Herbicide Resistance in Annual Bluegrass (Poa Annua L.) and its Management

Abstract

Annual bluegrass (Poa annua L.) is the most problematic weed of sports turf, particularly in temperate climates. Repeated use of PRE and POST-emergent herbicides in turf has resulted in evolution of resistance in this weed species. This study investigated the extent of herbicide resistance to six different herbicide modes of action used to control P. annua in turf in Australia. The herbicide resistance status was determined in 31 populations (18 from Victoria, 6 from NSW and 7 from SA) collected from golf courses in south-eastern Australia in 2017. The majority of populations were resistant to POST application ALS inhibitors (97%), PSII inhibitors (94%), endothall (100%), ACCase inhibitors (94%), and inhibitors of microtubule assembly (81%), whereas, only 7% of populations were resistant to PRE-application of inhibitors of microtubule assembly (propyzamide) and 13% of populations were resistant to POST application of EPSPS inhibitor (glyphosate). An additional population (HT) collected from a home garden in Adelaide, South Australia was confirmed resistant to glyphosate only. Four multiple resistant populations were selected for further research (P18, P27, P262-16 and P413-17) to determine the level of resistance, the mechanism of resistance and inheritance of resistance. A dose-response study showed that these four populations had a high level of resistance to the ALS inhibitors rimsulfuron (>19 fold) and foramsulfuron (>56 fold), but medium to low level of resistance to endothall >7-fold, the ACCase inhibitor pinoxaden >4.3-fold, the PSII inhibitor simazine, propyzamide and glyphosate. Two different target-site mutations in the ALS gene were identified in these populations, with Pro197Ser substitution in P18 and a Trp574Leu substitution in the other populations (P27, P262-16 and P413-17). A target-site mutation (Ile1781Leu) was identified in the ACCase gene in all four resistant populations of P. annua. However, target site mutations were not identified in the psbA gene, EPSPS or α-tubulin gene. The population HT collected from a home garden in Adelaide, South Australia was 2-fold resistant to glyphosate compared to S due to increased EPSPS gene copy number (1.7-9.3 fold) compared to the susceptible (S). EPSPS gene expression was also 3.5 to 16.3-fold higher in HT compared to the susceptible. Increase in EPSPS gene copy number was not observed in any other glyphosate-resistant population. Other common mechanisms of glyphosate resistance including target site mutations, reduced herbicide absorption or translocation and increased activity of aldo-keto reductase were not detected in any resistant population. The mechanism of resistance remains unknown in four glyphosate-resistant populations other than HT. Cross-pollination between one resistant population and the susceptible (P262-16♂ ✕ S♀) was successful and an investigation of the progeny showed that the inheritance of glyphosate resistance appears to be controlled by a single, nuclear dominant gene. Cross-pollination was successfully undertaken between two propyzamide resistant populations and a susceptible population. Inheritance of propyzamide resistance in P18 was due to a single dominant gene, but at least two genes contributed to resistance in P413-17. The differences in inheritance pattern between the resistant populations suggest different mechanisms might be involved in propyzamide resistance. No target-site mutation or other resistance mechanisms were identified in the resistant populations. Thus, the resistance mechanism to propyzamide remains unknown. Field experiments conducted in spring and autumn between 2018 and 2020 on a multiple-herbicide (ALS inhibitors, ACCase inhibitors, PSII inhibitors and endothall) resistant P. annua population in bermudagrass (Cynodon dactylon) turf at a golf course in South Australia. Amicarbazone was identified as the most effective alternative herbicide. Amicarbazone reduced the occurrence of P. annua by 98-100% followed by terbuthylazine >80%, indazaflam >63%, and pyroxasulfone >57%. Availability of multiple herbicides with different modes of action will enable greenkeepers to rotate herbicides in their management program to minimize the risk of resistance. The findings reported here provide improved understanding of the herbicide resistance status of P. annua populations in Australia, their resistance mechanisms and has identified some herbicide options that could be used to control multiple resistant populations in turf.