Introduction

Peanut (Arachis hypogaea L.) is an important crop in North Carolina and the southeastern United States (Brown, 2013). The relatively poor competitive ability of peanut necessitates season-long weed control to obtain maximize yield (Jordan, 2013; Wilcut et al., 1995). In addition to adverse effects of weed interference, diseases, insects, and nematodes can also be deleterious to yield. Mechanized production systems utilize a wide range of agrochemicals to manage peanut growth and development and minimize the impact of pests on peanut yield and quality (Lynch and Mack, 1995; Sherwood et al., 1995; Wilcut et al., 1995).

Monocotyledonous weeds, including annual and perennial grasses and sedges, as well dicotyledonous weeds are prevalent in peanut in the United States (Webster, 2009; Wilcut et al., 1995). Comprehensive herbicide programs are employed to manage weeds and minimize interference and prevent yield loss (Wilcut et al., 1995). Herbicides are often co-applied either prior to planting, immediately following planting, or after peanut and weeds have emerged to achieve optimum yield (Wilcut et al., 1995). Grasses are especially troublesome in peanut due to the requirement of digging and inversion associated with harvest because the dense, fibrous root systems of grasses can cause pods to be stripped from the peanut vines (Wilcut et al., 1995). Clethodim and sethoxydim control annual and perennial grasses when applied postemergence to peanut (Jordan, 2013). Palmer amaranth and other broadleaf weeds can be found throughout the major peanut production regions of the southeastern United States (Horak and Loughin, 2000). Although soil-applied herbicides are effective on Palmer amaranth (Grichar and Colburn, 1996; Grichar et al., 1999), season-long control is generally not adequate with soil-applied herbicides alone (Grichar, 2008). In addition to other postemergence herbicides, 2,4-DB is often applied several times during the growing season to suppress Amaranthus species and sicklepod [Senna obtusifolia (L.) Irwin and Barneby] (Jordan, 2013; Wilcut et al., 1995).

Fungicides are applied routinely to peanut to control the foliar diseases early leaf spot (caused by Cercospora arachidicola Hori) and late leaf spot (caused by Cercosporidium personatum Berk. & Curtis) and the soil-borne diseases stem rot (caused by Sclerotium rolfsii Sacc.), rhizoctonia limb rot (caused by Rhizoctonia solani), and Sclerotinia blight (caused by Sclerotinia minor Jagger) (Brenneman et al., 1994; Culbreath et al., 2008; Shew, 2013; Smith et al., 1992). Although variation is noted, fungicides are applied either singly or in combination beginning approximately 45 days after peanut emergence and continuing throughout the remainder of the growing season (Sherwood et al., 1995; Shew, 2013; Smith and Littrell, 1980).

A diversity of pesticide active ingredients is available for use in peanut production systems (Brandenburg, 2013; Jordan, 2013; Shew, 2013). Pests often occur simultaneously during the peanut growing season, and timing of application for many agrochemicals overlap (Brandenburg, 2013; Jordan, 2013; Shew, 2013). Therefore, there is often a possibility to co-apply herbicides and fungicides in peanut production systems. This approach is preferable if peanut is not injured, pest control is not compromised, and physicochemical compatibility exits. Research has been conducted to define interactions between herbicides (Burke et al., 2004; Culpepper et al., 1998; Wehtje et al., 1992) and herbicides and fungicides (Jordan et al., 2003; Lancaster et al., 2005a, 2005b, 2008). Although interactions described in the literature have been defined for some herbicide-fungicide mixtures (Lancaster et al., 2008), reports on the interactions of herbicides and prothioconazole plus tebuconazole, tebuconazole plus trifloxystrobin, and flutriafol with clethodim or 2,4-DB have not been reported.

Defining interactions among agrochemicals is important in assisting growers and their advisors in making decisions on co-application of these products. Therefore, the objectives of this study were to define interactions when clethodim and 2,4-DB are applied alone or in combination with fungicides not previously reported in the literature and to determine if changes in physicochemical characteristics of spray solutions occur that could influence efficacy.

Materials and Methods

Results and Discussion

Interactions of herbicides with fungicides in the field

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).

Table 1.

Control of large crabgrass 21 days after the application when clethodim was applied alone or in combination with selected fungicides.^a,b

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.

Table 2.

Control of Palmer amaranth 21 days after application of 2,4-DB applied alone or in combination with selected fungicides.^a,b

Physicochemical compatibility of herbicides with other agrochemicals

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.

Table 3.

Solution pH determined when clethodim and 2,4-DB were applied alone or in combination with selected fungicides at four sampling intervals.^a,b,c

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.

Table 4.

Presence or absence of precipitates on mixing of clethodim and 2,4-DB with other selected fungicides at four sampling intervals. ^a,b,c

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.

This research was supported financially by the North Carolina Peanut Growers Association and USAID Peanut CRSP grant LAG-G-00-96-90013-00. Gary Little and staff at the Upper Coastal Plain Research Station provided technical assistance.

Literature Cited

Brandenburg R.L 2013 Peanut insect management, Pages 81–100 in 2013 Peanut Information. North Carolina Coop. Ext. Ser. Publication AG-331 .

