll-bearing plant (Scrophuladiaceae) which attacks corn, sorghum, sugarcane, and more than 60 other important crop plants and weeds belonging to the grass family, was found in North Carolina and South Carolina in 1956. It presents a potential hazard to the production of grain crops with an annual value of more than 5 billion dollars in the United States. Continued extensive surveys indicate that witchweed is now present in 20 counties in North Carolina and 10 in South Carolina on a total of 9,566 farms containing 210,671 acres. Federal interstate and state intrastate quarantines (the first for a weed in the U. S.) were established to reduce its spread. Witchweed produces up to 500,000 microscopic longlived seeds per plant. These do not usually germinate until stimulated by a chemical stimulant secreted by the roots of certain plants. A stimulant has been isolated from corn but its exact chemical structure has not be determined. More than 300 chemicals have been evaluated in the laboratory in attempts to find a synthetic stimulant. Of these, 18 stimulated witchweed seed germination in the laboratory but they have not been evaluated for their effectiveness in controlling witchweed under field conditions. After 4 years of catch or trap crops, excellent control of witchweed has been obtained and highly profitable crops of corn have been produced on heavily infested land. Corn grown continuously with witchweed seed production prevented by 2,4-dichlorophenoxyacetic acid [2,4-D] has resulted in excellent witchweed control and high corn yields. Corn grown continuously receiving 2,3,6-trichlorophenylacetic acid [fenac] as a pre-planting soil-incorporated treatment and 2,4-D as a post-emergence spray as needed have proved effective full-season control of witchweed. Tobacco and certain other crops cannot be grown in rotation with corn for a period of 2 years after this treatment because of fenac residues in the soil. Other herbicides as pre-planting soil-incorporated or premergence treatments to control broadleaved and grass-type weeds plus 2,4-D to prevent witchweed seed production have been highly effective combinations for witchweed control. Quick eradication of witchweed will not be easy but effective control now and future eradication look favorable.

Chemical control of weeds in the Hawaiian sugar industry began in 1913 when experiments with sodium arsenite were first conducted on the Island of Hawaii. Sodium arsenite and some sodium chlorate were the main herbicides in use until a concentrated, activated diesel emulsion (CADE) was developed in 1944. From that time on the chlorophenoxyacetic acids and their compounds, sodium trichloroacetate, sodium 2,2-dichloropropionate, and the substituted ureas and triazines have, in turn, been incorporated into use on plantations for weed control. At present, over a half million acres are sprayed each year at a cost of nearly seven million dollars for labor and material.

When one talks about Latin America, one is talking about approximately 200 million people of a variable kind disproportionately divided among 20 republics comprising an area about 2.6 times the size of Continental United States, not including Alaska.

During the first season, tubers of purple nutsedge, Cyperus rotundus L. planted in Piedmont Plateau soil at Experiment, Georgia at 1-ft intervals produced 3,090,000 plants and 4,420,-000 tubers and bulbs per acre. Tubers planted at 3-ft intervals produced 2,320,000 plants and 2,760,000 tubers and bulbs. The differences were not pronounced at the end of the second season. A single cultivation at the beginning of the second year did not appreciably affect the number of tubers and bulbs formed. Continuous heavy shade during the second year reduced tuber-bulb formation. No apparent interaction between shading and cultivation occurred.

Oats, sesbania (Sesbania macrocarpa Muhl.) and purple nutsedge (Cyperus rotundus L.) were grown as test plants in nine different soil types treated with 0, 0.5, 1, 2, 4, 8, and 16 ppm of ethyl N,N-di-n-propylthiolcarbamate (EPTC). Toxicity of EPTC was negatively correlated with the organic matter of the soil. A positive correlation was found between the sand and silt content of the soil and EPTC toxicity to oats and nutsedge. Toxicity of EPTC was not correlated with soil pH, cation exchange capacity, clay content, general reaction, or parent material. EPTC induced a dormancy of nutsedge tubers, of which the length was inversely related to the organic content of the soil.

