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Bala Rathinasabapathi, James Ferguson, and Mark Gal

Shredded and chipped wood mulches are used for weed suppression in perennial fruit crops, in urban landscapes, and occasionally in vegetable crops. Wood chip mulches with weed-suppressing allelochemicals may be more effective for weed control, especially under sustainable and organic production systems, than mulches without such properties. The objective of this study was to test for the presence of water-soluble allelochemicals in wood chips derived from tree species, often found in wood resource recovery operations in the southeastern US. Presence of allelochemicals in water eluates of woodchips and leaves was evaluated in a lettuce bioassay. Eluates of wood chips from red maple (Acer rubrum L.), swamp chestnut oak (Quercus michauxii Nutt.), red cedar (Juniperus silicicola L.H. Bailey), neem (Azadirachta indica A. Juss.), and magnolia (Magnolia grandiflora L.) highly inhibited germinating lettuce seeds, as assessed by inhibition of hypocotyl and radicle growth. The effects of wood chip eluates from these five species were more than that found for eluates from wood chips of black walnut (Juglans nigra L.,) a species previously identified to have weed-suppressing allelochemicals. Tests on red cedar, red maple, and neem showed that water-soluble allelochemicals were present not only in the wood but also in the leaves. In greenhouse trials, red cedar wood chip mulch significantly inhibited the growth of florida beggarweed (Desmodium tortuosum DC.), compared to the gravel-mulched and no-mulch controls.

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Cécile Bertin, Andy F. Senesac, Frank S. Rossi, Antonio DiTommaso, and Leslie A. Weston

weedy and crop species show the promise of allelopathic potential for the suppression of surrounding vegetation. However, few studies have been conducted to further evaluate the allelopathic potential of these additional species ( Hoffman et al., 1996

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Michael J. Adler and Carlene A. Chase

allelopathic suppression of weed seed germination and seedling establishment. Future studies should focus on isolating and identifying the putative allelochemicals in shoots and roots of sunn hemp and cowpea and determining whether the allelochemicals are

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N.G. Creamer, M.A. Bennett, J. Cardina, and E.E. Regnier

Little research has been conducted to quantify allelopathic suppression of weeds in the field. The objectives of this study were to develop an adequate control for separating physical from allelochemical effects, use the control to quantify allelochemical suppression in the field, and determine whether a mixture of cover crops would provide a broader spectrum of weed control than single species. Hairy vetch, rye, crimson clover, and barley were cut into 5-cm pieces, shaken in distilled water (pH 6) to leach allelochemicals, and redried. A seed germination bioassay confirmed that leached cover crops were nontoxic to germinating seeds. Physical suppression of Eastern black nightshade by the four cover crop species occurred in the field study, as did allelochemical suppression by crimson clover. Only rye physically suppressed yellow foxtail, and none of the cover crops suppressed yellow foxtail allelochemically.

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Charles Macharia and Ellen B. Peffley

The genus Amaranthus contains many species which are common weeds found on the Texas high plains. In a field experiment plant height and numbers of plants of Amaranthus varied when grown with different Allium genotypes: Allium fistulosum var. `Heshiko' and an interspecific F1 hybrid 81215 (Heshiko × A. cepa cv. `New Mexico Yellow Grano'). The genotypes that showed no allelopathic effect were A. cepa cv `New Mexico Yellow Grano', A. fistulosum var. `Ishikura', and their F1 hybrid 8273. On the basis of these observations experiments have been done to quantify the degree of suppression. A randomized complete block design was used under greenhouse conditions in order to measure growth characters of Amaranthus.

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Mathieu Ngouajio and Milton E. McGiffen Jr.

