Materials and Methods

Field studies were conducted at the University of Guelph Ridgetown Campus, Ridgetown, Ontario, from 2003 to 2005. The soil was a Normandale fine sandy loam with 53.9% sand, 29.3% silt, 16.7% clay, 4.7% organic matter, and pH of 7.5 in 2003; a Brookston clay loam with 29.4% sand, 36.2% silt, 34.5% clay, 5.8% organic matter, and pH of 5.8 in 2004; and a Normandale very fine sandy loam with 80.6% sand, 12.3% silt, 7.2% clay, 6.1% organic matter, and pH of 7.2 in 2005. Each trial area was mouldboard plowed in the fall and cultivated twice with a field S-tine cultivator and rolling basket harrows in the spring before transplanting.

The experimental design was a randomized complete block split-plot design with four replications. The main plot treatments were s-metolachlor (1200 and 2400 g·ha⁻¹ a.i.), dimethenamid-p (750 and 1500 g·ha⁻¹ a.i.), sulfentrazone (100 and 200 g·ha⁻¹ a.i.), s-metolachlor + sulfentrazone (1200 + 100 and 2400 + 200 g·ha⁻¹ a.i.), and dimethenamid-p + sulfentrazone (750 + 100 and 1500 + 200 g·ha⁻¹ a.i.). An untreated control was included for comparison. Treatments were applied 1 d before transplanting using a CO₂-pressurized backpack sprayer calibrated to deliver 200 L·ha⁻¹ at 207 kPa using 8002 flat-fan nozzles (Spraying Systems Co., Wheaton, IL). The boom was 1.0 m long with three nozzles spaced 50 cm apart.

Plots were 1.5 m wide by 8 m long. ‘Enterprise’ pepper transplants, ≈6 to 8 cm tall at transplanting, were planted at a depth of 5 cm in twin rows (30 cm apart) at a rate of 29,000 transplants/ha on 29 May 2003, 7 June 2004, and 1 June 2005. There was no need for irrigation in these studies because rainfall provided adequate moisture. Plots were fertilized according to recommended Ontario crop production practices (OMAFRA, 2006b). Annual grass weeds were removed from the entire trial with an application of sethoxydim (150 g·ha⁻¹ a.i.) plus a surfactant/solvent (1 L·ha⁻¹) when the grasses reached the three to four leaf stage or pulled by hand if necessary.

The subplot factor was weed-free or weedy; one half of each plot (including the untreated control) was kept weed-free by hoeing on a weekly basis from transplanting until harvest, whereas the other half of each plot was left weedy.

Visual crop injury was rated on the weed-free half of each plot 7, 14, and 28 d after transplanting (DAT) using a scale of 0% to 100%. A rating of 0 was defined as no visible plant injury, and a rating of 100 was defined as total plant necrosis. At 56 DAT, broadleaf weed counts were recorded in two 0.5-m² quadrats placed in the middle of the weedy half of each plot. Weed density was taken as the average of these two readings and extrapolated to 1 m². Peppers were hand-harvested at crop maturity from the middle 3 m of the weedy and the weed-free halves of each plot at all locations and separated into marketable and nonmarketable fruit. Fruit number, fruit size, and yield were determined. Fruits were harvested on 28 Aug. and 11 Sept. 11 2003, on 20 and 31 Aug. in 2004, and on 17 and 24 Aug. in 2005.

Data were subjected to analysis of variance to determine the effect of year and year-by-treatment interactions on visual injury, broadleaf weed control, marketable fruit number, marketable fruit size, and marketable yield. Because the year-by-treatment interactions were not significant, data were combined over years and analyzed using the PROC MIXED procedure of Statistical Analysis Systems (1999). Variances were partitioned into random effects of years, blocks within years, and their interactions with fixed treatment effects. Significance of random effects was tested using a Z-test of the variance estimate and fixed effects were tested using F-tests. Error assumptions of the variance analyses (random, homogeneous, normal distribution of error) were confirmed using residual plots and the Shapiro-Wilk normality test. Percent visual injury at 7, 14, and 28 DAT was subjected to log transformation, whereas percent weed control for each weed species at 56 DAT was subjected to arcsine transformation (Bartlett, 1947), compared on the transformed scale, and converted back to the original scale for presentation of results (Bartlett, 1947). Treatment means for visual injury, broadleaf weed control, marketable fruit number, marketable fruit size, and marketable yield were separated using Fisher's protected least significant differences (α = 0.05).

Results and Discussion

Pepper tolerance to pretransplant herbicides.

