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James W. Shrefler, William M. Stall and Joan A. Dusky

Three field studies on high-organic-matter soils were conducted to determine the zone of influence of spiny amaranth on lettuce head quality. Spiny amaranth reduced lettuce head firmness at all distances from the weed, ≤105 cm. Lettuce ribbiness increased at 15 and 45 cm compared with the weed-free control. Untrimmed lettuce head weight was not affected by spiny amaranth presence beyond 45 cm. Trimmed lettuce head weight was reduced at all distances compared with the control. Stem diameter and core length were not affected by spiny amaranth competition. The presence of a single spiny amaranth plant significantly influenced some lettuce quality traits at ≤105 cm.

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Dorm G. Shilling, Joan A. Dusky, Mark A. Mossier and Thomas A. Bewick

Poor emergence of commercially grown lettuce has been observed when planted immediately after the removal of a celery crop. Greenhouse experiments were conducted to evaluate the possible allelopathic effects of celery residue on the emergence and growth of lettuce. The influence of amount and type of celery tissue, growth medium and fertility, incubation time in soil, and amendment of growth medium containing celery residue with activated charcoal was evaluated with respect to the allelopathic potential of celery. Celery root tissue was 1.8 and 1.6 times more toxic to lettuce seedling growth than was celery petiole or lamina tissue, respectively. Lettuce shoot growth was inhibited to a greater extent when grown in sand amended with celery residue rather than either amended vermiculite or potting soil. Incubation of celery root residue in soil for 4 weeks increased phytotoxicity at 1% (v/v) and decreased it at 4% (v/v). Increasing the fertility of pure sand with varying amounts of Hoagland's solution did not reverse the allelopathic effects of celery residue. The addition of activated carbon to the medium increased the growth of lettuce exposed to celery residues. Celery residues possess allelopathic potential to developing lettuce seedlings. Celery tissue type and concentration, soil type, incubation of celery root residue in soil, and addition of activated carbon to the growing medium influenced the magnitude of the observed phytotoxicity.

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Bielinski M. Santos, Joan A. Dusky, William M. Stall, Donn G. Shilling and Thomas A. Bewick

The effects of different populations densities of smooth pigweed and common purslane were determined in field trials conducted in organic soils. `South Bay' lettuce was planted in twin rows on 90-cm planting beds. Weed densities used were 0, 2, 4, 8, and 16 weeds per 6 m of row (5.4 m2). Phosphorus (P) was applied broadcast (1200 kg P/ha) and banded 2 inches below each lettuce row (600 kg P/ha). Lettuce fresh weights were collected 8 weeks after emergence. Data collected indicated that P regime and density had significant effects on lettuce yield and quality. For both weeds, yield decreased as density increased. In all cases, lettuce showed greater yields at a given density when grown with P banded than when P was applied broadcast. Critical density for smooth pigweed for P broadcast was between 2 and 4 plants per 5.4 m2, whereas this critical density occurred between 8 and 16 plants per 5.4 m2 when P was banded. Yield reductions of up to 24.4% and 20.1% occurred at the highest smooth pigweed density for broadcast and banded P, respectively. Two common purslane plants per 5.4 m2 were enough to reduce lettuce yields. Banding P helped lettuce to produce significantly more within each common purslane density. Yield reductions of 47.8% and 44.3% occurred at the highest common purslane density for broadcast and banded P, respectively. Apparently, banding P gives an additional advantage to the crop against smooth pigweed and common purslane.

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Bielinski M. Santos, Joan A. Dusky, William M. Stall, Donn G. Shilling and Thomas A. Bewick

The effects of different smooth pigweed and common purslane removal times and two phosphorus (P) fertility regimes were studied under field conditions. Head lettuce (cv. South Bay) in organic soils low in P fertility. Smooth pigweed and common purslane were grown at a density of 16 plants per 6 m of row (5.4 m2) and five removal times (0, 2, 4, 6, and 8 weeks) after lettuce emergence. Phosphorus (P) was applied broadcast (1200 kg P/ha) and banded 2 inches below each lettuce row (600 kg P/ha). Lettuce fresh weights were collected 8 weeks after emergence. When smooth pigweed was removed after 4 weeks, significant reductions (–17%) were observed for P banding. However, these reductions occurred after 2 weeks if P was broadcast. No significant differences were observed if removal was imposed later for P broadcast, whereas lettuce yields gradually decreased as removal time was delayed. These findings indicate that P banding can counteract the negative impact of smooth pigweed on lettuce and may allow farmers to delay weed control (if necessary) for another 2 weeks without significant yield reductions. Common purslane interference did not cause significant lettuce yield reductions as compared to the weed-free control for 6 weeks when P was banded, whereas this was true for P broadcast up to 4 weeks. Phosphorus fertility regime significantly influenced the period of weed interference of common purslane with lettuce, reducing its impact when P was banded.

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Russell T. Nagata, Joan A. Dusky, Robert J. Ferl, Antonio C. Torres and Daniel J. Cantliffe

Six transgenic `South Bay' lettuce lines (Lactuca sativa L.) with elevated levels of 5-enolpyruvyl shikimate-3-phosphate synthase (EPSPS) were evaluated for tolerance to the herbicide glyphosate. The six lines were selected from ≈150 independent transformation events using an Agrobacterium tumefaciens system. Three assay methods were used to identify gene expression with regard to glyphosate resistance. Leaf disks of the transgenic lines were cultured on media containing 0 to 1280 μm glyphosate. Leaf disks of the control had lower dry weight (DW) at 40 μm and greater glyphosate than all the transgenic lines. The transgenic lines continued to grow even at 1280 μm. Plants 21 days old were sprayed in the greenhouse with rates of glyphosate at 0 to 35.84 kg·ha-1. DW of all the lines were similar to the control, with a few exceptions, at glyphosate concentrations from 0 to 0.56 kg·ha-1. At 2.24 to 8.96 kg·ha-1 all of the transgenic lines had DW greater than the control, while at 17.92 and 35.84 kg·ha-1 only B-32, B-33, C-3, and C-14 had DW greater than the control. The resistant line from the greenhouse experiment, B-32, grew normally in field trials at the highest glyphosate rate, 17.92 kg·ha-1, while control plants died at 0.56 kg·ha-1 glyphosate. Lines A-11 and C-3 had lower DW than B-32 at 2.24 kg·ha-1 glyphosate and greater. While leaf disk assays can identify potential transformed lines expressing the EPSPS and glyphosate oxidase (GOX) gene, and greenhouse screening can evaluate seedling vigor after glyphosate application, field trials are necessary to evaluate plant growth and yield through the growing season. Chemical name used: N-(phosphono-methyl) glycine (glyphosate).