A survey of ethofumesate resistant annual bluegrass (Poa annua) on US golf courses

Annual bluegrass (Poa annua L.) is the most troublesome weed on golf courses in the US. Many agronomic practices intended to promote high-quality playing surfaces favor the growth and development of annual bluegrass, resulting in high weed pressure. One commonly used herbicide for annual bluegrass control on golf courses is ethofumesate, which is a very long chain fatty acid inhibitor. Annual bluegrass resistance to this herbicide is documented and confirmed in grass seed production systems, but potential resistance on golf courses was previously unknown. The objective of this study was to determine the frequency and magnitude of potential ethofumesate resistance from a sample of US golf courses. A dose-response experiment was initiated at Purdue University using 30 annual bluegrass populations collected from Alabama, California, Indiana, and Oregon golf courses. Ten ethofumesate doses included 0, 0.5, 1.0, 2.5, 5.0, 7.5, 10.0, 15.0, 20.0, and 40.0 lb a.i. acre⁻¹, with 1.0 to 2.0 a.i. acre⁻¹ as the standard label application rate for perennial ryegrass turf. A low level of resistance (R/S < 3) was found in several populations collected in each state. The mean effective dose necessary to kill 50% of the populations (ED₅₀) was 5.1, 9.2, 3.5, and 3.4 lb a.i. acre⁻¹ for populations from Alabama, California, Indiana, and Oregon, respectively. The most resistant population originated from California, with an ED₅₀ of 13.2 lb a.i. acre⁻¹. To reduce selection pressure from ethofumesate populations, golf course superintendents are encouraged to develop site-specific weed control programs that rotate herbicide sites of action, as well as utilize diverse control tactics.