Purple nutsedge is the most problematic weed in vegetable crops in the southern United States (Webster, 2002). The success of purple nutsedge is associated with its perennial nature, prolific tuber and rhizome production, C4 carbon-fixation pathway, and release of allelopathic compounds (Anderson, 1999; Stoller and Sweet, 1987). All the above characteristics together make purple nutsedge a highly competitive and difficult-to-control weed in vegetable crops. Season-long interference with purple nutsedge has reduced the yield of cucumber (Cucumus sativus) by 43%; tomato (Solanum lycopersicum) by 44%, lettuce (Lactuca sativa) by 54%, cilantro (Coriandrum sativum) by 61%, bell pepper (Capsicum annuum) by 73%, and radish (Raphanus sativus) by 100% (Morales-Payan et al., 1996, 1997a, 1997b, 1998; Santos et al., 1998; William and Warren, 1975).
Methyl bromide, a preplant soil fumigant, is widely used in the southern United States for weed control in polyethylene-mulched vegetable production (Duniway, 2002). However, methyl bromide is considered to be a contributor to ozone depletion, and therefore will be banned in the United States in the near future (U.S. Environmental Protection Agency, 2008). Weed control will be challenging in polyethylene-mulched vegetables in the absence of methyl bromide, especially for purple nutsedge, which can easily penetrate polyethylene mulch (Patterson, 1998). Thus, there is an urgent need to develop an effective alternative to methyl bromide.
Isothiocyanates (ITC) are a family of compounds composed of nitrogen (N), carbon (C), and sulfur (S), with an alkyl or aryl side chain (R) having the basic structure of R–N=C=S. ITC have shown herbicidal activity on several weed species, including purple nutsedge (Norsworthy and Meehan, 2005a, 2005b; Norsworthy et al., 2006; Peterson et al., 2001). Although the mode of action of ITC is still unknown, they are believed to interact with enzymes related to germination to prevent weed seed germination, including purple nutsedge tubers (Drobinca et al., 1977; Peterson et al., 2001; Norsworthy et al., 2006). Additionally, Norsworthy et al. (2006) showed the negative impact of various ITC on shoot density and shoot biomass production of purple nutsedge.
Isothiocyanates are highly volatile compounds, and >90% of ITC are lost primarily through volatilization (Brown and Morra, 1995). Hence, ITC are vulnerable to rapid loss in an open environment similar to field conditions, thereby linking weed control efficacy with the ability to reduce volatile losses. Volatilization losses of ITC can be partially overcome by using low-permeability polyethylene mulches, which aid ITC retention and enhance herbicidal efficacy (Norsworthy et al., 2006). In earlier experiments, virtually impermeable film (VIF) mulch retained higher concentrations of fumigants (methyl bromide and 1,3-dichloropropene + chloropicrin) compared with conventionally used low-density polyethylene mulch (LDPE) (Santos et al., 2007; Yates et al., 2002). Although ITC are comparatively less volatile than the above-mentioned fumigants, VIF reduces the loss of methyl ITC by 40% compared with LDPE mulch under laboratory experiments (Austerweil et al., 2006). Therefore, VIF mulch is expected to reduce the rate of loss of ITC under field conditions, in turn improving weed control.
Phenyl ITC was evaluated in the present study because of the lack of available data on phenyl ITC efficacy against purple nutsedge. In a previous greenhouse experiment, phenyl ITC at a concentration of 1352 ppm (w/w) in dry soil reduced yellow nutsedge emergence by >90% (Norsworthy and Meehan, 2005b). Because of the morphological similarities between yellow nutsedge and purple nutsedge, it was hypothesized that phenyl ITC would provide effective purple nutsedge control, and its efficacy would be enhanced by increasing the phenyl ITC concentration and by using VIF mulch. The objectives of this research were to determine the lethal concentration and exposure period of phenyl ITC needed for purple nutsedge tuber mortality, to compare the retention of phenyl ITC in soil under LDPE and VIF mulches, and to determine the efficacy of phenyl ITC under VIF mulch against purple nutsedge under field conditions.