Visual injury was less than 4% in all the treatments tested, including the tank mix combinations of s-metolachlor + sulfentrazone or dimethenamid-p + sulfentrazone (Table 1). The authors are unaware of any studies that have examined pepper tolerance to dimethenamid-p. Grey et al. (2002) observed significant visual injury, including stunting, chlorosis, and necrosis as well as significant reductions in plant stand and yield in pepper with sulfentrazone at a rate of 420 g·ha⁻¹ a.i. At the rates tested in the current study, pepper exhibited excellent tolerance to sulfentrazone. In addition to a lack of visual injury, fruit number per 3 m of row, mean fruit size, and marketable yield were not less than the untreated, weed-free control in any of the treatments (Table 2). It is concluded that transplanted pepper has excellent tolerance to pretranplant applications of dimethenamid-p (750 and 1500 g·ha⁻¹ a.i.), sulfentrazone (100 and 200 g·ha⁻¹ a.i.) as well as tank mixes of dimethenamid-p or s-metolachlor plus sulfentrazone.

Table 1. Mean percent injury 7, 14, and 28 d after transplanting (DAT) peppers for each herbicide treatment in weed-free subplots from 2003 to 2005 at Ridgetown, Ontario^z.

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Table 2. Marketable fruit number per 3 m of row, fruit size, and yield of pepper for each herbicide treatment in weed-free subplots from 2003 to 2005 at Ridgetown, Ontario^z.

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Weed control in pepper with pretransplant herbicides.

S-metolachlor alone (1200 g·ha⁻¹ a.i.) provided greater than 81% control of redroot pigweed, 75% control of eastern black nightshade, but less than 50% control of velvetleaf, common ragweed, and common lambsquarters (Table 3). Dimethenamid-p alone (750 g·ha⁻¹ a.i.) provided better than 80% control of redroot pigweed and eastern black nightshade; however, control of velvetleaf, common ragweed, and common lambsquarters was less than 70% (Table 3). The pretransplant application of sulfentrazone provided 80% control of common lambsquarters, 64% and 70% control of velvetleaf and redroot pigweed, but less than 60% control of common ragweed and eastern black nightshade (Table 3). At the rates used in this study, s-metolachlor, dimethenamid-p, and sulfentrazone each applied on their own did not provide commercially acceptable control (i.e., greater than 80%) of the major broadleaf weed species typically found in pepper fields in southwestern Ontario. The poor control provided by each of these herbicides when applied alone corresponded with significant reductions in fruit number per 3 m of row, mean fruit size, and marketable yield (Table 4).

Table 3. Percent control of Abutilon theophrasti (ABUTH), Amaranthus retroflexus (AMARE), Ambrosia artemisiifolia (AMBEL), Chenopodium album (CHEAL), and Solanum ptycanthum (SOLPT) at 56 d after transplanting pepper in weedy subplots from 2003 to 2005 at Ridgetown, Ontario^z.

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Table 4. Marketable fruit number per 3 m of row, fruit size, and yield of pepper for each herbicide treatment in weedy subplots from 2003 to 2005 at Ridgetown, Ontario (comparable values in the untreated, weed-free control are also shown)^z.

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The tank mix combination of s-metolachlor + sulfentrazone (1200 + 100 g·ha⁻¹ a.i.) provided greater than 80% control of common ragweed, redroot pigweed, common lambsquarters, and eastern black nightshade and 70% control of velvetleaf (Table 3). The dimethenamid-p + sulfentrazone (750 + 100 g·ha⁻¹ a.i.) treatment provided similar control of the major broadleaf weed species as the s-metolachlor + sulfentrazone treatment (Table 3). Velvetleaf control was not commercially acceptable in any of the treatments tested; however, research is currently underway to register clomazone in transplanted peppers, which would provide growers with an excellent tool to control this weed. The level of control provided by the tank mix combinations of s-metolachlor + sulfentrazone and dimethenamid-p + sulfentrazone resulted in yields that were similar to the untreated, weed-free control (Table 4).

The data from this study indicate that transplanted pepper possesses acceptable levels of tolerance to dimethenamid-p applied at 750 and 1500 g·ha⁻¹ a.i. and sulfentrazone applied at 100 and 200 g·ha⁻¹ a.i. Furthermore, when s-metolachlor or dimethenamid-p was tank mixed with sulfentrazone (100 or 200 g·ha⁻¹ a.i.) and applied to the soil before transplanting pepper, there was an acceptable level of crop tolerance and control of redroot pigweed, common ragweed, common lambsquarters, and eastern black nightshade. Based on these results, sulfentrazone and dimethenamid-p at rates evaluated can be useful herbicide options for weed management in pepper.