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Eric B. Bish, Thomas A. Bewick and Donn G. Shilling

Laboratory experiments were conducted to evaluate the relationship between leaf area, leaf number, dry weight, and mg extract of Lycopersicon hirsutum (LA 1777) leaf washes and germination or root growth of common purslane (Portulaca oleracea) seeds. Additional experiments were conducted to determine the relationship between L. hirsutum (accessions 1777 and 1625) leaf washes and germination or root growth of common purslane seeds. Activity of separated sesquiterpenes from trichomes were compared to crude leaf washes. Results from the leaf washes of the L. hirsutum accessions (1777 and 1625) indicated that there was no significant difference between hexane leaf washes, methanol leaf washes, or crude leaf extracts when common purslane was used as the assay species. The accession 1777 was greater than 800 x more inhibitory to germination and greater than 300x more inhibitory to root growth of purslane seeds than accession 1625.

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Ernest R. Terry Jr., William M. Stall, Donn G. Shilling, Thomas A. Bewick and Steven R. Kostewicz

Studies were conducted to determine the critical period of smooth amaranth interference in watermelon (Citrullus lunatus L.) and muskmelon (Cucumis melo L. var. reticulatus). Best-fit linear or exponential regression models were used to predict the maximum period of competition and the minimum weed-free period for 10% yield loss. The maximum period of competition and minimum weed-free period was 0.50 and 2.97 weeks after watermelon emergence, respectively, and 1.0 and 3.9 weeks after muskmelon emergence, respectively. The critical periods of smooth amaranth interference for the crops were between those intervals. In both crops, late emerging smooth amaranth had little effect on total yield. Smooth amaranth introduced at crop emergence reduced total yield. The effect of competition on yield components, i.e., fruit number per hectare and fruit mass, varied by crop. Muskmelon fruit count was more sensitive to smooth amaranth competition than was watermelon fruit count. Conversely, mass per fruit of muskmelon was less sensitive to this competition than was mass per fruit of watermelon.

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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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Lambert B. McCarty, D. Wayne Porter, Daniel L. Colvin, Donn G. Shilling and David W. Hall

Greenhouse studies were conducted at the Univ. of Florida to evaluate the effects of preemergence herbicides on St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] rooting. Metolachlor, atrazine, metolachlor + atrazine, isoxahen, pendimethalin, dithiopyr, and oxadiazon were applied to soil columns followed by placement of St. Augustinegrass sod on the treated soil. Root elongation and biomass were measured following application. Plants treated with dithiopyr and pendimethalin had no measurable root elongation and root biomass was severely (>70%) reduced at the study's conclusion (33 days). Root biomass was unaffected following isoxaben and oxadiazon treatments, but oxadiazon applied at 3.4 kg·ha-1 reduced root length by 50%. Atrazine at 2.2 kg·ha-1 and metolachlor + atrazine at 2.2 + 2.2 kg·ha-1, did not reduce root length in one study, while the remaining atrazine and metolachlor + atrazine treatments reduced cumulative root length and total root biomass 20% to 60%. Metolachlor at 2.2 kg·ha-1 reduced St. Augustinegrass root biomass by >70% in one of two studies. St. Augustinegrass root elongation rate was linear or quadratic in response to all treatments. However, the rate of root elongation was similar to the untreated control for plants treated with isoxaben or oxadiazon. Chemical names used: 6-chloro-N-ethyl-N'-(l-methylethyl)-1,3,5-triazine-2,4-diamine(atrazine);S,S-dimethyl2-(difluoromethyl)-4-(2-methylpropyl)-6-(t∼fluoromethyl)-3,5-pyridinecarbothioate (dithiopyr); N-[3-(1-ethyl-1-methylpropyl)-5-isoxazolyl]-2,6-dimethoxybenzamide (isoxaben); 2-chloro-N-(2-ethyl- 6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide (metolachlor); 3-[2,4-dichloro-5-(1-methylethoxy)phenyl]-5-(1,1-dimethylethyl)-1,3,4-oxadiazol-2-(3H)-one (oxadiazon); N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine (pendimethalin).