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Leslie A. Weston

Allelopathy can be defined as an important mechanism of plant interference mediated by the addition of plant-produced secondary products to the soil rhizosphere. Allelochemicals are present in all types of plants and tissues and are released into the soil rhizosphere by a variety of mechanisms, including decomposition of residues, volatilization and root exudation. Allelochemical structures and modes of action are diverse, and may offer potential for development of future herbicides. In the past, allelopathy was described by the Romans as a process resulting in the “sickening” of the soil; in particular, chickpea (Cicer arietinum) was described as problematic when successively cropped with other species. Other early plant scientists, such as De Candolle in the 1800s, first described the ability of plant roots to produce toxic exudates. More recently, research has focused on development of weed management strategies using allelopathic crop residues, mechanism of allelochemical action, and gene regulation of allelochemical production. This paper briefly describes a variety of weed and crop species that establishes some form of potent allelopathic interference, either with other crops or weeds, in agricultural settings, in the managed landscape, or in naturalized settings. Recent research suggests that allelopathic properties can render one species more invasive to native species and thus potentially detrimental to both agricultural and naturalized settings. In contrast, allelopathic crops offer strong potential for the development of cultivars that are more highly weed suppressive in managed settings. A new challenge that exists for plant scientists is to generate additional information on allelochemical mechanisms of release, selectivity and persistence, mode of action, and genetic regulation. Armed with this specific information, we can further protect plant biodiversity and enhance weed management strategies in a variety of ecosystems.

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C. Scott, R.K. Nishimoto and C.S. Tang

Cyperus kyllingia and Cyperus brevifolius are problematic turfgrass weeds in Hawaii. Both are closely related weed species with similar morphology and growth characteristics. C. kyllingia appears to be a more successful weed with regards to interference than C. brevifolius. Greenhouse experiments were conducted to compare the levels of interference exerted by C. kyllingia and C. brevifolius upon Cynodon dactylon turfgrass. C. kyllingia reduced the growth of C. dactylon by about 50 %, while C. brevifolius did not significantly reduce C. dactylon growth. These results correspond with the chemical profiles of C. kyllingia and C. brevifolius. Analysis has shown that C. kyllingia contains two sesquiterpenes which have been identified as potentially allelopathic components of Cyperus rotundus. C. brevifolius contains waxes and the two sesquiterpenes found in C. kyllingia are absent. This suggests that allelopathy may be the mechanism responsible for the different levels of interference exhibited by C. kyllingia and C. brevifolius, and these species may provide an important model for the study of allelopathy.

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Lantian Du, Baojian Huang, Nanshan Du, Shirong Guo, Sheng Shu and Jin Sun

ability (e.g., pathogenic bacteria, fungi, and nematodes), and the number of beneficial microbes decreases, thus affecting the normal growth of crops and causing yield reductions. The use of reasonable cropping systems and the employment of allelopathy

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Michael J. Adler and Carlene A. Chase

by competing with weeds for available resources and by promoting conditions that are unfavorable for weed germination and establishment ( Teasdale, 1998 ). The latter mechanism includes allelopathy, which is the inhibitory or stimulatory effect of a

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Mathieu Ngouajio

Over the last century, climate change, adoption of new regulations, and changes in cropping systems have significantly impacted weed and pest management in horticultural crops. The objective of this workshop was to provide a critical review of major changes and discuss current and future trends for weed and pest management. Speakers touched on a broad range of topics including climate change and disease dynamics, the use of disease resistance inducers, soil management for pest management, and the role of allelopathy in weed management. Major recommendations included 1) increased grower education related to the impact of climate change on plant diseases; 2) more research directed towards a better understanding of the interaction of plant–pathogen–inducer; 3) use of organic soil amendments, cover crops, crop rotations, and resistant cultivars to enhance the weed and disease suppressive effect of soils; and 4) enhancement of allelochemical production and subsequent weed suppression through conventional breeding and molecular techniques.