Anderson, W.P. 1999 Perennial weeds: Characteristics and identification of selected herbaceous species Iowa State Univ. Press Ames, IA
Austerweil, M., Steiner, B. & Gamliel, A. 2006 Permeation of soil fumigants through agricultural plastic films Phytoparasitica 34 491 501
Drobinca, L., Kristian, P. & Augustin, J. 1977 The chemistry of the NCS group 1003 1197 Patai S. The chemistry of cyanates and their derivatives Wiley New York
Gardiner, J.B., Morra, M.J., Eberlein, C.V., Brown, P.D. & Borek, V. 1999 Allelochemicals released in soil following incorporation of rapeseed (Brassica napus) green manures J. Agr. Food Chem. 47 3837 3842
Holm, L.G., Plucknett, D.L., Pancho, J.V. & Herberger, J.P. 1977 The world's worst weeds, distribution and biology University Press of Hawaii Honolulu, HI
Morales-Payan, J.P., Santos, B.M. & Bewick, T.A. 1996 Purple nutsedge (Cyperus rotundus) interference on lettuce under different nitrogen levels Proc. Southern Weed Sci. Soc. 49 201 (Abstr.).
Morales-Payan, J.P., Santos, B.M. & Stall, W.M. 1997a Effect of increasing purple nutsedge (Cyperus rotundus) densities on cilantro (Coriandrum sativum) yield Proc. Florida State Hort. Soc. 110 318 320
Morales-Payan, J.P., Santos, B.M., Stall, W.M. & Bewick, T.A. 1997b Effect of purple nutsedge (Cyperus rotundus) on tomato (Lycopersicon esculentum) and bell pepper (Capsicum annum) vegetative growth and fruit yield Weed Technol. 11 672 676
Morales-Payan, J.P., Santos, B.M., Stall, W.M. & Bewick, T.A. 1998 Interference of purple nutsedge (Cyperus rotundus) population densities on bell pepper (Capsicum annum) yield as influenced by nitrogen Weed Technol. 12 230 234
Norsworthy, J.K. & Meehan J.V. IV 2005a Herbicidal activity of eight isothiocyanates on Texas panicum (Panicum texacum), large crabgrass (Digitaria sanguinalis), and sicklepod (Senna obtusifolia) Weed Sci. 53 515 520
Norsworthy, J.K. & Meehan J.V. IV 2005b Use of isothiocyanates for suppression of palmer amaranth (Amaranthus palmeri), pitted morningglory (Ipomoea lacunosa), and yellow nutsedge (Cyperus esculentus) Weed Sci. 53 884 890
Norsworthy, J.K., Malik, M.S., Jha, P. & Oliveira, M.J. 2006 Effect of isothiocyanates on purple (Cyperus rotundus) and yellow nutsedge (Cyperus esculentus) Weed Biol. Manage. 6 131 138
Peterson, J., Belz, R., Walker, F. & Hurle, K. 2001 Weed suppression by release of isothiocyanates from turnip-rape mulch Agron. J. 93 37 43
Price, A.J., Charron, C.S., Saxton, A.M. & Sams, C.E. 2005 Allyl isothiocyanate and carbon dioxide produced during degradation of Brassica juncea tissue in different soil conditions HortScience 40 1734 1739
Santos, B.M., Gilreath, J.P. & Siham, M.N. 2007 Comparing fumigant retention of polyethylene mulches for nutsedge control in Florida spodosols HortTechnology 17 308 311
Santos, B.M., Morales-Payan, J.P., Stall, W.M. & Bewick, T.A. 1998 Influence of purple nutsedge (Cyperus rotundus) density and nitrogen on radish (Raphanus sativus) yield Weed Sci. 46 661 664
Stoller, E.W. & Sweet, R.D. 1987 Biology and life cycle of purple and yellow nutsedge (Cyperus rotundus and C. esculentus) Weed Technol. 1 66 73
U.S. Environmental Protection Agency 2008 Ozone layer depletion: Regulatory programs: The phase-out of methyl bromide Montreal protocol 15 Sept. 2008 <http://www.epa.gov/ozone/mbr/index.html>.
Webster, T.M. 2002 Weed survey: Southern states: Vegetable, fruit and nut crops subsection Proc. Southern Weed Sci. Soc. 55 237 258 (Abstr.).
Yates, S.R., Gan, J., Papiernik, S.K., Dungan, R. & Wang, D. 2002 Reducing the fumigant emission after soil application Phytopathology 92 1344 1348