Date of Graduation

12-2022

Document Type

Thesis

Degree Name

Master of Science in Horticulture (MS)

Degree Level

Graduate

Department

Horticulture

Advisor/Mentor

Matthew Bertucci

Committee Member

Nilda R. Burgos

Second Committee Member

Amanda McWhirt

Third Committee Member

Aaron Cato

Keywords

Blackberries, Herbicide, Ouachita

Abstract

This field trial assesses some of the preemergence herbicide options available to growers and their effect on newly transplanted blackberries (Rubus L. subgenus Rubus Watson). Weed control has recently been surveyed as a top priority for blackberry growers; however, limited preemergence herbicides are registered for new blackberry plantings. Weed control is an ongoing component of blackberry production and with few in-season postemergence herbicide options available, growers rely on preemergence herbicides to maintain clean fields. The preemergent herbicides assessed in this trial were chosen with the intention to broaden the chemical control options available to growers in new plantings. A two-year field trial was initiated in 2021 and conducted at two locations: Milo J. Shult Research and Extension Center in Fayetteville, AR (36.09 ºN, 94.17 ºW) and the University of Arkansas Fruit Research Station in Clarksville, AR (35.53 ºN, 93.40 ºW). Seven treatments consisting of six preemergence herbicides (mesotrione, flumioxazin, oryzalin, S-metolachlor, pendimethalin, and napropamide) and one hand-weeded check were applied to field plots of newly transplanted tissue culture propagated blackberry plugs (var. ‘Ouachita’). Preemergence herbicide treatments were reapplied to the same plots in 2022. Data were collected on visual injury, plant height, leaf chlorophyll content, and green coverage of blackberry canopies and of bare ground portions of each plot. Yield data were collected in the second year, and fruit were analyzed for soluble solids content (°Brix), pH, and average berry weight. In the first year mesotrione and flumioxazin treatments caused the most injury to the primocanes. Injury by flumioxazin was not detectable at the final rating of the first year, but injury by mesotrione was high 84 days after treatment (DAT). Napropamide, S-metolachlor, oryzalin, and pendimethalin did not cause injury over 6% throughout the 2021 season. In the second year (2022) no damage was incurred by any treatments, from the treated or the non-chemical weed-free (NCWF) check. The mesotrione treatment affected plant height the most in 2021 at the end of the season compared to the NCWF check. In 2022 plant height was not assessed. Yield measurements taken in 2022 exhibited no significant differences in response to preemergence herbicide treatments. A greenhouse experiment was conducted to investigate the effects of a broad selection of preemergence herbicides at two rates. This screening was initiated August 2021 and repeated March 2022 in Fayetteville, AR in a horticultural greenhouse at the Milo J. Shult Research and Extension Center. Tissue cultured ‘Ouachita’ blackberry plugs were transplanted into utility pots that were treated with a preemergence herbicide treatment. Twenty-five treatments in total consisted of twelve preemergence herbicides at 1× and 2× rates and one untreated control. Data were collected on plant height, visual injury ratings, internode length, leaf chlorophyll content, and destructive harvest including leaf count, leaf dry biomass, specific leaf area (SLA), and leaf area to dry matter ratio (LADMR). Halosulfuron, rimsulfuron, and mesotrione treatments showed progressively increasing visual injury from 7 days after treatment (DAT) until 42 DAT. Flumioxazin, napropamide, S-metolachlor, and pendimethalin treatments exhibited similar responses to the untreated control regarding height and visual injury and may be acceptable for use in young blackberry weed management programs. Data obtained from this screening characterized the physiological response of new blackberries treated with these preemergence herbicides. Both trials demonstrated the deleterious effects of mesotrione on young plants and why it is not recommended for use in first year plantings. Both trials demonstrated the validity of the 24(c) labeling of S-metolachlor. These findings validate many of the regional recommendations and provide new evidence to consider expanding registration and labeled usage requirements for select preemergence herbicides. This knowledge and further field investigation have the potential to lead to more informed IPM strategies.

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