Survey of target site resistance alleles conferring resistance in Poa annua

Poa annua L. (annual bluegrass) is a common weed in turfgrass and has been reported resistant to 12 different herbicide sites of action, with various combinations of multiple-herbicide resistance having been identified. To quantify the extent of herbicide-resistant P. annua, the ResistPoa Project (resistpoa.org) surveyed 1349 P. annua populations for resistance to nine sites of action and one plant growth retardant. Herein, we report results from sequencing of known target site mutations found in 5-enolpyruvylshikimate-3 phosphate synthase (EPSPS), acetolactate synthase (ALS), photosystem II protein D (psbA), and ???-tubulin genes. Populations were sequenced using either capillary or amplicon sequencing (AmpSeq), depending on the complexity of the gene, and were analyzed for target-site resistance. After additional resistance screening, a total of 389 suspected resistant populations were sequenced-131 for ALS, 83 for EPSPS, 93 for psbA, and 82 for ???-tubulin. From the resistant populations, 64 displayed resistance to multiple sites of action. After sequencing, it was determined that target-site resistance was the common form of resistance for all sites of action outside of psbA with 65.6% of ALS populations, 73.5% of EPSPS, 39.8% of psbA, and 91.5% of ???-tubulin having presented a target-site mutation.