Research was conducted in North Carolina during 2008 and 2009 at the Upper Coastal Plain Research Station located near Rocky Mount. Soil was a Goldsboro fine sandy loam (fine-loamy, siliceous, subactive, thermic Aquic Paleudults). Efficacy of clethodim and 2,4-DB was evaluated in separate experiments in non-crop areas. Plot size was 2.4 by 5 m.

Treatments consisted of clethodim (Select Max, Valent Corp., Walnut Creek, CA) at 140 g ai/ha applied alone or in combination with chlorothalonil (Bravo Weather Stik, Syngenta Crop Protection, Inc., Greensboro, NC) at 840 g ai/ha, flutriafol (Topguard fungicide, Cheminova Inc., Research Triangle Park, NC) at 64 g ai/ha, pyraclostrobin (Headline, BASF Corp., Research Triangle Park, NC) at 170 g ai/ha, prothioconazole plus tebuconazole (Provost, Bayer CropScience LP, Research Triangle Park, NC) at 80 + 160 g ai/ha, and tebuconazole plus trifloxystrobin (Absolute, Bayer CropScience LP, Research Triangle Park, NC) at 67 + 67 g ai/ha. Clethodim alone or with fungicides was applied with crop oil concentrate (Agri-Dex, Helena Chemical Company, Collierville, TN) at 1.0% (v/v).

Palmer amaranth control by 2,4-DB (Butyrac 200, Albaugh Inc., Ankeny, IA) at 280 g ai/ha applied alone or with these fungicides was also evaluated. Adjuvant was not included. Pesticides were applied with a CO₂-pressurized backpack sprayer calibrated to deliver 140 L/ha using flat-fan nozzles (TeeJet TP8002 flat-fan spray nozzles, Spraying Systems Co., Wheaton, IL) at 275 kPa.

Large crabgrass control by clethodim was evaluated in two separate fields at Rocky Mount during 2008 and in one field during 2009 when plants were of 15 to 20 cm in height. Palmer amaranth control by 2,4-DB was evaluated at Rocky Mount in two separate fields during both 2008 and 2009. Palmer amaranth was 25 to 30 cm in height at timing of application.

Visual estimates of percent weed control were recorded 21 days after treatment using a scale of 0 to 100% where 0  =  no weed control and 100  =  complete weed control (Frans et al., 1986). Foliar chlorosis, necrosis, and plant stunting were considered when making the visual estimates.

Laboratory experiments were conducted to compare physicochemical compatibility of the herbicide combinations at rates and spray volume described in the field experiments. In contrast to field experiments where a municipal water source was used, deionized water at pH 6.3 was used in the laboratory experiments. Agrochemicals were mixed in the following order: flowables (chlorothalonil, flutriafol, prothioconazole plus tebuconazole, and tebuconazole plus trifloxystrobin), emulsifiable concentrates (clethodim and pyraclostrobin), and soluble liquids (crop oil concentrate 2,4-DB). Herbicide solutions were prepared in a final volume of 80 ml in sterilized plastic specimen cups (Specimen cup120 ml-53 ST ORG CAP, Fisher Scientific, Fairlawn, NJ) of 120 ml capacity. After mixing different agrochemicals, solution was vortexed (Vortex Genie 2, Fisher Scientific, Fairlawn, NJ) immediately and examined for precipitates followed by determining pH using a portable pH meter (Oakton portable pH meter, Fisher Scientific, Fairlawn, NJ). Solutions were allowed to sit for 6 h after the time of mixing, examined visually with the naked eye for precipitates, vortexed, and re-examined for precipitates followed by pH determination. Similarly, mixtures were allowed to sit for 24 and 72 h after the initial solution preparation using the same procedure. Presence or absence of precipitates were determined visually and described as yes or no, respectively. Any visual depositions on the bottom of the specimen cup or the presence of suspended solids in the solution were considered as precipitates. Two types of precipitates, temporary and permanent, were noticed when herbicides were mixed with other agrochemicals. Temporary precipitates went back into solution on vortexing while permanent precipitates did not go into solution after vortexing.

The experimental design was a randomized complete block with treatments replicated four times in the experiments evaluating large crabgrass and Palmer amaranth control. The experimental design in the laboratory experiment was a completely randomized design with two replications and the experiment was repeated once. Data for visual estimates of percent weed control and solution pH were subjected to ANOVA using the PROC GLM procedure of SAS (Statistical Analysis Systems, version 9.1, SAS Institute Inc., Cary, NC) using expected mean squares to test fixed and random effects. Means of significant main effects and interactions were separated using Fisher's Protected LSD test at p ≤ 0.05. F-statistic and Pearson correlation coefficients between pooled weed control for each herbicide and solution pH determined at four time intervals were calculated using the PROC CORR procedure in SAS.

