Fumigants alone or in combination with herbicide for weed management in bell pepper (Capsicum annuum)

Highlights

• DMDS + Pic was more effective than 1,3-D + Pic for nutsedge control.

• Neither DMDS + Pic nor 1,3-D + Pic effectively controlled weeds at season end.

• Applications of metam potassium or S-metolachlor improved weed control.

Abstract

Weed management is a significant challenge faced by pepper (Capsicum annuum L.) growers in the Southeastern United States. Field experiments were implemented to study the combined effects of soil fumigants with supplemental metam potassium or S-metalochlor on weed control and bell pepper growth and yield. Experiments were designed as a completely randomized split plot with primary soil fumigant (PF) as the main plot factors and supplemental weed management (SWM) as the subplot factors. The PF included a nontreated control, 396 kg ha⁻¹ dimethyl disulfide (DMDS) + Chloropicrin (Pic) (79:21 w/w), and 336 kg ha⁻¹ 1,3-dichloropropene (1,3-D) + Pic (40:60 w/w). The SWM included a nontreated control, 475 kg ha⁻¹ metam potassium, and 1.06 kg ha⁻¹ S-metolachlor. PF and SWM treatments independently affected weed densities with no significant interactions occurring. At 12 weeks after transplanting (WATP), DMDS + Pic significantly reduced purple nutsedge (Cyperus rotundus L.) density in areas where the plastic mulch was removed after transplant to evaluate weed control, whereas 1,3-D + Pic did not significantly reduce purple nutsedge density. Neither 1,3-D + Pic nor DMDS + Pic significantly reduced broadleaf and grassy weed densities compared to the nontreated control at the end of the growing season. Metam potassium reduced purple nutsedge density by 48% compared to the nontreated control at 2 WATP but nutsedge densities did not differ at subsequent dates. S-metolachlor reduced purple nutsedge density by 50 and 69% at 4 and 6 WATP in areas where the mulch was removed after transplant compared to nontreated control. At 12 WATP, metam potassium and S-metolachlor reduced grassy weed density by 80 and 49%, respectively, and reduced broadleaf weed density by 55 and 67% compared to the nontreated control, respectively. Overall, we conclude that DMDS + Pic was more effective at reducing purple nutsedge density as compared to 1,3-D + Pic. Supplemental applications of metam potassium or S-metolachlor following the fumigation of 1,3-D + Pic or DMDS + Pic enhanced the control of purple nutsedge, broadleaves, and grasses. The evaluated treatments did not promote or adversely affect bell pepper growth or yield.

Introduction

Bell pepper (Capsicum annuum L.) is an economically significant vegetable crop in the state of Florida. As of 2016, growers in Florida planted 5463 ha of fresh market bell pepper, harvested 5220 ha, and generated a total of $209 million farmgate value (USDA-NASS, 2017). Florida bell pepper production represents approximately 18% of the state vegetable production. Florida was the second largest producer of bell peppers in 2016 just behind California. In the United States, bell pepper is among the top 10 vegetable crops in harvested area, and produced a gross value of 735 million in 2016 (USDA-NASS, 2017).

One of the most significant challenges faced by pepper producers is weed management (Webster, 2010). Purple (Cyperus rotundus L.) and yellow (Cyperus esculentus L.) nutsedges are the problematic weeds for plasticulture vegetable production throughout the southern United States. They interfere with vegetable growth by competing for sunlight, water, and nutrients (Webster, 2005a, 2005b, 2010). In many regions of the United States, bell peppers are grown on plastic-mulched raised beds that are fumigated for pest control. Polyenthylene mulch can effectively suppress broadleaf and grassy weeds (McAvoy and Freeman, 2013; Patterson, 1998), except the planting holes for transplant (Boyd and Reed, 2016). However, plastic mulch is not effective in suppressing nutsedge (Cyperus spp.) because nutsedge shoots are able to penetrate the plastic film.

