Introduction

There is interest in growing certified organic peanut in the southeastern U. S. to support increasing demand for organic peanut food products (Culbreath, 2005). Weed management is universally considered to be the major limiting factor in organic crop production (Organic Farming Research Foundation, 2001) and this is the case with organic peanut. Weed management in organic cropping systems is conceptually based on the same principles of weed management in conventional crop production; an integration of crop rotations, cultural practices, mechanical controls, and herbicides. However, the only herbicides allowed for use in certified organic crop production are those approved by the Organic Materials Resource Institute (OMRI; Box 11558; Eugene, OR 97440). In general terms, OMRI certification ensures that growers can use the herbicide, without compromising their organic crop production certification. While OMRI herbicides are approved to be used in certified organic crop production, such approval does not consider weed control efficacy, crop injury, or cost.

Essential oils are aromatic, volatile oils extracted from plants and have been shown to have herbicidal properties and potential weed management uses in organic cropping systems (Chase et al., 2004; Tworkoski, 2002). Similarly, some organic acids have herbicidal properties and, if derived from organic sources, may offer potential for weed control in organic cropping systems (Comis, 2002; Weber et al., 2005). Two herbicidal formulations² one containing clove oil³ and the other containing citric and acetic acids⁴, have been approved by OMRI for use in organic crop production. Both herbicides are considered to be nonselective postemergence herbicides (Anonymous 2007a, 2007b; Boyd and Brennan, 2006; Chase et al., 2004), but no research has been published regarding their efficacy or injury to peanut. Therefore, field trials were initiated in 2004 to evaluate the tolerance of peanut to direct applications of these herbicides.

Materials and Methods

Field trials were conducted at the Coastal Plain Experiment Station Ponder Farm, near Tifton, GA in 2004 and 2005. The soil was a Tifton loamy sand (fine-loamy, Kaolinitic, thermic Plinthic Kandiudults); 88% sand, 6% silt, 6% clay, with 0.2% organic matter. The experimental design was a factorial arrangement of treatments in a split-plot with four replications. Main plots were two OMRI-approved herbicides; clove oil (70 l/ha;) and a mixture of acetic and citric acid (65 l/ha).

Sub-plots were herbicides applied at vegetative emergence of peanut (VE), two weeks after VE (2 wk), four weeks after VE (4 wk), sequentially VE/2 wk, sequentially VE/4 wk, sequentially VE/2 wk/4 wk, and a nontreated control. All treatments were applied with a tractor mounted CO₂ pressurized plot sprayer calibrated to deliver 234 l/ha using low-drift spray tips (Turbo TeeJet® 11003; Spraying Systems CO.; P. O. Box 7900; Wheaton, IL) treating a swath 1.8 m wide. No adjuvants were added to the herbicide treatments.

DP-1 peanut were planted 18 May 2004 and 23 May 2005. DP-1 was chosen for use in these trials since it has excellent host plant resistance to early leafspot [Cercospora arachidicola S. Hori], late leafspot [Cercosporidium personatum (Berk. & M. A. Curtis) Deighton], stem rot (Sclerotium rolfsii Sacc.), and spotted wilt (tomato spotted wilt tospovirus), which makes the cultivar an excellent choice for reduced input peanut production (Cantonwine et al., 2006). Plots were 1.8 m wide and 6.1 m in length. Peanut were seeded in rows 91 cm apart at 112 kg/ha, which produced a final stand of 20 plants/m row. All plots were irrigated sufficiently to prevent drought stress.

All plots were maintained weed-free by using biweekly sweep cultivation for six weeks and weekly hand-weeding. Throughout the duration of the season, neither insecticides nor fungicides were applied to the plots. However, these plots were not able to be certified organic because the peanut seed were treated with non-approved protectants and suitable isolation from adjacent areas routinely treated with pesticides was not maintained.

Observations were made within 12 h of treatment application to note the nature of any immediate and noticeable injury to the peanut. Visual estimations of crop injury were taken mid-season to assess the extent of permanent or prolonged injury to the plant compared with the nontreated control. Ratings were recorded on a scale of 0 to 100, where, 0  =  no crop injury and 100  =  complete crop injury. Peanut yields were measured by digging, inverting, air-curing, and combining peanut from the entire plot using commercial two-row equipment. Yield samples were mechanically cleaned to remove foreign material. Yields are reported as cleaned farmer stock peanut. All data were analyzed using analysis of variance. Means were separated using Fisher's LSD (P≤0.05).

Results and Discussion

Data were analyzed separately by year due to differences in meteorological conditions between years. Weather conditions were generally more humid in 2004 compared to 2005, which affected peanut response and recovery from herbicide phytotoxicity.

Visual Injury

Injury symptoms from clove oil appeared less than 12 hours after treatment and were characterized by necrotic peanut tissue (data not shown). New peanut growth did not have foliar necrosis. Peanut treated with citric plus acetic acid had minimal (≤4%) foliar necrosis.

In general, clove oil was more injurious than citric plus acetic acid when rated mid-season (Table 1). Clove oil applied at VE stage of peanut growth did not injure peanut. Clove oil applied later than VE tended to stunt peanut, with applications at 4 wk being the most injurious time of application. Sequential applications were as injurious as the later applications alone. Clove oil applied three times was the most injurious (19 to 28% visual injury) treatment when rated mid-season. The maximum phytotoxicity from citric plus acetic acid averaged 4% (Table 1), which was nonsignificant. Citric plus acetic acid applied from VE to 4 wk after emergence, alone or sequentially, did not injure peanut.

Table 1

Injury and yield response for weed free DP-1 peanut to naturally derived herbicides in Tifton, GA; 2004 to 2005.

The technical efforts of Daniel R. Evarts and Andy M. Hornbuckle are recognized as a significant contribution to the success of this research project. This research was partially supported by a grant from the Southern Regional Sustainable Agricultural Research and Education Program, through Hebert Green Agroecology, Inc.

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