Synthetic Auxin Resistant Weeds

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Synthetic Auxin biochemical mechanism of resistance in Mustard. Synthetic auxin resistance in two these cases appears to differ from other other species, tall waterhemp and common Resistant Weeds auxin resistance. Sixteen of the 27 species lambsquarters, are not widespread yet, but Despite synthetic auxin herbicides being have documented resistance to have the potential to become serious used longer and on a greater area than any 2,4-D, seven to MCPA, and six to dicamba. problems in the United States if they are other herbicide mechanism of action the In the United States six weed species have not managed properly. evolved resistance to synthetic auxin area infested with synthetic auxin resistant This fact sheet is an introduction to a series herbicides, with only one, Kochia scoparia, weeds is low in comparison to many other of fact sheets on auxin resistant weeds, being widespread and a serious economic herbicide mechanisms of action. Twenty covering kochia, wild radish, corn poppy, problem. Globally the most important seven weeds have evolved resistance to wild mustard, tall waterhemp, and common synthetic auxin resistant weeds are Kochia, synthetic auxins. This excludes grasses lambsquarters. resistant to quinclorac since the Wild Radish, Corn poppy, and Wild

Table 1. The occurrence of synthetic auxin resistant weeds worldwide.

Species First Year Herbicides Country Amaranthus tuberculatus 2009 2,4-D United States Carduus nutans 1981 2,4-D New Zealand Carduus pycnocephalus 1997 2,4-D, MCPA, MCPB New Zealand Centaurea cyanus 2012 dicamba Poland Centaurea solstitialis 1988 picloram United States Chenopodium album 2005 dicamba New Zealand Cirsium arvense 1985 2,4-D, MCPA Hungary, Sweden Commelina diffusa 1957 2,4-D United States Daucus carota 1957 2,4-D Canada, United States Descurainia sophia 2011 MCPA China Fimbristylis miliacea 1989 2,4-D Malaysia Galeopsis tetrahit 1988 dicamba, fluroxypyr, MCPA Canada Galium aparine 2014 fluroxypyr China Galium spurium 1996 quinclorac Canada Kochia scoparia 1994 dicamba, fluroxypyr United States Lactuca serriola 2007 2,4-D, dicamba, MCPA United States Limnocharis flava 1995 2,4-D Indonesia, Malaysia Limnophila erecta 2002 2,4-D Malaysia Papaver rhoeas 1993 2,4-D Italy, Spain Ranunculus acris 1988 MCPA New Zealand Raphanus raphanistrum 1999 2,4-D Australia Sinapis arvensis 1990 2,4-D, dicamba, dichlorprop, MCPA, mecoprop, picloram Canada, Turkey Sisymbrium orientale 2005 2,4-D Australia Soliva sessilis 1999 clopyralid, picloram, triclopyr New Zealand Sonchus oleraceus 2015 2,4-D Australia Sphenoclea zeylanica 1983 2,4-D Malaysia, Philippines, Thailand Stellaria media 1985 fluroxypyr, MCPA, mecoprop China, United Kingdom

Additional information on herbicide resistance can be found at weedscience.org Synthetic Auxin Resistant Weeds

Rarity of Synthetic Auxin Transgenic Crops and Synthetic Auxin Herbicides

Resistant Weeds Synthetic Auxin Synthetic auxins were the first highly effective, and selective organic herbicides. Gressel and Segel (1982) suggested that Resistant Weeds They have been used for over 60 years the low incidence of synthetic auxin Synthetic auxin resistant crops (corn, and are still being used extensively resistant weeds may be due to the auxinic cotton, and soybeans) have been herbicides having multiple sites of action, worldwide for control of broadleaf weeds in developed to provide an additional many crops and non-agricultural areas. requiring multiple mutations within an herbicide mechanism of action to help Synthetic auxins are structurally similar to individual to confer resistance. This seems manage and mitigate the evolution of the natural plant hormone IAA and mimic a reasonable assumption, however in most herbicide resistant weeds. The synthetic their effects to kill weeds. cases (Sinapis arvensis, Kochia scoparia, auxin herbicides are inherently low risk Centaurea solstitialis, and Galium spurium) herbicides for the selection of herbicide- research have shown resistance to be due Best Management Practices resistant weeds. However low risk does not to a single mutation (single gene) with only mean “No Risk” and as we have seen with Diversity in weed control practices is key to one case where resistance was confirmed glyphosate (another low risk herbicide) if delay and manage herbicide resistance in by two additive genes (Galeopsis tetrahit). synthetic auxins are used over large areas weeds. This involves: Alternative theories for the paucity of without sufficient rotation with other synthetic auxin resistant weeds are that 1. Rotation or mixtures of herbicide herbicide mechanisms of action and use of resistant mutations are extremely rare, or mechanisms of action. non-herbicidal weed control strategies then that mutations conferring resistance are they are likely select for resistance 2. Using at least two herbicides a year lethal. relatively rapidly. If used wisely they will from different herbicide mechanisms provide a way to extend the life of existing of action that are still effective on the herbicides used in these crops. particular population of the target weed. This may include use of pre- emergence herbicides.

3. Using cultural/mechanical weed control methods including shallow tillage in the spring, crop rotation, and cleaning equipment.

4. Using full herbicide rates applied at the correct weed size and to carefully monitor results.

5. Scouting fields after herbicide application and controlling escapes.

See Norsworthy et. Al. Reducing the Risks of Herbicide Resistance: Best Management Practices and Recommendations, Weed Science, 2012, 60, sp1, 31 for detailed information on resistance management.

REFERENCES

Mithila J., Hall J.C., Johnson W.G., Kelley K.B., and Riechers D.E. 2011. Evolution of Resistance to Auxinic Herbicides: Historical Perspectives, Mechanisms of Resistance, and Implica- tions for Broadleaf Weed Management in Agronomic Crops. Weed Science 59:445