Isopropyl N-phenylcarbamate (IPC) and isopropyl N-(3-chlorophenyl)carbamate (CIPC) were the most effective herbicides for controlling downy brome without serious injury to perennial grasses grown for seed production in eastern Washington. The disodium salt of 3,6-endoxohexahydrophthallic acid (endothal) gave good downy brome control at Spokane, but was not satisfactory for control at Hooper and Pomeroy, Washington.

The relative tolerance of cucumber and squash to amiben was established for germinating seeds, and for young plants with both foliage and root applications. In all cases, the dosage required for 50% inhibition of growth was 6 to 8 times greater for squash than for cucumber. Root applications of carbon-14 labeled amiben on reciprocal grafts of cucumber and squash plants indicated that the shoots were spared in these experiments by differential blocking of amiben transport from the roots. Shoots of either species appeared tolerant on squash roots and susceptible on cucumber roots. Radioactivity in the shoots of both species was much greater when grafted on cucumber than on squash. Foliage applications of carbon-14 labeled amiben showed more entry into cucumber than squash, but little movement to untreated plant parts in either species.

White mustard (Brassica hirta Moench) and foxtail millet (Setaria italica (L.) Beauv.), thinned to uniform stands, varied in a three-year study in their effects on peas depending on weed population, duration of competition, relative time of pea and weed emergence, and year to year variations in seasonal rainfall. Full-season competition from 3 mustard plants per square foot reduced pea stands 0 to 25%, pea vine weight 39 to 71%, and yields of shelled peas 0 to 64%, during the three years. In one direct comparison, 27 foxtail plants per square foot caused reductions in peas comparable to those caused by 3 mustard plants. When mustard emerged 3 days before peas, fresh weight of pea vines was reduced 54 percent; but when mustard emerged 4 days after peas, only 17 percent.

The relative growth rates of peas, Brassica hirta, B. Kaber, Amaranthus retroflexus, Polygonum Persicaria, Setaria italica, S. viridis, S. glauca, and Echinochloa crusgalli grown without intra- and inter-species competition suggest that the Brassica species would be the more severe competitors with peas and that weed competition usually would be more severe in late-seeded peas.

The effect of 4,6-dinitro-o-sec-butylphenol(DNBP) and several phenylureas, s-triazines, chlorinated phenoxyacetic acids and chlorinated benzoic acids on the photolytic activity (Hill reaction) of chloroplasts isolated from turnip greens (Brassica spp.) was studied. Photochemical activity was measured potentiometrically with ferricyanide as the electron acceptor.

For the various members of each herbicidal group close agreements were obtained between inhibitions expressed against the Hill reaction and their relative phytotoxicities, except for the chlorinated benzoic acids. Of the five phenylureas tested, 1-n-butyl-3-(3,4-dichlorophenyl)-1-methyrurea (neburon) and 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron) were most inhibitory and 3-phenyl-1,1-dimethylurea (fenuron) was least inhibitory to the Hill reaction. 2-Chloro-4,6-bis(isopropylamino)-s-triazine (propazine) and 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine) were the most inhibitory and 2-chloro-4,6-bis(diethylamino)-s-triazine (chlorazine) was the least inhibitory of seven s-triazines to the Hill reaction. DNBP inhibited the Hill reaction by 50 percent (I50) at 1.3 × 10-5 M. An I50 value of 5.1 × 10-4 M was obtained for 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) which was the most inhibitory of the chlorinated phenoxyacetic acids tested. 3,4,5-Trichlorobenzoic acid (3,4,5-TBA) was the most inhibitory chlorinated benzoic acid tested (I50 #x2A75; 2 × 10-3M).

I50 values were established and compared for chloroplasts isolated from turnip greens, corn and soybeans for diuron, simazine, DNBP, isopropyl N-(3,4-dichlorophenyl)carbamate (3,4-DCIPC), isopropyl N-(3-chlorophenyl)carbamate (CIPC), 2,4,5-T and 3,4,5-TBA. Chloroplasts from the three species were essentially equally sensitive to each of the seven herbicides. Hence, no evidence was obtained which would explain species tolerance or susceptibility.