Brenneman T.B Sumner H.R Chandler L.R Hammond J.M and Culbreath A.K 1994 Effect of application techniques on performance of propiconazole for peanut disease control Peanut Sci. 21 : 134 – 138 .

Brown A.B 2013 Virginia type peanuts: Situation and outlook, Page 4 in 2013 Peanut Information. North Carolina State University Coop. Ext. Ser. Publication AG-331 .

Burke I.C Price A.J Wilcut J.W Jordan D.L Culpepper A.S and Ducar J.T 2004 Annual grass control in peanut (Arachis hypogaea) with clethodim and imazapic Weed Technol. 18 : 88 – 92 .

Culbreath A.K Kemerait R. C and Brenneman T.B 2008 Management of leaf spot diseases of peanut with prothioconazole applied alone or in combination with tebuconazole or trifloxystrobin Peanut Sci. 35 : 149 – 158 .

Culpepper A.S York A.C Jennings K.M and Batts R.B 1998 Interaction of bromoxynil and postemergence graminicides on large crabgrass (Digitaria sanguinalis) Weed Technol. 12 : 554 – 559 .

Frans R.E Talbert R Marx D and Crowley H 1986 Experimental design and techniques for measuring and analyzing plant responses to weed control practices In Camper N.D (ed.) Research Methods in Weed Science South. Weed Sci. Soc. , Champaign, IL pp. 29 – 46 .

Grichar W.J and Colburn A.E 1996 Flumioxazin for weed control in Texas peanuts (Arachis hypogaea L.) Peanut Sci. 23 : 30 – 36 .

Grichar W.J Dotray P.A and Sestak D.C 1999 Diclosulam for weed control in Texas peanut Peanut Sci. 26 : 23 – 28 .

Grichar W.J 2008 Herbicide systems for control of horse purslane (Trianthema portulacastrum L.), smellmelon (Cucumis melo L.), and Palmer amaranth (Amaranthus palmeri S. Wats) in peanut Peanut Sci. 35 : 38 – 42 .

Horak M.J and Loughin T.M 2000 Growth analysis of four Amaranthus species Weed Sci. 48 : 347 – 355 .

Jordan D.L Peanut weed management 2013 Pages 45–80 in 2013 Peanut Information. North Carolina Coop. Ext. Ser. Publication AG-331 .

Jordan D.L Culpepper A.S Grichar W.J Ducar J.T Brecke B.J and York A.C 2003 Weed control with combinations of selected fungicides and herbicides applied postemergence to peanut (Arachis hypogaea L.) Peanut Sci. 30 : 1 – 7 .

Lancaster S.H Jordan D.L York A.C Burke I.C Corbin F.T Sheldon Y.S Wilcut J.W and Monks D.W 2005a Influence of selected fungicides on efficacy of clethodim and sethoxydim Weed Technol. 19 : 397 – 403 .

Lancaster S.H Jordan D.L York A.C Wilcut J.W Brandenburg R.L and Monks D.W 2005b Interaction of late-season morningglory (Ipomoea spp.) management practices in peanut (Arachis hypogaea L.) Weed Technol. 19 : 803 – 808 .

Lancaster S.H Jordan D.L and Johnson P.D 2008 Influence of graminicide formulation on compatibility with other pesticides Weed Technol. 22 : 580 – 583 .

Lynch R.E and Mack T.P 1995 Biological and biotechnical advances for insect management in peanut In Pattee H.E and Stalker H.T eds. Advances in Peanut Science Stillwater: Am. Peanut Res. and Educ. Soc. 95 – 159 .

Sherwood J.L Beute M.K Dickson D.W Elliot V.J Nelson R.S Opperman C.H and Shew B.B 1995 Biological and biotechnical control in Arachis diseases In Pattee H.E and Stalker H.T eds. Advances in Peanut Science Raleigh: Am. Peanut Res. and Educ. Soc. pp. 160 – 206 .

Shew B.B 2013 Peanut disease management, Pages 101–131 in 2013 Peanut Information. North Carolina Coop. Ext. Ser. Publication AG-331 .

Smith F.D Phipps P.M and Stipes R.J 1992 Fluazinam: A new fungicide for control of Sclerotinia blight and other soilborne pathogens of peanut Peanut Sci. 19 : 115 – 120 .

Smith D.H and Littrell R.H 1980 Management of peanut foliar diseases with fungicides Plant Dis. 64 : 356 – 361 .

Wanamarta G and Penner D 1989 Identification of efficacious adjuvants for sethoxydim and bentazon Weed Technol. 3 : 60 – 66 .

Webster T.M 2009 Weed survey- southern states Proc. South. Weed Sci. Soc. 62 : 509 – 524 .

Wehtje G Wilcut J.W and McGuire J.A 1992 Paraquat behavior as influenced by 2,4-DB in peanut (Arachis hypogaea L.) and selected weeds Peanut Sci. 19 : 51 – 55 .

Wilcut J.W York A.C Grichar W.J and Wehtje G.R 1995 The biology and management of weeds in peanut (Arachis hypogaea) In Pattee H.E and Stalker H.T eds Advances in Peanut Science. Stillwater: Am. Peanut Res. and Educ. Soc. pp. 207 – 224 .