Successful production of bell pepper was traditionally reliant on methyl bromide for control of various soilborne diseases, nematodes, and weeds (Gilreath and Santos, 2004a, 2004b; Zasada et al., 2010). However, methyl bromide depletes the stratospheric ozone and its’ use in agricultural systems was banned (USEPA, 2017). Alternative fumigants for pest management have been studied extensively (Boyd et al., 2017; Eure and Culpepper, 2017; Gilreath and Santos, 2004a; 2004b, 2005; Gilreath et al., 2005; Qiao et al., 2010; Miller et al., 2014; Stevens and Freeman, 2018) with the majority of growers now relying on 1,3-dichloropropene (1,3-D), chloropicrin (Pic), dimethyl disulfide (DMDS), or methyl isothiocyanate generators such as metam-sodium and metam-potassium.

Unfortunately, registered methyl bromide alternatives lack broad-spectrum activity and tend to effectively control only one or two classes of pests (Duniway, 2002; García-Méndez et al., 2008; Hanson and Shrestha, 2006). To address this shortcoming, multiple fumigants are typically applied as simultaneous combinations, either as mixtures or as sequential applications (García-Méndez et al., 2008; Gilreath and Santos, 2004b). Pic is effective against insects and soil-borne pathogens but lack of efficacy against nematodes and weeds (Hanson and Shrestha, 2006; Hutchinson et al., 2000, 2003; Li et al., 2014; Zasada et al., 2010). Seed coat impermeability and hardness are documented to be the primary barriers reducing the weed control efficacy of soil fumigation (Egley et al., 1986). Pic, at low rate, may stimulate weed seed germination/nutsedge tuber sprouting (Santos and Gilreath, 2006). As a result, Pic performed better as a weed control synergist and commonly applied in conjunction with other soil fumigants such as 1,3-D or DMDS (Boyd et al., 2017; Eure and Culpepper, 2017; Hutchinson et al., 2003; Zasada et al., 2010). Hutchinson et al. (2003) documented that the addition of Pic to 1,3-D, propargyl bromide, methyl bromide, metam-sodium, and methyl iodide resulted in greater efficacy against yellow nutsedge tubers.

Previous research has suggested that soil fumigants combined with a preemergence herbicide can improve purple nutsedge control (Gilreath et al., 1994; Gilreath and Santos, 2004a, 2005). For example, Gilreath and Santos (2005) reported that 1,3-D + Pic controlled 85% purple nutsedge, but when applied with pebulate achieved 99% control. However, pebulate is injurious to pepper (Stall and Gilreath, 1996). Thus, other preemergence herbicides need to be evaluated with fumigants to expand the tool available for weed control in pepper. S-metolachlor, a chloroacetamide herbicide, primarily targets the fatty acid elongase enzymes in the biosynthesis of very long-chain fatty acids (Trenkamp et al., 2004). S-metolachlor is third party registered for preemergence weed control in bell pepper (Anonymous, 2014). Its weed control efficacy in conjunction with soil fumigants needs to be evaluated.

Metam potassium, a soil fumigant, controls a broad spectrum of pests (Anonymous, 2010) and can be used alone for weed control (Hanson and Shrestha, 2006). In previous investigations, Boyd et al. (2017) documented that broadcast application of metam potassium at 10 cm depth from the bed top using a fumigation rig with six shanks reduced the purple nutsedge density in plasticulture tomato production but was generally less effective compared to 1,3-D + Pic or DMDS + Pic. The authors also reported that deep applications (30 cm) of metam potassium were generally less effective compared to shallow (10 cm) applications at reducing various broadleaf and grassy weed densities in the planting holes where they are most problematic. Banded applications of metam potassium where the holes will be punched for transplant may reduce metam potassium usage but weed control efficacy is unknown. The goal of this research was to investigate the efficacy of 1,3-D + Pic or DMDS + Pic applied as the primary fumigants in conjunction with band applications of metam potassium or broadcast applications of S-metolachlor for weed control and evaluate their effect on pepper growth and yield.