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Abstract

Three studies were conducted to evaluate preemergence herbicides for weed control efficacy and crop tolerance in radishes (Raphanus sativus L.) grown on organic soils. Herbicides evaluated were CDEC, metolachlor, alachlor, pendimethalin, thiobencarb, propachlor, metribuzin, and pronamide. Control of both broadleaf and grass weed species was provided by most herbicides evaluated with little or no apparent visual loss in crop vigor except mebribuzin. Weed control efficacy was reduced during the 2nd study due to excessive rainfall. Weedy and hand-weeded checks consistently produced some of the highest yields, indicating possibly some toxicity to the crop due to herbicide treatment or an apparent lack of weed competition. Chemical names used: 2-chloroallyl diethyl-dithiocarbamate (CDEC); 2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-l-methylethyl)acetamide(metolachlor); 2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide (alachlor); N-(l-ethylpropyl)-3,4-dimethyl-2,6-dinitro benzenamine (pendimethalin); S-[(4-chlorophenyl)methyl] diethylcarbamothioate (thiobencarb); 2-chloro-N-isopropylacetanilide (propachlor); 4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one(metribuzin), and 3,5-dichloro(N-l,l-dimethyl-2-propynyl)benzamide (pronamide).

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).

Field trials were conducted in Gainesville, Fla., to determine the influence of nitrogen fertilization on the interference effect of purple or yellow nutsedge on the yield of fresh tomato. Nitrogen (N) rates of 50, 100, 150, 200, 250, 300, and 350 kg·ha^–1 were applied broadcast to the soil. Before transplanting, 1-m-wide soil beds were covered with plastic and fumigated with methyl bromide to suppress the growth on undesired weeds. Nutsedge-free and purple or yellow nutsedge-infested tomato plots were separately established. `Solar Set' tomatoes were transplanted in the middle of the soil beds, 50 cm apart in a single row. In nutsedge-infested plots, weed densities known to cause significant yield reduction in tomato (100 purple nutsedge plants/m² and 50 yellow nutsedge plants/m²) were uniformly established perforating the plastic and transplanting viable tubers in the perforations. Purple and yellow nutsedge tubers were transplanted the same day as tomatoes and were allowed to interfere during the whole crop season. Results indicate that N rates had a significant effect on tomato fruit yield in both nutsedge-free and nutsedge-infested treatments. The presence of either purple or yellow nutsedge significantly reduced the fruit yield of tomato at all N rates. As N rates increased, tomato fruit yield reduction caused by the interference of either nutsedge species also increased. When yellow nutsedge was allowed to interfere with tomato, fruit yield loss was as low as 18% at 50 kg N/ha and as high as 42% at 350 kg N/ha. In purple nutsedge-infested tomato, fruit yield reductions ranged from 10% at 50 kg N/ha to 27% at 350 kg N/ha. N effects on nutsedge-free and nutsedge-infested tomato yields were described by quadratic equations, with maximum tomato fruit yield values being reached between 200 and 250 kg N/ha in both nutsedge-free and nutsedge-infested treatments.