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C. Hamel, F. Morin, A. Fortin, R.L. Granger, and D.L. Smith

Herbicides are increasingly used in orchards. Since apple trees strongly depend on mycorrhizae, the effects of three commonly used herbicides on the host plant and endophyte were examined. Symbiosis between tissue-cultured P16 apple rootstocks and Glomus versiforme (Karsten) Berch was established under greenhouse conditions. Simazine (1, 2, 10, and 20 μg a.i./g), dichlobenil (1, 5, 10, and 25 μg a.i./g), paraquat (0.5, 1, 10, and 100 μg a.i./g), or water was applied to mycorrhizal and nonmycorrhizal plants as a soil drench. The response of mycorrhizal plants to herbicide was greater, and the relative elongation rate was more sharply reduced in mycorrhizal (76%) than in nonmycorrhizal plants (33%). Six weeks after herbicide application, dry mass reduction due to herbicides was similar (39% and 36%) for mycorrhizal and nonmycorrhizal plant shoots, respectively, while root dry mass reduction was larger for mycorrhizal (63%) than nonmycorrhizal plants (46%). None of the herbicide treatments affected root colonization. However, an in vitro hyphal elongation test with G. intraradices Schenck & Smith and herbicide-amended (0, 1, 10, 100, and 1000 μg a.i./g) gellan gum solidified water showed that either dichlobenil or paraquat, even at the lowest concentrations, could significantly reduce hyphal elongation. Simazine did not affect hyphal elongation in vitro, a result suggesting that improved absorption capacity of mycorrhizae explains, at least in part, the increased phytotoxicity of some herbicides. It was found that plant mortality was higher among mycorrhizal than nonmycorrhizal apple trees for all herbicide treatments. The increased CO2 assimilation rates of dichlobenil-treated mycorrhizal plants contrasted with the decreased rates of control plants measured 1 week after dichlobenil treatment. This indicates a physiological interaction between mycorrhizal colonization and dichlobenil in the toxic response of apple plants. Chemical names used: 2-chloro-4,6-bis-ethylamino-s-triazine (simazine), 2,6-dichlorobenzonitrile (dichlobenil), 1,1'-dimethyl-4,4'bipyridinium (paraquat).

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Patrick E. McCullough, Ted Whitwell, Lambert B. McCarty, and Haibo Liu

poor shade tolerances, heavy thatch/mat accumulation, and disease susceptibility ( Bunnell et al., 2005 ; White, 1998 ; White et al., 2004 ). These cultivars are also sensitive to herbicides and plant growth regulators used for managing higher

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Christopher A. Proctor and Zachary J. Reicher

along sidewalks and drives, herbicide control of purslane may be necessary. Labels of many turfgrass herbicides list purslane as a weed species controlled, but limited published research is available on herbicide control of purslane in turf. Several

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Shital Poudyal and Bert M. Cregg

). In a study by Gilliam et al. (1992) , 80% of applied granular herbicide landed off-target when empty 2.8-L containers were spaced 30 cm apart. Typically, only a small fraction of pesticides leach out of containers because of high pesticide retention

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Megh Singh, Mayank Malik, Analiza H.M. Ramirez, and Amit J. Jhala

, herbicides are an important choice commonly used by citrus growers either as strip applications within the crop row or as broadcast applications to the grove floor ( Sharma et al., 2008 ). Nonbearing young citrus trees (<4 years old) require greater attention

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Michele R. Warmund, David H. Trinklein, Mark R. Ellersieck, and Reid J. Smeda

Auxin herbicides have been registered for postemergence control of broadleaf weeds in agricultural crops for more than 50 years ( Peterson et al., 2016 ). Since their initial development, different formulations of auxin herbicides, such as 2,4-D

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S. Christopher Marble, Andrew K. Koeser, and Gitta Hasing

Since 2,4-D was discovered in the 1940s, development and use of herbicides has continually increased ( Timmons, 2005 ). Use of herbicides in residential landscapes is also increasing. From 1994 to 2007, there has been a 60% increase in sales of

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Carl W. Coburn, Albert T. Adjesiwor, and Andrew R. Kniss

or hayfields that receive frequent mowing whereas plants growing in undisturbed areas may exhibit varying degrees of sexual and asexual reproduction. Herbicides are important tools for weed control in turfgrass and other areas and can be particularly

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Jayesh B. Samtani, John B. Masiunas, and James E. Appleby

stressors such as adverse environments, air pollution, or pests ( Haugen et al., 2000 ). Haugen et al. (2000) and WDATCP (2003) proposed that insect feeding, environmental factors, or herbicide drift could cause leaf tatters. Our preliminary research

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Bernard H. Zandstra, Sylvia Morse, Rodney V. Tocco, and Jarrod J. Morrice

( Centaurea maculosa ), and wild carrot ( Daucus carota ). Herbicide resistance in annual weeds is always a potential problem, and resistance to herbicides targeting photosystem II (PS II) has been confirmed for redroot pigweed ( Amaranthus retroflexus ) and