Seed samples of triazine-resistant and susceptible pigweed were received from the four areas of the United States and Canada where resistant pigweed populations have appeared. The reported resistance or susceptibility of all biotypes was verified, and several plants from each seed source were grown to maturity for species identification. Pigweed samples from Maryland and Pennsylvania were smooth pigweed (Amaranthus hybridus L.). One resistant and one susceptible biotype from Washington were also smooth pigweed. All other samples from Washington and the sample from Ontario were Powell amaranth (A. powellii S. Wats.). None of the triazine-resistant seed samples were redroot pigweed (A. retroflexus L.), despite earlier reports of resistance in this species. Taxonomic characteristics important in the identification of smooth pigweed, Powell amaranth, and redroot pigweed are discussed.

To study rainfall effects, simulated rainfall was applied to velvetleaf (Abutilon theophrasti Medic.), common lambsquarters (Chenopodium album L.), wild mustard [Brassica kaber (DC.) L. C. Wheeler var. pinnatifida (Stokes) L. C. Wheeler], soybean [Glycine max (L.) Merr. ‘Hodgson’], and redroot pigweed (Amaranthus retroflexus L.) in greenhouse and field studies following foliar applications of the alkanolamine (AKA) salt or the butoxyethanol (BE) ester of 2,4-D [(2,4-dichlorophenoxy)acetic acid] at rates that induced equivalent levels of phytotoxicity. Simulated rainfall less than 1 min after herbicide treatment reduced the phytotoxicity of the AKA salt of 2,4-D to a much greater extent than that of the BE ester with effects ranging from elimination of all injury from the AKA salt to soybeans to no reduction in phytotoxicity of the BE ester to common lambsquarters. The quantity of simulated rainfall required to induce maximum reductions in phytotoxicity of the BE ester ranged from 1 mm on common lambsquarters to 15 mm on velvetleaf. The time interval from 2,4-D treatment until rainfall required to achieve a phytotoxic response level of 80% of that attained without rainfall varied greatly among plant species and herbicide formulations; ranging from less than 1 min for the BE ester on common lambsquarters to more than 24 h for the AKA salt on velvetleaf. The addition of an alkylarylpolyoxyethylene glycol surfactant to 2,4-D spray solutions reduced herbicide rates required to induce equivalent levels of phytotoxicity, increased losses in phytotoxicity of the BE ester caused by rainfall, and reduced the time interval from treatment to rainfall required to attain an equivalent level of phytotoxicity with the AKA salt.

Four field experiments conducted over 3 yr indicated that cultivation alone failed to prevent johnsongrass [Sorghum halepense (L.) Pers.] from reaching densities that severely reduced yields of cotton (Gossypium hirsutum L. ‘Acala SJ-2’). Density of johnsongrass in plots cultivated four times and hoed weekly for 8 weeks after emergence was reduced to 1 shoot/m2 at harvest compared to 74 shoots/m2 for plots that were only cultivated. In addition to a 60% average yield loss of seed cotton, yield losses ranging from 40 to 76%, ginning losses were also greater from cultivated than from hand-weeded plots. Compared to cultivated plots, supplementing cultivation with two postemergence applications of 3.0 kg/ha of DSMA (disodium methanearsonate) increased the average yield of cotton by 20% and reduced perennial johnsongrass densities by 64% at harvest. Although yields were improved by applying DSMA, they averaged 40% less than those of hand-weeded plots. The temporary weed control obtained with DSMA was profitable in terms of the additional lint and seed obtained, but insufficient cotton was produced to pay expenses for producing the crop by any of the methods of weed control. High labor costs for hoeing prevented this treatment from being profitable.

The experiment was conducted during 3 yr to measure the effect of corn cockle (Agrostemma githago L.) competition on winter wheat (Triticum aestivum L. ‘McDermid’) grain yield. In all years corn cockle was removed monthly in separate treatments from emergence (October) to harvest (July). Hand-weeded and weedy controls were compared to herbicidal control using treatments of bromoxynil (3,5-dibromo-4-hydroxybenzonitrile). Reductions in yield of wheat as affected by competition from corn cockle were not significant if weeds were removed before February of each year in winter wheat that had been seeded the previous October. Bromoxynil controlled more weed plants when applied on corn cockle that had less than four leaves (fall treatment). Corn cockle densities of 170 to 340 plants/m2 at the Pendleton, Oregon site lowered wheat yields an average of 18% when competition was eliminated by March. Competition that continued until wheat harvest (July) reduced yields 60%.

The effects of temperature and dew period on infectivity of Puccinia chondrillina Bubak and Syd. and the effect of light intensity on uredospore germination were studied under controlled environmental conditions. When rush skeletonweed (Chondrilla juncea L.) plants were inoculated with uredospores at 8, 16, and 24 C, larger pustules developed faster at 24 C than at the cooler temperatures. The greatest amount of infection occurred at 8 and 16 C (75% of the leaf area infected compared to 25% at 24 C). The greater number of infection sites at cooler temperatures was coincident with an increase in uredospore germination at 8 and 16 C compared to 24 C. Increasing the dew period from 0 to 6 h increased the amount of infection by the rust. Light intensities as low as 0.5 klux significantly reduced uredospore germination on water agar. Germ tube growth was restricted significantly by light intensities of 2.5 to 4.5 klux. Applications of uredospores to aid in biological control of this serious weed would be most successful if made at dusk under cool temperatures when an extended dew period is expected.