Field study.

Studies were conducted at the Eastern Shore Agricultural Research and Extension Center near Painter, VA, in 2001 and 2002. The soil type was a Bojac sandy loam (Typic Hapludults) with less than 1% organic matter and a pH of 6.1 to 6.2. A conventional seedbed was prepared by chisel plowing followed by disking. Before planting, the experimental area was tilled with an S-tine field cultivator with double rolling baskets. Fertilizer was applied in accordance with regional recommendations (Donohue and Heckendorn, 1994). Pepper variety Camelot was transplanted 1 ft apart within each row with a single row transplanter on 14 June 2001 and 20 June 2002. A PPI application of trifluralin (Treflan®; Dow Agrosciences, Indianapolis, IN) at 8 oz/acre plus clomazone (Command®; FMC Corp., Philadelphia, PA) at 4.5 oz/acre was applied to the entire test area (including the control areas) before pepper transplant to aid in the control of grass weeds.

Plots for POST herbicide treatments consisted of two treated rows spaced 3 ft apart with a treated area of 8 × 20 ft; a single guard row of peppers separated each plot. Herbicides were applied POST with a propane-powered backpack sprayer calibrated to deliver 220 L·ha⁻¹ at 210 kPa through flat fan nozzles (Teejet 8003 flat fan nozzle; Spraying Systems, Wheaton, IL). POST applications were made on 12 July 2001 and 12 July 2002. Five ALS-inhibiting herbicides were applied at rates in which they selectively controlled broadleaf weeds in certain broadleaf crops or at rates that provide limited herbicidal activity on Solanaceous weeds, such as black nightshade (Solanum nigrum), which is closely related to pepper (Stall 1999; Vencill 2002). Herbicide treatments included trifloxysulfuron (Envoke®; Syngenta Crop Protection, Greensboro, NC) at 4 g·ha⁻¹, halosulfuron (Sandea®; Gowan Co., Yuma, AZ) at 13, 20, and 27 g·ha⁻¹, sulfosulfuron (Maverick®; Monsanto Co., St. Louis, MO) at 46 g·ha⁻¹, cloransulam (FirstRate™; Dow Agrosciences) at 18 g·ha⁻¹, and tribenuron (Express®; DuPont Crop Protection, Wilmington, DE) at 17 g·ha⁻¹. All treatments in the field studies included 0.25% nonionic surfactant (Induce®; Helena Chemical Co., Memphis, TN).

Plots were established in 2001 into sites that contained pigweed species and carpetweed. Pigweed in 2001 consisted primarily of prostrate pigweed (Amaranthus blitoides), but in 2002, smooth pigweed was the major species present. Pigweed and carpetweed were up to 4 inches tall at POST applications in 2001; however, most of the pigweed and carpetweed were not emerged at the time of POST applications in 2002. Pepper was ≈7 to 12 inches tall at POST herbicide applications in both years.

Pepper injury from herbicide applications was evaluated 1, 2, and 4 weeks after treatment (WAT). Weed control was evaluated 4 WAT. Pepper yields were determined by hand harvesting fruit from both rows of each plot between two and three times per season.

Greenhouse study.

Greenhouse studies were conducted in 2002 to evaluate pepper tolerance to POST applications of the ALS inhibitors halosulfuron at 27, 54, and 134 g·ha⁻¹, trifloxysulfuron at 8, 15, and 38 g·ha⁻¹, sulfosulfuron 46, 92, and 230 g·ha⁻¹, cloransulam 18, 36, and 90 g·ha⁻¹, and tribenuron 17, 34, and 84 g·ha⁻¹. Herbicide rates were selected based on the lowest registered use rate for these herbicides in their primary crop. Two additional rates of each herbicide were also included that corresponded to two and five times these primary use rates.