When turnip-green chloroplasts were treated with inhibitory concentrations of diuron, simazine, DNBP, CIPC, 2,4,5-T and 3,4,5-TBA in the dark, the herbicides could be removed by washing without seriously impairing the photolytic activity of the chloroplasts. However, irreversible changes occurred when the chloroplasts were illuminated while in contact with the herbicides, i.e., photolytic activity was not completely restored when the chloroplasts were washed, except with simazine.

Disking, relisting, cultivation and no repreparation were employed in repreparing for planting seedbeds that had received preemergence treatment with 3-(3,4-dichlorophenyl)1,1-dimethylurea (diuron). Only the relisting method of seedbed repreparation appeared hazardous with respect to crop injury from herbicidal residues. Retreatment with diuron tended to increase injury symptoms in replanted cotton but yields were not significantly reduced by retreatment. Early-season weed competition, preventable by preemergence treatment, appeared to be more hazardous to yields than slight to moderate herbicidal injury to the cotton. Cotton replanted on plots treated once or treated twice significantly outyielded that on untreated plots in one of the two years in which all plots were replanted.

Annual treatment with 1 lb/A of 2,4-D (2,4-dichlorophenoxyacetic acid) repeated for several years during mid- to late June gave excellent control of ironweed (Vernonia baldwinii). Later dates of treatment were not as effective. Mowing was not satisfactory as a weed control measure for ironweed. The populations in mowed plots in long-term experiments closely paralleled populations of untreated plots. Where numbers were reduced by spray treatment, recovery or reinfestation was comparatively slow. Quadrat counts were compared with visual estimation as methods of evaluating effects of treatments. The estimation method did not permit quantitative evaluation of year-to-year treatment or environmental effects.

Aqueous solutions of sodium and calcium polysulfides convert labeled simazine, 2-chloro-4,6-bis(ethylamino)-s-triazine-2,4,6-C14, to a chloroform insoluble, charcoal adsorbable derivative which is thought to be hydroxysimazine, 2-hydroxy-4,6-bis(ethylamino)-s-triazine-2,4,6-C14. In soil, calcium polysulfide accelerates the breakdown of simazine as shown by oat bioassay. These results suggest the possibility of using polysulfides to remove unwanted residues of chlorotriazine herbicides from crop lands.

Seeds of white spruce (Picea glauca, Moench), Colorado spruce (P. pungens, Engelm.), and Scotch pine (Pinus sylvestris L.) were treated with simazine in water at rates up to 1000 ppm for 20 hours, washed 10 min., rinsed twice, and sown. There was no effect on Colorado spruce or Scotch pine, but white spruce seeds were seriously affected above 250 ppm. Simazine has been considered not to affect seeds.

Salmonberry (Rubus spectabilis Pursh.) was basally treated in Western Oregon during the dormant season and at bud bursting in April, using a 1:1 mixture of 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), and the same mixture of 2,4-D and 2,4,5-T monobutyl ether glycol esters, and 2,3,6-trichlorobenzoic acid (2,3,6-TBA), in two carriers and with different additives, and with diesel oil alone. January applications gave generally poorer results than those in March and April. March and April applications produced 100% defoliation and kill for all treatments, with only erratic exceptions. Diesel oil alone gave effective control only in April. Sprays containing 2½% active ingredients gave as good results as those containing 5%.

A year's study of the kinds of weed seeds caught in an 18-mesh copper screen intercepting half the canal width found seeds of 20 species or genera. Bermudagrass (Cynodon dactylon) was found every month of the year; saltgrass (Distichlis spicata) only in March; others in 2 to 10 months of the year.

Carbon14 carboxy labeled 2,4-D with varied amounts of Tween 20 (polyoxyethylene sorbitan monolaurate) was applied to excised Kalmia angustifolia L. (Ericaceae) leaves and, after an absorption period, the radioactivity of the dried ground leaf tissue was measured by counting. Entry rate of 2,4-D into recently formed leaves decreased with increase in Tween 20, within the range of 2,4-D/Tween 20 ratios used (roughly 5/1 to 0.5/1). Entry rate of 2,4-D into older leaves increased with increase in Tween 20.