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Albert H. Markhart III, Milton J. Harr, and Paul Burkhouse

Weed control in organic vegetable production is a major challenge. During Summer 2004, we conducted field trials to manage weeds in organic sweet corn, carrots and onions. In sweet corn, we evaluated the efficacy of transplanting greenhouse-grown sweet corn seedlings. In carrots and onions, we tested vinegar and several concentrations of acetic acid. Studies were conducted in southwestern Minnesota at the Lamberton Research and Outreach Center and in eastern Minnesota at Foxtail Farm in Shaefer. Ten-day-old corn transplants were effective at both locations. Stand establishment was greater, less tillage was needed, and yield was greater than in the seeded plots. Straight vinegar was not very effective in controlling weed populations. Although there was greater damage to broadleaf weeds than grasses, straight vinegar did not reduce the need for tillage. Although 10% to 20% acetic acid did provide better weed control, it significantly damaged carrot and onion seedlings. These results suggest that using sweet corn transplants is time and cost effective for small acreage sweet corn production such as CSAs. Vinegar and acetic acid are problematic. Nonselectivity, potential danger in handling, and poor control at low concentrations were all considered significant disadvantages.

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Roger F. Albach, Michael V. Hickman, and Darius Swietlik

Simazine is a pre-emergent herbicide of low water solubility (5 ppm) that is long-lasting and has high crop selectivity. Simazine residues in a citrus orchard, portions of which have been under chemical weed control for an extended period, were quantified. The orchard is located in the semi-tropical region of South Texas on alluvial Hidalgo sandy clay soil of relatively uniform profile. The orchard investigated had been included in a long-term weed control study lasting 30 years. Three treatments were established based upon the history of the herbicide use. Treatment A had received herbicide applications for 30 years and simazine applications during the last 8 years (22.8 kg·ha-1 total simazine application). Treatment B was mechanically cultivated without herbicides for 28 years and received annual simazine applacations the last 2 years (7.6 kg·ha-1 total). Treatment C was maintained in grass sod and received no herbicide. Soil fractions were taken at 15-cm increments by soil probe to a total depth of 1.5 m. Three probes in each of three replicate plots for each treatment were analyzed. The highest concentration of simazine was found near the surface and decreased with depth. The rate of this decrease was less with treatment A than with B. In the depth range of 15 to 105 cm, treatment A had a greater simazine content than with treatment B. Simazine was detected in some samples below the 120 cm depth.

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S.A. Fennimore, M.J. Haar, and H.A. Ajwa

The loss of methyl bromide (MB) as a soil fumigant has created the need for new weed management systems for crops such as strawberry (Fragaria ×ananassa Duchesne). Potential alternative chemicals to replace methyl bromide fumigation include 1,3-D, chloropicrin (CP), and metam sodium. Application of emulsified formulations of these fumigants through the drip irrigation system is being tested as an alternative to the standard shank injection method of fumigant application in strawberry production. The goal of this research was to evaluate the weed control efficacy of alternative fumigants applied through the drip irrigation system and by shank injection. The fumigant 1,3-D in a mixture with CP was drip-applied as InLine (60% 1,3-D plus 32% CP) at 236 and 393 L·ha-1 or shank injected as Telone C35 (62% 1,3-D plus 35% CP) at 374 L·ha-1. Chloropicrin (CP EC, 95%) was drip-applied singly at 130 and 200 L·ha-1 or shank injected (CP, 99%) at 317 kg·ha-1. Vapam HL (metam sodium 42%) was drip-applied singly at 420 and 700 L·ha-1. InLine was drip-applied at 236 and 393 L·ha-1, and then 6 d later followed by (fb) drip-applied Vapam HL at 420 and 700 L·ha-1, respectively. CP EC was drip-applied simultaneously with Vapam HL at 130 plus 420 L·ha-1 and as a sequential application at 200 fb 420 L·ha-1, respectively. Results were compared to the commercial standard, MB : CP mixture (67:33) shank-applied at 425 kg·ha-1 and the untreated control. Chloropicrin EC at 200 L·ha-1 and InLine at 236 to 393 L·ha-1 each applied singly controlled weeds as well as MB : CP at 425 kg·ha-1. Application of these fumigants through the drip irrigation systems provided equal or better weed control than equivalent rates applied by shank injection. InLine and CP EC efficacy on little mallow (Malva parviflora L.) or prostrate knotweed (Polygonum aviculare L.) seed buried at the center of the bed did not differ from MB : CP. However, the percentage of weed seed survival at the edge of the bed was often higher in the drip-applied treatments than in the shank-applied treatments, possibly due to the close proximity of the shank-injected fumigant to the edge of the bed. Vapam HL was generally less effective than MB : CP on the native weed population or on weed seed. The use of Vapam HL in combination with InLine or CP EC did not provide additional weed control benefit. Chemical names used: 1,3-dichloropropene (1,3-D); sodium N-methyldithiocarbamate (metam sodium); methyl bromide; trichloro-nitromethane (chloropicrin).

