MNLA Regulatory Update – Palmer Amaranth (Amaranthus palmeri) Discovered in Winona County
James Calkins, MNLA Regulatory Affairs Manager; July 23, 2020
On July 17, 2020, the Minnesota Department of Agriculture (MDA) announced that Palmer amaranth (Amaranthus palmeri) was discovered in a soybean field in Winona County. Upon further inspection, additional plants that were also believed to be Palmer amaranth were found and removed from the field and these suspicions have since been confirmed by genetic testing. The field has subsequently been treated with herbicide, and follow-up scouting has not revealed any new plants. The site will now be monitored for up to three years to ensure the infestation has been eliminated.
Palmer amaranth was first discovered in western Minnesota in Yellow Medicine County and Lyon County in September 2016, and since these initial finds additional infestations have been found in seven more counties including Douglas and Todd County in 2017, Jackson and Redwood County in 2018, Houston and Lincoln County in 2019, and now Winona County in 2020. So far, none of these infestations are widespread, and the Minnesota Department of Agriculture is actively coordinating efforts to eradicate these infestations in an attempt to prevent the spread of this serious weed to new areas.
Palmer amaranth was designated as a Prohibited/Eradicate Noxious Weed in Minnesota in 2014 and as a Prohibited Weed Seed in 2016. As a result of these regulatory designations, existing plants may not be transported off site and must be destroyed and absolutely no palmer amaranth seed is allowed in any seed sold in the state. Because Palmer amaranth is an emerging threat in Minnesota, suspected finds should be immediately reported to the MDA for confirmation and eradication. Suspicious plants should be reported to the MDA’s Arrest the Pest Line at 1-888-545-6684 or firstname.lastname@example.org (mailto:email@example.com).
Before the 2018 growing season, all of the infestations of Palmer amaranth that had been found in Minnesota were in conservation plantings rather than crop production fields and were believed to have originated from a single, conservation mix, seed source. Unfortunately, the more recent finds in Jackson, Redwood, Houston, and Lincoln Counties, and this new find in Winona County, have been found in agricultural fields – a specifically, soybean fields – and the sources of thee infestations are unknown.
Several species of amaranth (also called pigweeds) are present in Minnesota and of these species Palmer amaranth most closely resembles, and is most commonly confused with, tall waterhemp (Amaranthus tuberculatus), a native species that is also a serious agricultural weed. Both species are dioecious (male and female flowers produced on separate plants; individual plants male or female), warm-season annuals that grow very tall (6-8 feet or taller). Having petioles (leaf stalks) that are longer than the leaf blade is a key characteristic that can be used to separate Palmer amaranth from other pigweeds including waterhemp. Palmer amaranth produces copious amounts of seed (as many as 250,000 seeds can be produced by a single plant), is a fast-growing and highly competitive weed, and has developed resistance to multiple herbicides with different modes of action including dinitroanilines, triazines, acetolactate synthase (ALS) inhibitors, 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors, and glyphosate-based herbicides. As a result, Palmer amaranth poses a serious threat to agriculture in Minnesota. MNLA members should be aware of this important weed and help prevent its spread through contaminated seed and other means. It is also important that suspected finds be reported so they can be verified and treated by the MDA. Since it is a warm-season annual, this is the time when Palmer amaranth starts to be noticed in agricultural fields and other disturbed sites including roadsides, vacant lots, and gardens.
The MDA news release announcing the discovery of Palmer amaranth in Winona County is available on the MDA website at https://www.mda.state.mn.us/palmer-amaranth-found-winona-county.
Additional information about Palmer amaranth, including how to identify Palmer amaranth and differentiate it from other is available on the MDA website at https://www.mda.state.mn.us/plants/pestmanagement/weedcontrol/noxiouslist/palmeramaranth and https://www.mda.state.mn.us/plants-insects/palmer-amaranth-minnesota
If you have questions or comments regarding this MNLA Regulatory Update or the status of EAB in Minnesota, neighboring states, or North America, contact Jim Calkins, MNLA Regulatory Affairs Manager, at firstname.lastname@example.org or 952-935-0682.
Figure 1. Palmer amaranth (Amaranthis palmeri), a member of the Amaranth or Pigweed Family (Amaranthaceae) and native to the southwestern United States and northern Mexico, is a new and potentially serious weed in Minnesota that nursery and landscape professional should be on the lookout for and report to the Minnesota Department of Agriculture; note the sharp bracts that are present on the flowers and seed heads of female plants; Palmer amaranth was found in Winona County in July 2020 (Photo Credit: Rebekah D. Wallace, University of Georgia, Bugwood.org).
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