Biology and Herbicide Resistance Dynamics of Annual Bluegrass (Poa annua L.) in Turfgrass Systems

Abstract

Annual bluegrass (Poa annua L.) is one of the most common and pervasive weeds in turfgrass systems across the United States. It causes severe economic and aesthetic damage in managed turfgrass systems and is the most herbicide-resistant weed in the world. Multiple studies were conducted between the Fall of 2020 and the Spring of 2023, including a preemergence herbicide dose-response assay, a seedbank persistence study, and a common garden study, to elucidate the many phenotypic and adaptive traits of annual bluegrass. Two annual bluegrass populations were confirmed to exhibit resistance to 32X prodiamine (1X=735.6 g ai ha⁻¹) and 16X pronamide (1X=1155.9 g ai ha⁻¹) and observed to survive a 1X field rate of indaziflam (1X=54.5 g ai ha⁻¹). The populations evaluated in the seedbank persistence experiment were buried at two depths [surface (0 cm) and 5 cm] in seven locations. Generally, seed on the soil surface did not persist as long as the seed buried at the 5-cm depth. Annual bluegrass exhibited a short-lived seedbank, with a ≥95% reduction in seed viability within two years of burial in the majority of the populations. To evaluate 15 morphological and reproductive traits, seven common gardens were established across a geographical gradient (USDA Plant Hardiness Zones 5b–8b) in the fall of 2020 and harvested in the spring of 2021. The annual bluegrass populations exhibited a wide range of inter- and intra-population diversity for the evaluated traits. In a principal component analysis involving these 15 variables, the first two components explained 63% of the total variation. A cluster analysis grouped these populations according to their origin: warm-season, cool-season, or transition zone. There were strong positive correlations between some traits; specifically, flag leaf length was positively (P=≤0.05) associated with seed count plant⁻¹, and plant height was positively (P=≤0.01) associated with seed shattering. These findings indicate that annual bluegrass is an extremely diverse plant that can adapt to nearly any environment. Implementation of diverse management tactics that involve chemical and non-chemical tools, especially in a site-specific manner, is expected to thwart adaptation and offer a sustainable approach to the long-term management of this species.