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A.W. Caylor, W.A. Dozier Jr., G. Wehtje, D.G. Himelrick, J.A. McGuire, and J.A. Pitts

The postemergence-active herbicides lactofen, fomesafen, and acifluorfen were applied to established matted-row strawberry plants (Fragaria × ananassa) and evaluated for broadleaf weed control and foliar phytotoxicity. Strawberries were evaluated for yield and fruit quality. Treatments were applied following June renovation. All herbicide treatments resulted in acceptable control of broadleaf weeds present at the time of application; however, sicklepod (Cassia obtusifolia) germinated after herbicide application. All treatments caused foliar injury within 3 days after application. No injury symptoms were evident 21 days after treatment due to new foliage development. Fomesafen and acifluorfen were the only herbicides to suppress runner count. Yields the following year were not reduced by herbicide treatments. Chemical names used: (±)-2-ethoxy-l-methy1-2-oxoethyl 5-[2-chloro-4-(trifluoromethyl) phenoxy]-2-nitrobenzate (lactofen); 5-[2-chloro-4-(trifluoromethyl)phenoxy] -N -(methylsu1fonyl)-2-nitrobenzamide (fomesafen); 5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid (acifluorfen).

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Bielinski M. Santos, James P. Gilreath, Maria de L. Lugo, and Luis E. Rivera

A field study was conducted in Gurabo, P.R., to examine the potential of drip-applied herbicides for weed control in polyethylene-mulched tomato. The herbicide treatments were a) metolachlor at a rate of 1.1 kg a.i./ha; b) napropamide at 2.2 kg a.i./ha; c) pebulate at 4.5 kg a.i./ha; and d) trifluralin at 0.8 kg a.i./ha. A nontreated control was added. Each herbicide plot was split in two application methods: preemergence application and through the drip lines with 100 m3 water. In both cases, herbicides were delivered three weeks before tomato transplanting. There was no significant difference between the two delivery methods. Metolachlor showed the best control of broadleaf weeds (>80%) and highest tomato fruit yield. Applying herbicides through the drip lines is a viable alternative in mulched tomato.

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Angela K. Tedesco, Gail R. Nonnecke, John J. Obrycki, Nick E. Christians, and Mark L. Gleason

Field plots of four production systems of `Tristar' dayneutral and `Earliglow' Junebearing strawberry (Fragaria xananassa Duch.) were established in 1993. Productions systems included conventional practices (CONV), best-management practices including integrated crop management (ICM), organic practices using corn gluten meal, a natural weed control product, (ORG-CGM), and organic practices using a natural turkey manure product (ORG-TM). `Earliglow' plants grown with ORG-CGM showed the highest number of runners and total vegetative biomass. Plots with CONV and ICM systems using standard herbicide treatments had lower total weed numbers (11 and 18, respectively) than ORG-CGM (63) and ORG-TM (58). `Tristar' plant growth, yield and berry number were reduced when plants were grown under straw mulch in ORG-CGM and ORG-TM compared to CONV and ICM plots with polyethylene mulch.

