Investigation of Palmer Amaranth (Amaranthus palmeri S. Watson) Resistance to Mesotrione (Group 27: HPPD Inhibitors) in Mississippi

Palmer amaranth is one of the most problematic and damaging weeds in agronomic crops throughout the southern United States. Greenhouse research was conducted at the Delta Research and Extension Center in 2020 and 2021, to investigate possible Palmer amaranth resistance to mesotrione (Callisto) and its distribution in Mississippi. Palmer amaranth from various Counties (random) of Mississippi were sampled in 2017. Seedheads from about 52 samples (from different County) were processed to obtain clean seed. In 2020, forty of the Palmer amaranth populations out of 52 samples tested had survivors ranging from 1 to 53% from 1 X rate (0.105 kg ai ha-1) of mesotrione application. Palmer amaranth control from mesotrione application was less than 60% for the Bolivar-1, Bolivar-2, Bolivar-4, Holmes-1, Leflore-1, Leflore-3, Tonica-4, and Washington-5 Counties (ranging from 41 to 53% survivors). In 2021, seeds from survivor plants (F1 progeny) were planted on September 21, 2021, in a tray and emerged on September 24. Seedlings were thinned to about 100 plants per population and sprayed at 2- to 4-leaf stage on October 13 with mesotrione at 0.105 (1 X = recommended rate), 0.21 (2 X), and 0.42 (4 X) kg ai ha-1 + COC at 1% v/v. The F1 progeny from Warren-3, Leflore-1, Sunflower-3, Tallahatchie-3, and Tunica-4 had 56, 57, 70, 86, and 88% survivors from mesotrione application at 4X rate.