Herbicide resistance is a constantly growing threat to agriculture in the parts of the world that heavily rely on chemical weed control. Traditionally, weed managers use a reactive approach to failures in weed control, including herbicide resistance, where first a problem occurs and then the issue is remediated (Robert et al., 2016). However, it may be possible to take a more proactive approach to weed management, where weed control issues are predicted using epidemiological approaches and/or rapidly identified using modern diagnostics to adjust agricultural practices and reduce the impact of weeds on production. To this end, laboratory experiments and field validations were conducted to 1) optimize rapid molecular assays that identify resistant individuals of horseweed (Erigeron canadensis L.), Palmer amaranth (Amaranthus palmeri S. Watson), waterhemp (Amaranthus tuberculatus (Moq.) Sauer), and common ragweed (Ambrosia artemisiifolia L.), and 2) identify specific mechanisms conferring resistance to acetolactate synthase (ALS) and 5- enolpyrovylshikimate-3-phosphate synthase (EPSPS) inhibiting herbicides in these species. Nine different single nucleotide polymorphisms (SNPs) were identified in five ALS positions known to confer herbicide resistance among all species surveyed. One SNP in EPSPS gene in horseweed as well as increased EPSPS copy numbers in Palmer amaranth (20-160 copies) and waterhemp (2-7 copies) accessions were identified, all of which are known to confer resistance to glyphosate. The genotyping assays identified resistance mutations in >98% of cases if they were present and could be generated in approximately two days. We confirm that rapid molecular resistance diagnostics can be valuable tool in herbicide resistance diagnostics, providing growers with a new, more rapid tool for confirming herbicide resistance in Michigan weed populations. In another study, we set out to understand the relationship between management practices and herbicide resistance evolution in horseweed to glyphosate, dicamba and 2,4-D. Greenhouse experiments were conducted to 1) assess current resistance spectrum of horseweed in Michigan to those herbicides, and to 2) determine main factors that contribute to the presence of resistance in horseweed. Out of the 20 accessions screened, 60% were resistant to glyphosate, 35% to 2,4-D, and 20% to dicamba. Dose-response data were integrated with previous management history and environmental factors using odds-ratio analyses to rank which factors influenced the presence of resistance. Out of all significant pairwise comparisons, 44% were related to crop rotation frequency, 33% to previous herbicide-resistance status, and 22% to location. Results highlight that growers can proactively manage herbicide-resistance evolution of horseweed in Michigan by continuing to adopt integrated weed management techniques, especially crop rotations and herbicide rotations, to prevent the successive selection events that occur in low diversity management systems.
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