Effect of Bradyrhizobia Inoculant Formulation with Phorate in New Peanut Fields

ABSTRACT Peanut (Arachis hypogaea L.) is a legume and requires nodulation by Bradyrhizobia to convert atmospheric N into a plant usable form. Peanut inoculants place large volumes of viable Bradyrhizobia near the emerging root of the plant to infect it ensuring N-fixation. Peanut seedlings are susceptible to feeding by thrips (Thysanoptera: Thripidae), which transmit Tospovirus leading to tomato spotted wilt (TSW) and can result in yield decline. Phorate is a common in-furrow insecticide used to reduce thrips feeding on the plants, and hence reduce the risk of TSW infection. However, placing phorate in the same furrow with living Bradyrhizobia has raised concerns about phorate potentially reducing efficacy of inoculants. Experiments were designed to test liquid and sterile peat formulations of Bradyrhizobia inoculant with two peanut cultivars in-furrow at planting, both with and without phorate insecticide to determine yield, grade, and growth parameters on peanut. Research was conducted on a non-irrigated loamy sand with no prior history of growing peanut in Tifton, GA in 2013 and 2014. There were no negative effects on peanut yield or grade, or other measured parameters when phorate was included in-furrow with inoculants. Yields with inoculants were equal or greater (up to 32% more) than non-inoculated peanut, and grade was improved (+1.2 to 1.4%) when using the liquid formulation instead of the sterile peat formulation or non-inoculated peanut. Other indicators of improved plant performance using inoculants over non-inoculated plants included the abundance and activity of nodules on the roots and time to row closure (vegetation overlap of bare soil). These data indicate there are no detrimental effects to peanut inoculant when also including phorate insecticide in-furrow.