The search for methyl bromide (MBr) alternatives has been a vast source of research during the last decade, in which hundreds of trials have been conducted nationwide to examine the efficacy of different soil fumigants on soil-borne pests in polyethylene-mulched tomato (Lycopersicon esculentum), pepper (Capsicum annuum), strawberry (Fragaria ×ananassa), cucurbits, cut flowers, and other commodities. Although a great deal of progress has been achieved in this field, currently there is no a single molecule to replace MBr. Instead, ongoing research focuses not only on the efficacy of the combination of certain fumigants and herbicides, but also on application techniques and formulations (Duerksen, 2002; Noling and Gilreath, 2001). Purple nutsedge and yellow nutsedge (Cyperus esculentus) are the most troublesome weeds to control in polyethylene-mulched vegetable crops and have the ability to emerge through the mulch, causing yield and quality losses (Gilreath and Santos, 2005; Gilreath et al., 2005). In the past, MBr applications have effectively reduced nutsedge populations below damage thresholds. However, other fumigants do not have consistent efficacy against these weeds.
Metam sodium and metam potassium (MK) are among the most promising MBr alternatives (Ajwa et al., 2002; Martin, 2003). The primary breakdown product of these fumigants is methyl isothiocyanate, which is a potent biocide that react with amines and thiols in biological molecules (Duniway, 2002; Lam et al., 1993; Pruett et al., 2001). These fumigants are available in liquid formulations, which provide application flexibility because they can be either directly sprayed on the soil or drip injected (Duniway, 2002; Ou et al., 2006). The efficacy of these fumigants against nutsedge has been tested with mixed results (Ajwa et al., 2003; Martin, 2003). Previous research showed that metam efficacy against nutsedges increases when partnered with preemergence herbicides in tomato (Gilreath and Santos, 2004a, c). In contrast, Locascio et al. (1997), examining the effect on tomato yield of two forms of application of metam in comparison with MBr plus chloropicrin (Pic), found that fruit yield in the drip-applied metam plots was ≈60% of that for MBr + Pic, whereas the performance of the soil-applied metam was even lower.
Recently, various reports have suggested improved MK performance on nutsedges (Vaculin et al., 2003). However, MK rates, distribution in the soil, water delivery volumes, and flow rates could be among the reasons for the inconsistent results. Duniway (2002) suggested that metam must be delivered carefully to avoid either leaching, when excessive water volumes are used, or rapid volatilization, with application of insufficient water. Ou et al. (2006) indicated that water volumes play a significant role in the distribution of metam within the first 8 inches of the soil. However, further characterization of the influence of water volumes is needed to provide definite answers on its role on weed control. Therefore, the objectives of these studies were to determine: 1) the effect of water delivery volumes and flow rates on purple nutsedge control with MK, and 2) the influence of MK rates and concentrations on purple nutsedge control.
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Duerksen, C. 2002. Vapam and metam potassium: Making the difference with precise application. 2002 Annu. Intl. Res. Conf. Methyl Bromide Alternatives Emissions Reductions. Orlando, Fla., 6–8 Nov. 2002. Methyl Bromide Alternatives Outreach. Fresno, Calif. p. 26–1.
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Vaculin, P.D., R.C. Hochmuth, and E.H. Simmone, 2003. Effect of bed width on drip applied metam potassium for purple nutsedge control. 2003 Annu. Intl. Res. Conf. Methyl Bromide Alternatives Emissions Reductions. San Diego, 3–6 Nov. 2003. Methyl Bromide Alternatives Outreach, Fresno, Calif. p. 20-1-3.