Abstract

One of the most critical issues for weed scientists today is the management of herbicide-resistant weeds. Herbicide-resistant Palmer amaranth [Amaranthus palmeri (S.) Watson] is the most problematic pest problem for corn (Zea mays L.), cotton (Gossypium hirsutum L.), and soybean (Glycine max (L.) Merr.) producers in Mississippi. Greenhouse research was conducted at the Delta Research and Extension Center from 2019 to 2021 to investigate Palmer amaranth accessions potentially resistant to glufosinate and its distribution in Mississippi. In the 2019 screening, 20 out of 54 Palmer amaranth populations tested had individuals exhibiting 2 to 20% survival from a 1X rate (0.68 kg ai ha^-1 as recommended rate) of glufosinate. Bolivar-3 (9% survival), Coahoma-1 (11% survival), Holmes-3 (11% survival), Issaquena-2 (13% survival), Tunica-4 (17% survival), and Yazoo-3 (20% survival) accessions had the highest rate of survival. Twelve of those accessions were selected for a dose-response study in August 2020. Palmer amaranth plants that survived a 1X rate of glufosinate (suspected glufosinate-resistant Palmer amaranth) in 2019 were transplanted to larger pots and grown for seed production in 2020. In 2020, seedheads from female Palmer amaranth were harvested, threshed, and cleaned to obtain progeny, which were then stored until further evaluation. Only one Palmer amaranth plant (F1 progeny) from Quitman-1, Tunica-4, and Washington-5 survived glufosinate (1X) in 2020. No Palmer amaranth plants survived 2X and 4X rates of glufosinate in 2020. In the 2021 screening, four out of five original Palmer amaranth accessions selected (seed from 2017) survived, except for Issaquena-2 accession. Palmer amaranth individuals from accession Coahoma-1, Holmes-3, Tunica-4, and Yazoo-3 exhibited 3.7, 3.2, 1, and 1% survival, respectively, from a 1X rate of glufosinate. No F1 Palmer amaranth survivors were identified from any of those accessions evaluated. At this time, we cannot say with certainty that these survivors are resistant to glufosinate. Additional research (including physiological and molecular studies) needs to be conducted to confirm whether offspring of these survivors have developed resistance to glufosinate.

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