The interaction of experiment (year and location combinations) and treatments was significant for large crabgrass control by clethodim, therefore, data are presented for each experiment. When compared to clethodim alone, application of clethodim with pyraclostrobin reduced large crabgrass control in all three experiments by 6 to 48 percentage points (Table 1). The reduction in control was greater in 2009 than 2008 and this may be due to higher rainfall during the 2008 growing season which resulted in plants undergoing less stress after combinations of clethodim and fungicides were applied. Reduction in grass control by graminicides caused by other pesticides is often exacerbated when plants are under water stress (Burke et al., 2004; Wanamarta and Penner, 1989).

Although control was not affected during 2008, applying chlorothalonil with clethodim decreased control from 84% to 69% compared with clethodim alone in 2009 (Table 1). Lancaster et al. (2005a) reported that large crabgrass control was reduced when clethodim was applied with chlorothalonil and pyraclostrobin. Applying clethodim with tebuconazole plus trifloxystrobin reduced large crabgrass in 2009 but did not affect control during 2008 compared with clethodim alone (Table 1). Control with clethodim was not affected by prothioconazole plus tebuconazole or flutriafol. Tebuconazole often does not reduce control with clethodim (Jordan et al., 2003; Lancaster et al., 2005a). The influence of prothioconazole plus tebuconazole or tebuconazole plus trifloxystrobin as well as flutriafol on efficacy of clethodim has not been reported previously.

The interaction of experiment by treatment was significant for Palmer amaranth control with 2,4-DB; therefore, data are presented for each experiment. In one experiment in 2009, control with 2,4-DB increased when pyraclostrobin was included compared with 2,4-DB alone (Table 2). Palmer amaranth control by 2,4-DB was not affected by chlorothalonil, flutriafol, prothioconazole plus tebuconazole, or tebuconazole plus trifloxystrobin. Lancaster et al. (2005b) reported that co-application of pyraclostrobin with 2,4-DB reduced entireleaf morningglory (Ipomoea hederacea var. integriuscula Gray) control compared with 2,4-DB alone.

Interactions of sampling time by treatments were noted for solution pH for clethodim and 2,4-DB combinations, therefore, data are presented for each sampling time. The average pH of solution with clethodim plus crop oil concentrate was 4.42, 4.49, 4.61, and 4.53 at 0, 6, 24, and 72 h sampling intervals, respectively (Table 3). When compared with initial pH of the aqueous solution (pH  =  6.30), clethodim plus crop oil concentrate reduced solution pH regardless of fungicide across all sampling intervals. Including chlorothalonil, flutriafol, and prothioconazole plus tebuconazole with clethodim plus crop oil concentrate increased solution pH at 0, 6, and 24 h sampling times compared to clethodim plus crop oil concentrate. When compared to solution with clethodim plus crop oil concentrate, the addition of pyraclostrobin and tebuconazole plus trifloxystrobin increased solution pH at 0 and 6 h sampling times.

The average solution pH value of 2,4-DB alone was 6.99, 6.93, 6.84, and 6.78 at 0, 6, 24, and 72 h sampling times, respectively (Table 3). When compared with 2,4-DB alone, solution pH was slightly increased across sampling times when flutriafol and pyraclostrobin were included in the mixture. In contrast, chlorothalonil decreased solution pH compared with 2,4-DB alone at all sampling times. Also, solution pH slightly decreased at 0, 6, and 72 h sampling intervals when prothioconazole plus tebuconazole was added to solution with 2,4-DB. When compared with 2,4-DB alone, tebuconazole plus trifloxystrobin decreased solution pH at 0 h sampling time only.

Combinations of clethodim with flutriafol, prothioconazole plus tebuconazole, and pyraclostrobin formed temporary precipitates; however, inclusion of chlorothalonil produced permanent precipitates across all sampling intervals (Table 4). Precipitates were formed with all 2,4-DB combinations with fungicides but solutions were reestablished after vortexing at each sampling interval (Table 4). No precipitates were formed in solution with 2,4-DB alone across sampling intervals.

Correlations for large crabgrass control by clethodim, pooled over experiments, and solution pH across sampling times, were not significant (p-values ranging from 0.3162 to 0.9934, data not shown in tables). However, significant positive correlations were noted for pooled Palmer amaranth control by 2,4-DB combinations and solution pH determined at four sampling times (p-values ranging from 0.0173 to 0.0529 with coefficients ranging from 0.81 to 0.88, data not shown in tables).

In summary, field experiments demonstrated that pyraclostrobin consistently reduced large crabgrass control by clethodim. In many instances, combinations of clethodim plus crop oil concentrate with chlorothalonil and tebuconazole plus trifloxystrobin reduced large crabgrass control by clethodim alone. The fungicides flutriafol and prothioconazole plus tebuconazole had the least affect on clethodim efficacy. Fungicides did not reduce Palmer amaranth control by 2,4-DB. Although several combinations of clethodim or 2,4-DB with fungicides formed temporary precipitates at different time intervals, the only permanent precipitates were formed when chlorothalonil, clethodim, and crop oil concentrate were mixed. Large crabgrass control by clethodim was not correlated with solution pH although significant correlations of Palmer amaranth control by 2,4-DB and solution pH were noted.