One ‘Camelot’ pepper plant was transplanted into a 4 × 4-inch pot (T.O. Plastics 4” Fill Pots; Wetzel, Virginia Beach, VA) filled with a commercial potting mix (Pro-Mix BX; Premier Horticulture, Red Hill, PA). Plants were watered and fertilized (Excel All Purpose 21N–2.2P–41.6K; Wetzel) as needed to facilitate maximum plant growth and vigor. Pepper plants were 7 to 9 inches tall at herbicide application and similar to pepper heights when POST applications were made in the field study. Herbicides were applied using a greenhouse cabinet sprayer at a spray volume of 220 L·ha⁻¹ and 210 kPa pressure. A single even flow nozzle (Teejet 8002 EVS flat fan spray tip; Spraying Systems) was placed 12 inches above the uppermost part of the pepper plants. All treatments in the greenhouse studies included 0.25% (v/v) nonionic surfactant (Induce®; Helena Chemical Co.).

Pepper injury was visually rated 1 and 3 WAT. Chlorophyll content was measured at 1 WAT on a representative leaf in the meristematic regions of each plant with a chlorophyll meter (Spad 502; Konica Minolta, Tokyo). Pepper heights were measured 3 WAT from the potting mix surface to the uppermost point on the pepper plant. Shoot biomass was then harvested and plants were dried to constant moisture and weighed. Heights, chlorophyll content, and biomass ratings are presented as the percentage of reduction in comparison with the nontreated control.

Field and greenhouse studies were arranged in a randomized complete block design. Treatments in the field were replicated three times and greenhouse treatments were replicated four times; all studies were repeated once. Pepper injury and weed control were visually rated on a scale of 0 to 100, where 0 equals no pepper injury or weed control and 100 equals pepper death or complete weed control. Data from the field and greenhouse studies were subjected to analysis of variance (ANOVA) and Fisher's protected least significant difference (lsd) (P = 0.05) was used for mean separation. Data were pooled over studies when no study by treatment interaction occurred. The nontreated control was not included in the analyses of the greenhouse studies.

Field study.

In the field study, no year by treatment interaction occurred for pepper injury 1 WAT; therefore, these data were pooled over years (Table 1). All other data from these field studies are presented by year.

Table 1.

Pepper injury, control of pigweed species and carpetweed, and pepper yield with postemergence applications of several acetolactate synthase inhibitors.

View Table

All herbicides except tribenuron provided greater than 95% control of pigweed species in 2002 (Table 1). This low control of pigweed species with tribenuron is likely due to its limited soil residual activity in comparison with the other ALS inhibitors evaluated (Vencill, 2002). When prostrate pigweed was the predominant species in 2001, only tribenruon at 17 g·ha⁻¹ and trifloxysulfuron at 4 g·ha⁻¹ provided 85% or greater control of pigweed species. This type of differential response of ALS inhibitors to pigweed species has been previously reported (Nelson and Renner, 1998; Trader et al., 2007; Umeda, 1999).

Carpetweed was controlled 85% to 99% with trifloxysulfuron, sulfosulfuron, and cloransulam (Table 1). Halosulfuron controlled emerged carpetweed 80% to 92% in 2001, but only 67% to 72% in 2002 when most carpetweed had not emerged at the time of POST herbicide applications. POST halosulfuron applied alone or in sequential applications at rates greater than 25 g·ha⁻¹ provided better control of carpetweed in other studies (MacRae et al., 2008). Tribenuron controlled carpetweed 33% to 68%, which was not considered commercially acceptable.

