Producers growing squash for markets who desire the use of more naturally occurring herbicides need alternatives that effectively provide season-long weed control. Although corn gluten meal has shown promise as an early-season preemergent herbicide in squash production, any uncontrolled weeds can cause serious yield reductions (Webber et al., 2010). In general, there are limited options for control of annual broadleaf weeds in summer squash and both organic and conventional producers would benefit from appropriate herbicides that effectively provide postemergent weed control (Kemble et al., 2013). Previous research with postemergence nonsynthetic contact herbicides in noncrop situations determined that these herbicides must be applied to very young or small weeds if acceptable weed control is expected (Webber et al., 2012a). A potential solution to this would be to increase weed control efficacy on larger weeds and decrease squash injury by using multiple or sequential postdirected herbicide applications (herbicides postdirected sprayed at the base of the crop rather than over the top).
Scythe® (Dow AgroSciences, Indianapolis, IN) is a fatty acid–based, nonselective, synthetic contact herbicide consisting of 57% PA and other related fatty acids. Pelargonic acid is a fatty acid naturally occurring in many plants and animals and present in many foods (U.S. Environmental Protection Agency, 2004). Pelargonic acid injures and kills plants by destroying the cell membranes, causing rapid desiccation of plant tissues. Chloroplast bleaching is seen within a short time after application (Senseman, 2007). The primary impact is the immediate decrease in intercellular pH with a resulting loss of cell membrane integrity (Senseman, 2007). Although PA is not listed as a “certified organic” herbicide, it is seen as a more natural type herbicide for use in “sustainable” crop production (Webber and Shrefler, 2007).
Previous field studies determined that PA applied at 5, 10.8, and 16.7 lb/acre as Scythe® at 3%, 6.5%, and 10% (v/v) in 100 gal/acre was effective as a broadcast postemergence contact herbicide when applied to small annual weeds (Webber and Shrefler, 2007). Webber et al. (2012b) determined that the application volume could be reduced from 100 to 40 gal/acre in postdirected applications in spring transplanted onion (Allium cepa) and maintain good weed control with PA applied at 5, 10, and 15 lb/acre . Unfortunately, crop injury and significant reduction in onion yields were observed with PA application rates compared with the weed-free control.
Pelargonic acid in commercial formulations has potential as a nonselective postemergent contact herbicide for weeds in vegetable crops if application method and rate can be customized to individual crops to maintain weed control efficacy without reducing yields because of crop injury (Webber and Shrefler, 2007; Webber et al., 2005, 2012a). Research was conducted to determine the impact of sequential postdirected applications of PA on weed control efficacy, crop injury, and yields in yellow squash.
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