Response of Stevia (Stevia rebaudiana) to Herbicides Applied Post
Stephen Ippolito, MS Seminar
Under the direction of Dr. Katherine Jennings
Thursday, November 3, 2022 @ 1:00pm
121 Kilgore Hall / Zoom

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Abstract:
Stevia (Stevia rebaudiana Bertoni) is a zero-calorie sweetener, 200 to 400 times sweeter than sucrose. Stevia has been grown commercially as a sugar substitute in much of Asia since 1995.  In addition, stevia has been consumed as a sweetener for hundreds of years; however, only recently has stevia been authorized for use as a food additive in the US by the FDA. With the authorization of stevia as a food additive, several companies have released stevia products including Coca-Cola (Truvia) and Pepsi (PureVia). Stevia is sensitive to weed competition, especially early in the season. However, few herbicides have been registered for use in stevia. As a result, POST weed control options are limited in Stevia. Thus, studies were conducted to evaluate the safety of organic and conventional herbicides for use post-transplant in stevia.

Field and greenhouse studies were conducted to evaluate the efficacy and safety of applying organic herbicides over-the-top and POST-directed to stevia. Treatments included caprylic acid plus capric acid, clove oil plus cinnamon oil, d-limonene, acetic acid (200 grain), citric acid, pelargonic acid, eugenol, ammonium nonanoate, and ammoniated soap of fatty acids. In field studies, D-limonene, pelargonic acid, ammonium nonanoate, and ammoniated soap of fatty acids-controlled Palmer amaranth > 90% 1 wk after treatment (WAT). Palmer amaranth was not adequately controlled (< 65%) by citric acid, acetic acid and eugenol 1 WAT. In addition, 1 WAT d-limonene and pelargonic acid treatments resulted in over 90% control of annual sedge. Regarding crop safety, caprylic acid plus capric acid, pelargonic acid, and ammonium nonanoate caused < 30% injury 2 WAT. D-limonene, citric acid, acetic acid, and ammoniated soap of fatty acids caused less than 18 to 25% injury 2 WAT. Clove oil plus cinnamon oil and eugenol caused < 10% injury. Despite being injurious, organic herbicides did not reduce yield compared to the nontreated check. Due to the injurious nature of many of the organic herbicides applied, they should be utilized as an emergency action to prevent greater yield loss from weeds rather than part of a typical weed management program. The application of clove oil + cinnamon oil over-the-top resulted in <10% injury 28 DAT in the greenhouse and 6 WAT POST-directed in the field. In addition, this treatment provided excellent control of Palmer amaranth 4 WAT. Clove oil plus cinnamon oil has potential for use for early season weed management for organic production systems.

Greenhouse studies were conducted to evaluate the safety of conventional herbicides applied over-the-top of stevia 2 wk after transplanting. Treatments included S-metolachlor, acifluorfen, linuron, halosulfuron, ethalfluralin, pendimethalin, metribuzin, trifloxysulfuron, pyroxasulfone, and carfentrazone. At 1 wk after treatment (WAT) aciflourfen, metribuzin and carfentrazone caused 30 to 45% injury across both experimental runs.  In contrast across both runs at 1 WAT, S-metolachlor (1%), linuron (20, 5%), halosufluron (16, 14%), ethalfluralin (3, 1%), pyroxasulfone (7, 11%), pendimethalin (1, 0%), and tryfloxysulfuron (16, 19%) caused 20% or less injury to stevia. By 4 WAT, injury to stevia from all treatments caused less than 20% injury, except metribuzin and trifloxysulfuron which caused 84 and 69%, respectively.

S-metolachlor, linuron, ethalfluralin, pendimethalin and pyroxasulfone did not cause a significant reduction in above ground biomass relative to the nontreated check 28 DAT. Below ground biomass was not impacted by linuron, ethalfluralin, pendimethalin, and pyroxasulfone treatments. Linuron, pendimethalin, and ethalfluralin appear to have potential for use in stevia and if registered in stevia would represent new modes of action for in-season weed management in the crop.