Palmer amaranth resistance to acetolactate synthase (ALS)–inhibiting herbicides was first identified in Georgia in 2000. Since then, complaints from peanut producers have increased concerning failure of ALS herbicides in controlling Palmer amaranth. Because efficacy of ALS herbicides can be compromised under adverse conditions, seeds from Palmer amaranth plants that escaped weed control were collected across the peanut-growing region in Georgia to investigate the cause of these reported failures. Greenhouse and growth-chamber studies were conducted using these seeds to evaluate whether weed escapes were a result of Palmer amaranth resistance to ALS herbicides. Each of the 61 accessions collected across Georgia exhibited varying levels of resistance to imazapic applied POST (< 55% control, relative to ALS-susceptible Palmer amaranth). Subsamples of the accessions were evaluated for their response to imazapic rates, which indicated variable levels of resistance across Palmer amaranth accessions. The rate of imazapic that provided 50% reduction in Palmer amaranth plant biomass (I50) for the known susceptible biotype was 0.9 g/ha of imazapic. Of the 10 accessions evaluated, 8 of them had I50values that ranged from 3 to 297 g/ha of imazapic. The other two accessions could not be fit to the log-logistic dose–response curve and had undeterminable I50values because of high levels of ALS resistance (> 1,400 g/ha of imazapic). Herbicide cross-resistance experiments indicated that 30 accessions were resistant to the ALS herbicides imazapic, chlorimuron, pyrithiobac, and diclosulam at the recommended field-use rates. However, each of these 30 accessions was susceptible to glyphosate. These data demonstrate that ALS-resistant Palmer amaranth occurs throughout the peanut-growing region of Georgia. Growers in Georgia will need to alter their weed-control programs in peanut to include herbicides with multiple modes of action that do not rely on ALS herbicides for effective Palmer amaranth control.

Tropical spiderwort (more appropriately called Benghal dayflower) poses a serious threat to crop production in the southern United States. Although tropical spiderwort has been present in the United States for more than seven decades, only recently has it become a pest in agricultural fields. Identified as an isolated weed problem in 1999, tropical spiderwort became the most troublesome weed in Georgia cotton by 2003. Contributing to the significance of tropical spiderwort as a troublesome weed is the lack of control afforded by most commonly used herbicides, especially glyphosate. Vegetative growth and flower production of tropical spiderwort were optimized between 30 and 35 C, but growth was sustained over a range of 20 to 40 C. These temperatures are common throughout much of the United States during summer months. At the very least, it appears that tropical spiderwort may be able to co-occur with cotton throughout the southeastern United States. The environmental limits of tropical spiderwort have not yet been determined. However, the rapid spread through Georgia and naturalization in North Carolina, coupled with its tolerance to current management strategies and aggressive growth habit, make tropical spiderwort a significant threat to agroecosystems in the southern United States.

The efficacy of reduced rates of fomesafen, bentazon plus acifluorfen, and chlorimuron, in combination with sethoxydim was investigated in New Jersey from 1989 to 1991. Herbicides applied at sequential 0.25x rates controlled common lambsquarters, common ragweed, and stinkgrass. Chlorimuron plus sethoxydim did not control common lambsquarters at any rate. Generally, herbicides applied at 0.5x rates were not as effective as the full or sequential 0.25x rates. Yields of plots treated with reduced rates were equal to or greater than those treated with full rates.

Research was conducted at eight locations across the United States peanut belt during 2008 to evaluate peanut response to postemergence applications of dicamba. Dicamba was applied at 0, 40, 70, 140, 280 and 560 g ai/ha at 30, 60, and 90 days after peanut planting (DAP). In 5 of 8 locations, peanut yield losses were greater when dicamba was applied at 30 and 60 DAP when compared to 90 DAP. Estimated yield losses for dicamba applied at 40 g ai/ha ranged between 2% to 29%. Estimated yield losses for dicamba applied at 560 g ai/ha ranged between 23% to 100%. These data may aid peanut growers in making appropriate management decisions in situations where offtarget movement of dicamba has occurred or sprayer contamination is suspected.Key Words: Arachis hypogaea L., crop tolerance, drift, herbicide injury, sprayer contamination.Concerns regarding glyphosate-resistant weeds has led to an interest in developing alternative herbicide-tolerant crops. Dicamba-tolerance is being developed in several broadleaf crops including soybean [Glycine max (L.) Merr.] and cotton (Gossypium hirsutum L.) (Behrens et al. 2007;Subramanian et al. 1997). Currently, dicamba is registered for postemergence broadleaf weed control use in various grass crops such as field corn (Zea mays L.), sorghum [Sorghum bicolor (L.) Moench], and wheat (Triticum aestivum L.) (Anonymous, 2011).Dicamba's reputation for off -target movement due to drift and volatility has been well documented (Al-Khatib and Peterson, 1999;Behrens and Lueschen, 1979). In the southeast, peanut is grown in close proximity to both soybean and cotton. Thus, the adoption of dicamba-tolerance in these crops increases the probability of drift, volatilization, and tank contamination problems that could negatively influence peanut development and yield.Peanut response to dicamba has not been well documented. Dicamba applied at approximately 2 g/ha had no effect on peanut yield in one field trial (Prostko et al. 2009). However, other studies on the control of volunteer peanut indicated that peanut is not tolerant of dicamba (York et al. 1994). In a related forage crop, rhizome peanut (Arachis glabrata Benth) yields were significantly reduced by a foliar application of dicamba + 2,4-D (Ferrell et al. 2006). Other systematic studies on the influence of dicamba rate and timing on peanut have not been published in the literature. Therefore, the objective of this research was to quantify the effects of various rates of dicamba, applied at 30, 60 or 90 days after planting (DAP), on peanut yield. Materials and MethodsField trials were conducted at eight locations across the United States peanut belt during 2008. A complete description of these locations is presented in Table 1. Production and pest management practices were followed according to local Cooperative Extension recommendations.All trials were conducted in a randomized complete block design with a three (application timing) by six (dicamba rate) factorial arrangement of treatments. Dicamba timings were 30, 60, and 90 DAP and d...

The efficacy of preemergence (PRE) applications of ethalfluralin or pendimethalin incorporated with irrigation was compared with mechanical preplant incorporated (PPI) applications at equivalent rates in peanut. PRE applications of herbicides followed by irrigation were as effective as PPI applications in controlling Texas panicum, southern crabgrass, and crowfootgrass. Split PPI/PRE applications of ethalfluralin were more effective than PPI applications in controlling Texas panicum in 2 of 3 yr. Sequential postemergence applications of clethodim or sethoxydim increased the control of Texas panicum when ethalfluralin or pendimethalin controlled Texas panicum less than 82%. No differences in peanut yield were observed between PPI or PRE applications of either ethalfluralin or pendimethalin.