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D. Brault, K.A. Stewart, and S. Jenni

Optical properties of paper and plastic mulches were determined in experiments on mulched head lettuce (Lactuca sativa L.) grown in organic soil in 1997-98. Mulches used in 1997 were a coextruded white/black polyethylene, a beige paper coated with latex on both sides and a black paper coated with latex on both sides. Three supplementary mulches were added in the 1998 experiment: beige paper coated with a biodegradable polymer either coated on the underside, on the top side or on both sides. Optical properties of the polyethylene mulch remained stable over the course of the experiment. As the paper mulches aged, they changed structurally, spectrally, or both, but remained in place until harvest. The black paper was the only mulch that offered complete weed control, although the weeds under the other mulches did not develop beyond the cotyledonary or two-true-leaf stage.

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Vasey N. Mwaja and John B. Masiunas

A three-year study determined the effect of winter cover crops on weeds and vegetable crops in a vegetable production system. Winter rye and hairy vetch were interseeded in the fall of 1990, 1991 and 1992 at 112 and 34-kg ha-1, respectively. The cover crops were killed by ether applying glyphosate at 1.1 kg a.i ha-1 [reduced tillage(RT)] or mowing and disking the cover crop (Disked). The conventional tillage (CT) was bare ground with a preplant incorporated application of 0.84 kg a.i ha-1 of trifluralin. During the three years, the greatest snap bean yields were in the CT; total yields of cabbage and tomato varied between the years; and were not affected by management systems. Weed control was similar in the RT and CT treatments during the three years. Disked cover crop treatments tended to have greater weed numbers than either RT or CT treatments.

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Charlotte Herman and Emily Hoover

The objective of our study was to establish first year strawberry plantings without using herbicides. `Honeyoye' transplants were set into plots measuring 6.1m × 7.32m on 21 May, 1993. Four treatments were established: winter wheat, a dwarf Brassica sp., napropamide (2.24kg/h), and no weed management. After the strawberry plants, cover crops (and some weeds) were fairly well established, (18 June) 6 week-old African “weeder” geese were put into half of each plot to graze. Weekly data was taken on the percentage of soil area covered with plant material, height and stage of development of plants, and weeds present. A weed transect was done in 6 July. Plant material was collected from each plot on 26 July and 16 Sept. in a 0.2m2 area, and dried. The most promising cover crop treatment was the dwarf Brassica for early season weed control. However, the herbicide treatment with no geese produced the best strawberry plant growth.

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Jeffrey G. Norcini, Melvin P. Garber, William G. Hudson, Ronald. K. Jones, Ann. R. Chase, and Kane Bondari

Members of the American Association of Nurserymen and the Society of American Florists were surveyed as to their use of herbicides and nonchemical alternative weed control practices for 1993. Glyphosate was the top-ranking herbicide among the total of 37 reported, in terms of number of respondents and estimated total amounts of active ingredients applied. It was used by all but two of the respondents that used herbicides in their operations. Oryzalin was the top-ranked preemergent herbicide, and was second only to glyphosate in number of respondents and amount of active ingredient applied. The highest estimated use in amounts of active ingredient applied was in the southeastern (43% of total) and north-central (27% of total) regions, nearly two to three times the estimated use in the northeastern or western regions. However, there were only about 50% more respondents in the southeastern or north-central regions compared to the other regions. About 56% of herbicide active ingredients used were in field sites, 22% in container sites, 19% in perimeter areas, and 3% in green-houses. Large firms (annual sales more than $2,000,000) used the greatest estimated total amount of active ingredients, while small firms (annual sales more than or equal to $500,000) tended to use nonchemical alternatives the most. Nearly all respondents used handweeding or hoeing as part of their weed control program. Mowing was used by 84% of the respondents, 71% used tractor cultivation, and 66% used mulches (includes gravel and black plastic). Alternative methods were rated as somewhat effective to very effective by 65% or more of the respondents who used them.