A field study was conducted in 1987 to 1991 to determine the effect of tillage and rainfall on distribution of atrazine in soil. Soil samples (10-cm increments to 50 cm) and crop residue samples were taken at regular intervals after application each year and analyzed for atrazine. Crop residue and living vegetation on no-till plots intercepted 60 to 70% of the applied atrazine; 3 to 16% of the atrazine remained in crop residue 1 to 2 wk later. The amount of atrazine recovered in soil, 1 to 2 wk posttreatment, ranged from 22 to 59 and 47 to 73% of the amount applied for no-till and conventional till, respectively. An average of 2.6 times more atrazine was recovered in the surface 10 cm of soil under conventional till than under no-till for all samplings and years. Total amounts of atrazine in the sampled profile (0- to 50-cm depth) were also generally lower under no-till than conventional till. More leaching below 10 cm occurred under no-till than conventional till, particularly in 1988 and 1990 when rain fell soon after application. Variation in soil atrazine levels among years was related to timing and amount of the first and subsequent rainfall after application.

A biotype of rigid ryegrass (Lolium rigidum Gaudin biotype VLR69) resistant to some ALS inhibitors was characterized to determine the mechanisms of resistance to the sulfonylurea herbicides. The biotype had a high level of resistance to chlorsulfuron (20×) and triasulfuron (25×), and an intermediate level of resistance to imazaquin (7×) and sulfometuron (7.5×) but exhibited a low level of resistance to imazapyr (2.5×). At 60 to 90 g ai ha-1 sulfometuron, 4% of the population survived without apparent herbicidal effect The same response to sulfometuron was also observed when seeds of the resistant biotype VLR69 were germinated on agar containing sulfometuron. At 27 nM sulfometuron, 4% of the seeds germinated and grew normally while the growth of the bulk of the population was retarded. This differential response to sulfometuron at the seedling stage allowed selection of sulfometuron-resistant individuals from the population. Activity of ALS extracted from these sulfometuron-resistant plants was less sensitive to inhibition by chlorsulfuron than ALS extracted from plants considered sulfometuron susceptible in the same system. Unselected plants from the VLR69 population were able to detoxify chlorsulfuron more rapidly than susceptible VLR1 plants. It is apparent that there are at least two mechanisms of sulfonylurea resistance in the resistant biotype VLR69 which occur at different frequencies within the population. These data show that more than one mechanism or resistance may develop in response to herbicide selection pressure and that the resulting populations are not necessarily homogeneous.

Weed germination studies often quantify only radicle emergence to indicate germinated seeds. Four categories were used to investigate root and shoot emergence from germinating seeds including: root (seeds with radicles emerged); shoot (seeds with plumules or cotyledons emerged); complete (both roots and plumules emerged); and total (any emergence: roots, shoots, or both). Medusahead and yellow starthistle germination tests illustrated that emergence was equal in the root and total categories. Shoot and complete categories had identical counts, but those counts were often different from the total and root categories. Emergence of medusahead shoots at 8 C was 3 d later than roots, but at 18 C roots and shoots emerged nearly simultaneously. Yellow starthistle roots were not affected by picloram, but emergence of cotyledons (shoot category) was reduced by 80 %. Yellow starthistle seeds stored for 74 and 90 mo produced more radicles than cotyledons after 28 d at 28 C. Medusahead and yellow starthistle root emergence in the laboratory sometimes exceeded seedling emergence from a greenhouse soil mixture. The presence of both roots and shoots (complete category) was the most accurate criterion for documenting yellow starthistle and medusahead seed response to laboratory test conditions.

Greenhouse experiments were conducted to evaluate phytotoxicity, absorption, and translocation of thifensulfuron when applied to pea at different rates, droplet sizes, and as dry particles. Thifensulfuron rates that reduced shoot dry weight by 25% were 0.46, 0.59, 0.98, and 1.21 g ai ha-1 for droplet diameters of 110, 155, 300, and 450 μm, respectively. The 14C-thifensulfuron absorption as percent of applied increased twofold, whereas translocation decreased 36% as herbicide concentration increased from 18 to 146 mg L-1 in 980-μm droplets. Small droplets (50 μm) that dried to particles before contact with leaves did not damage pea plants. However, under dew conditions, dry particles damaged peas to a similar degree as liquid droplets. We concluded that small and concentrated droplets of thifensulfuron can damage peas more than large and diluted droplets as a result of increased herbicide absorption.

In greenhouse tests, the host specificity of Alternaria crassa (Sacc.) Rands, a mycoherbicide for jimsonweed was altered by the addition of water-soluble filtrates of jimsonweed and hemp sesbania or fruit pectin to fungal conidia suspensions. Several crop and weed species that were resistant to the fungus alone exhibited various degrees of susceptibility following these amendments, with 100% mortality occurring to the weeds hemp sesbania, showy crotalaria, and eastern black nightshade when treated with the fungus/pectin mixture. Nonhost plants that were inoculated with conidial mixtures amended with sucrose or cellulose were not affected. Modification of host specificity of this pathogen could have an influence on its bioherbiddal potential

Downy brome is a grassy weed that infests millions of acres of winter wheat in the Western United States and costs farmers millions of dollars in lost yields. It can not be controlled consistently using chemicals or cultural practices. Isolates of naturally occurring rhizobacteria have shown potential as biological control agents for downy brome. Centrifuged supernatant from aerobic shake cultures of Pseudomonas fluorescens strain D7 suppressed germination of seeds and reduced root and shoot growth of downy brome in agar diffusion assays. The present article relates to the isolation and characterization of active compounds from strain D7. The active compounds were denatured and lost activity with commonly used techniques of extraction, concentration, and purification of microbial products. Two unusual techniques for isolation and partial purification of the active compounds from supernatant of strain D7 were developed. With one technique, supernatant was concentrated at 30 to 35 C under vacuum to 10% of the initial volume. Subsequently, the concentrate was chromatographed on sephadex gels to separate the active fraction. With the second technique, cell-free supernatant was frozen, with or without prior concentration, at −12 C for 12 to 24 h. The frozen supernatant was then thawed at 2 to 4 C, resulting in precipitation and crystallization of the active fraction. The active fraction isolated with either method inhibited downy brome. Inhibition was complete at concentrations as low as 1 mg total dry matter L-1 agar. Also, the active fraction inhibited the plant pathogenic fungus Gaeumannomyces graminis var. tritici. Chemical analysis of the active fraction complex revealed the presence of chromopeptides and other peptides, fatty acid esters, and a lipopolysaccharide matrix. Separation of any of the components from the complex resulted in nearly complete loss of activity against downy brome.

Rapid biochemical tests developed for measuring viability of nondormant crop seed have been used for measuring dormant weed seed viability. The objective of this experiment was to propose and test a new method for validating one of these assays, the tetrazolium assay, for determining dormant weed seed viability. Subsamples of partially dormant jointed goatgrass seed from the same seed source were periodically subjected to sequential germination and tetrazolium assays as they afterripened until seed were completely nondormant. Changes in percent germinated, percent tetrazolium positive (presumably dormant), and percent tetrazolium negative (presumably dead) seed were graphed versus time, and compared with models of valid and invalid cases. The validation method showed that the tetrazolium assay accurately determined the viability of seed remaining after a 3-wk germination period for jointed goatgrass seed removed from joints at the start, but underestimated the viability for seed afterripened and germinated in joints. The validation method showed that additional research is needed to refine the tetrazolium assay for this latter purpose.