Pepper injury differed greatly with the herbicides applied and the year in which these herbicides were applied. Halosulfuron at 20 and 27 g·ha⁻¹, sulfosulfuron at 46 g·ha⁻¹, and tribenuron 17 g·ha⁻¹ caused 20% to 30% injury to pepper by 1 WAT (Table 1). Halosulfuron at 13 g·ha⁻¹ and trifloxysulfuron 4 g·ha⁻¹ injured pepper 15% and 12%, respectively. In 2001, pepper injury generally declined over time with the exception of pepper treated with tribenuruon, which caused 42% injury by 4 WAT. In 2002, pepper injury generally increased over time, except for pepper treated with halosulfuron at 13 and 20 g·ha⁻¹. This differential injury over years is likely due to the increased amount of rainfall received within 8 d after treatment in 2002 (6.1 cm) compared with 2001 (0.38 cm), and therefore likely enhanced herbicide root absorption. By 4 WAT, the lowest amount of injury was observed on pepper treated with trifloxysulfuron at 4 g·ha⁻¹ and halosulfuron at 13 and 20 g·ha⁻¹.

Pepper treated with halosulfuron and cloransulam produced yields similar to those in the control in 2001 (Table 1). Trifloxysulfuron and sulfosulfuron treated pepper yielded 17.6 and 16.2 kg/plot, respectively, which was greater than pepper yield in the control (11.1 kg/plot) in 2001. When crop injury was most pronounced among all treatments in 2002, trifloxysulfuron was the only treatment that did not reduce pepper yield in comparison with the control. Tribenuron was the only treatment that produced lower pepper yield than the control in both years of this study.

Greenhouse study.

In the greenhouse study, no repetition by treatment interaction occurred for 1 and 3 WAT pepper injury, percentage of chlorophyll reductions, and percentage of height reductions; therefore, these data were pooled over repetition (Table 2). However, there was a repetition by treatment interaction for percentage of dry weight reductions, and these data are presented by repetition.

Table 2.

Pepper injury, percentage of height reductions, percentage of dry weight reductions, and percentage of chlorophyll reductions from postemergence applications of acetolactate synthase inhibitors in the greenhouse.

View Table

Pepper injury from these ALS inhibitors was generally a combination of stunting and chlorosis and ranged from 11% to 34% with all treatments by 1 WAT (Table 2). Injury increased with all treatments by 3 WAT. Trifloxysulfuron at 8 g·ha⁻¹ caused the lowest level of pepper injury (18%), which was similar to trifloxysulfuron at 15 g·ha⁻¹ and halosulfuron at 27 and 54 g·ha⁻¹. In addition, all herbicide treatments reduced chlorophyll content 35% to 69%. Chlorophyll reduction was least in pepper treated with trifloxysulfuron at 8 and 15 g·ha⁻¹, halosulfuron at 27 g·ha⁻¹, and cloransulam at 18 g·ha⁻¹.

Trifloxysulfuron at 8 and 15 g·ha⁻¹ and halosulfuron at 27, 54, and 134 g·ha⁻¹ caused the least amount of pepper height reductions (Table 2). All other treatments reduced pepper heights between 23% and 33%. Pepper dry weights were reduced 39% to 92% by all herbicide treatments. Trifloxysulfuron at 8 g·ha⁻¹ provided the lowest amount of pepper dry weight reductions over both repetitions of this study.

Variations in weed control and pepper injury, chlorophyll content, heights, dry weights, and yield were observed depending on the rate of each ALS inhibitor applied POST to pepper. Greater rainfall in 2002 likely increased pepper injury and yield loss from several herbicide treatments by increasing herbicide uptake through the roots. Trifloxysulfuron was the only herbicide that provided greater than 86% control of pigweed species and carpetweed in both years of the field study. In addition, trifloxysulfuron provided the lowest levels of injury and chlorophyll, height, and dry weight reductions when compared with most other treatments. Similarly, trifloxysulfuron was the only herbicide that did not reduce pepper yield in both years in comparison with the control. Therefore, future studies should include evaluations of trifloxysulfuron at different rates for safety to several varieties of pepper. In addition, future studies should include trifloxysulfuron treatments applied PP, PPI, PRE, or POST-directed to pepper for potential improvements in weed control while limiting potential pepper injury, as has been observed with other ALS inhibitors in vegetable crops (Dittmar et al., 2008).