Nutsedge Control with Drip-applied 1,3-Dichloropropene plus Chloropicrin in a Sandy Soil

An evaluation of the effect of bed width (24, 28, 32, and 36 inches) on the control of a mixed population of nutsedge [yellow nutsedge (Cyperus esculentus) and purple nutsedge (C. rotundus)] was conducted with an emulsifi able concentrate formulation of a 1,3-dichloropropene (1,3-D) and chloropicrin (CP) mixture (1,3-DCP) for application through drip irrigation systems. Beds were mulched with either 1.4-mil-thick virtually impermeable fi lm (VIF) or 0.75-mil-thick high-density polyethylene (HDPE) and 1,3-DCP was applied at 35 gal/acre by surface chemigation or via subsurface chemigation 6 inches deep within the bed. HDPE was more permeable to gaseous 1,3-D than VIF so that 1 day after treatment (DAT), 1,3-D gas concentration at the bed centers under VIF was signifi cantly higher than under HDPE. Dissipation of 1,3-D gas with HDPE occurred within 7 DAT, but dissipation with VIF took ~10 days. In bed centers, 1,3-D concentrations 1 DAT were in the range of 2.3 to 2.9 mg·L–1 whereas in bed shoulders concentrations ranged from 0.1 to 0.55 mg·L–1. In 2002 and 2003, 1,3-D concentration in shoulders of narrower beds was signifi cantly higher than in the wider beds, but dissipated more rapidly than in wider beds. Lower initial 1,3-D concentrations were observed with HDPE fi lm in shoulders than with VIF and the rate of dissipation was lower with VIF. At 14 DAT, nutsedge plants were densely distributed along bed shoulders (19 to 27 plants/m2) with little or no emergence in the centers of beds (fewer than 5 plants/m2), but with no response to bed width. Nutsedge density increased with time, but the nature of the increase differed with bed width. The most effective nutsedge suppression was achieved with 36-inch beds, which had densities of 11–13 plants/m2 on bed centers and 53 plants/m2 on bed shoulders by 90 DAT. Nutsedge suppression was initially more effective with VIF than with HDPE fi lm, so that no nutsedge emerged in the centers of beds mulched with VIF compared with 2–7 plants/ m2 with HDPE by 14 DAT. On bed shoulders there were 2–7 plants/m2 with VIF and 32–57 plants/m2 with HDPE. Increase in nutsedge density with time was greater with VIF so that by 90 DAT nutsedge densities on bed centers and shoulders were greater than with HDPE in 2002 and the same as with HDPE in 2003. Subsurface chemigation did not consistently improve suppression of nutsedge when compared with surface chemigation. Concentrations of 1,3-D in bed shoulders irrespective of bed width were nonlethal. Initial superior nutsedge suppression with VIF did not persist. Nutsedge control in a sandy soil with 1,3DCP chemigation is unsatisfactory with one drip-tape per bed. Chisel-applied volatile formulations of the soil fumigant mixture 1,3-DCP are among the more promising alternatives to methyl bromide for crop production (Locascio et al., 1997). 1,3-DCP is already registered for use in plasticulture production of vegetables and strawberry (Fragaria ×ananassa) in Florida for the control of plant pathogenic nematodes and soilborne pathogens. Its effect on nutsedges is weak or inconsistent when compared with methyl bromide. Under some conditions, the resulting concentrations of the fumigant and duration of exposure appear to be suffi cient to provide adequate control of nutsedge (Hochmuth et al., 2002). However, this is not always the case and these conditions are not well understood. Hence, one approach to controlling purple nutsedge has utilized fumigant/ herbicide combinations (Gilreath and Santos, 2004a, 2004b, 2005; Gilreath et al., 2004). In laboratory studies the effective concentration of 1,3-D + 17% chloropicrin resulting in 50% mortality of imbibed yellow nutsedge tubers was 20 μM (Hutchinson et al., 2004); while in greenhouse studies, 99% mortality of imbibed yellow nutsedge tubers was obtained with 327 L·ha–1 1,3-D + 35% chloropicrin (Motis et al., 2002). We hypothesized that the failure to achieve consistent results on nutsedges with 1,3-D-containing fumigants in the fi eld may be due to insuffi cient duration of exposure to lethal concentrations. Drip fumigation is a relatively new concept and Ajwa et al. (2002) identifi ed a number of areas in which more work is needed: minimizing application rates; selecting mulches that restrict emissions and maximize effi cacy; and optimum soil conditions, carrier amounts, and combinations of chemicals. Benefi ts of applying soil fumigants by chemigation include limited worker exposure since installation of polyethylene fi lm is done prior