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Xi Wang, Genhua Niu, Mengmeng Gu, Paul A. Baumann and Joseph Masabni

). The herbicidal activity of MSM is due to allelopathy induced by hydrolysis products of glucosinolates (GSLs) ( Hoagland et al., 2008 ). Certain plants in the Brassicaceae family are known to possess allelopathic properties, and are used as cover

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Orion P. Grimmer and John B. Masiunas

Winter-killed oats (Avena sativa) may have potential for use to suppress weeds in early seeded crops such as pea (Pisum sativum). Residue biomass and surface coverage are generally correlated with weed suppression. Oat residues also contain allelochemicals. Our objective was to determine if oat cultivars vary in residue production and allelopathy. Differences between oat cultivars were observed in residue production, and for effects on emergence of common lambsquarters (Chenopodium album) and shepherd's-purse (Capsella bursa-pastoris) in the greenhouse, and germination of pea and common lambsquarters in an infusion assay. Two of the oat cultivars producing the greatest biomass, `Blaze' (in the field) and `Classic' (in the greenhouse), interfered minimally with pea germination and were among the best cultivars in inhibiting common lambsquarters and shepherd's-purse. `Blaze' also greatly inhibited common lambsquarters germination in the infusion assay that measured allelopathy. Thus, `Blaze' and `Classic' possess suitable characteristics for use as a cover crop preceding peas.

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Thomas Björkman, Lisa M. Blanchard and Gary E. Harman

Sweet corn (Zea mays L.) varieties carrying the sh2 gene are in high demand, but such varieties have poor stress tolerance, especially during plant establishment. Trichoderma harzianum Rifai strain 1295-22 is a biocontrol fungus developed to provide season-long colonization of crop roots. It has the potential to reduce root rot and increase root growth. In the absence of detectable disease, colonization by Trichoderma increased root and shoot growth by an average of 66%. The enhancement was not uniform among the plants. Low- and intermediate-vigor plants were larger in the presence of Trichoderma, but high-vigor plants were not further enhanced by the fungus. Seeds that were subjected to oxidative stress with 0.05% NaOCI had much-reduced vigor; subsequent treatment with Trichoderma fully restored vigor. This result indicates that the damage caused by hypochlorite is specifically repaired by Trichoderma. Treatment of imbibed but unemerged seeds with cold (5/10 °C night/day) for varying periods reduced subsequent growth. Plants with Trichoderma-colonized roots were 70% larger at all durations of cold treatment. The absence of interation indicates the growth reduction due to cold and the growth enhancement due to Trichoderma are by different mechanisms. Allelopathic reduction in root growth by rye was mimicked by applying benzoxazolinone to the soil. Trichoderma-colonized roots grew faster, but the characteristic shortening of the radicle still occurred. There was no interaction between Trichoderma and allelopathy, indicating that these two treatments affect growth by independent mechanisms. The different ways that growth was enhanced by Trichoderma lead us to propose that this fungus acts, in part, by reversing injurious oxidation of lipids and membrane proteins. Root growth is markedly enhanced by colonization with Trichoderma harzianum. This enhancement can restore some stress-induced growth reduction and may directly reverse oxidative injury.

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J.P. Mitchell, C. Summers, T.S. Prather, J. Stapleton and L.M. Roche

Observations that tomato transplants died or were severely stunted when set into unincorporated sorghum-sudan hybrid surface mulch led us to further investigate the potential allelopathic impacts of this warm-season cover crop in a series of field experiments. Survival and dry weights of tomato, lettuce, and broccoli transplants were determined in fallow, incorporated sorghum-sudan-, and unincorporated sorghum-sudan-mulched soils. All three species transplanted into plots in which the sorghum-sudan had been cut and left on the soil surface had a significantly lower dry weight than plants transplanted into fallow soil or into soil where the sorghum-sudan had been incorporated. Additionally, fewer transplants survived in the mulch treatment. The surface mulch plots also significantly reduced weed biomass nearly 10-fold. We believe that a water-soluble compound that is leached out of the sorghum-sudan hybrid is toxic to all three of the plants tested. Further laboratory and greenhouse tests are under way to determine the exact nature of the toxic substance.

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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.