Organic agriculture is growing in importance worldwide. In the United States, the rate of increase of organic growers was estimated at 12% in 2000. However, many producers are reluctant to undertake the organic transition because of uncertainty of how organic production will affect weed population dynamics and management. The organic transition has a profound impact on the agroecosystem. Changes in soil physical and chemical properties during the transition often impact indirectly insect, disease, and weed dynamics. Greater weed species richness is usually found in organic farms but total weed density and biomass are often smaller under the organic system compared with the conventional system. The improved weed suppression of organic agriculture is probably the result of combined effects of several factors including weed seed predation by soil microorganisms, seedling predation by phytophagus insects, and the physical and allelopathic effects of cover crops.

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Bruce P. Bordelon and Jill Hubertz

In a previous study to determine the feasibility of using herbicide desiccated cover crops for weed suppression during vineyard establishment, we found that weed suppression is excellent for about 6 to 8 weeks after desiccation in fall-planted rye. By the end of the season, however, weed growth in rye plots was similar to weedy control plots. Vine growth was reduced in rye plots compared to weed-free bare ground plots. Because of the experimental design, no follow-up weed control was performed in the rye plots and weeds eventually became well-established. So, it was impossible to determine if reduced vine growth was due to weed competition or allelopathy from the rye residues. A second study was conducted to determine the effects of follow-up weed control (with glyphosate) in fall-planted rye plots and weedfree bare ground plots. Results indicate that vine shoot number, shoot length, leaf area, and top growth dry weight was greatest in weedfree bare ground, less, but not significantly so in rye with follow-up weed control, and significantly less in rye without follow-up weed control. Root dry weight was reduced in rye with and without follow-up weed control compared to weedfree bare ground. Root dry weight was reduced 37% in rye with follow-up weed control and 63% in rye without follow-up weed control compared to weedfree bare ground. These results suggest that weed competition is not the primary cause of vine growth reduction in herbicide desiccated rye cover crops, so there is likely allelopathic effects of the rye residues on grapevines, which would limit using rye as a desiccated cover crop during vineyard establishment. However, there may be some value in using rye in established vineyards to reduce vigor.

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Leslie A. Weston and Andrew F. Senesac

For the past 5 years, we have evaluated more than 100 herbaceous perennial groundcovers, including both grasses and grass mixtures, as well as ornamental broadleaf materials, for their ability to establish, suppress weeds, provide aesthetic appeal, and resist pests in various landscape and roadside settings across New York State. By working in cooperation with the NYSDOT, we have developed recommendations for materials that have performed well in difficult, potentially stressful, roadside and landscape settings. We have performed replicated research and demonstration trials that have clearly shown that certain species and cultivars provide effective weed suppression; great aesthetic appeal due to foliar texture, color, or flowering, resist pests and diseases; and require low maintenance over time. In addition, certain materials tolerate high levels of salt (NaCl), simulating roadside salt application exposure, in supplemental greenhouse studies. Materials generally suppressed weeds effectively by forming a dense canopy in a short period of time, and reducing light interception at the soil surface under this dense canopy. Certain groundcovers also appeared to exhibit strong potential allelopathic properties when grown either in field or laboratory settings. The selection of new plant materials for use in low-maintenance landscape settings offers potential to reduce time and maintenance inputs in difficult landscape or roadside settings, with the added benefit of reducing pesticide application in these settings for weed management. Additional studies are currently underway to develop further recommendations for use of warm- and cool-season turfgrasses in these settings.

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Muhammad Mansoor Javaid, Manish Bhan, Jodie V. Johnson, Bala Rathinasabapathi, and Carlene A. Chase

’s allelopathic properties for weed suppression in horticultural crop production systems, we examined the phytotoxicity of its aqueous foliar extracts and ground, dried residues in an earlier study ( Adler and Chase, 2007 ). Both sunn hemp extracts and ground

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Na Liu, Baoli Zhou, Xin Zhao, Bo Lu, Yixiu Li, and Jing Hao

may be resolved by the use of selected tomato rootstocks. Among other mechanisms for resistance of grafted eggplants to V. dahliae , the allelopathic suppression exhibited by the root exudates from the grafted eggplants has been suggested to play a