Confirmation and Control of Annual Bluegrass Resistant to Photosystem II Inhibiting Herbicides

An annual bluegrass (Poa annua L.) phenotype suspected to be resistant to photosystem II (PSII)-inhibiting herbicides was identified in a zoysiagrass [Zoysia matrella (L.) Merr., cv. ‘Royal’] sod production field during 2013. Research was conducted during 2013 to 2015 to confirm if this phenotype (R) was resistant to PSII-inhibiting herbicides and evaluate alternative herbicide options for effective preemergence control. In the glasshouse, treatment with the PSII-inhibiting herbicide simazine at the maximum labeled rate for use in sod production (2240 g ha⁻¹) only controlled this R phenotype 33%, and increasing the application rate to 9000 g ha⁻¹ did not improve control. Comparatively, simazine controlled a known susceptible phenotype (S) 100% at both rates. DNA sequencing identified a genetic mutation in R annual bluegrass that resulted in a serine-to-glycine substitution at amino acid residue 264 on the D1 protein targeted by simazine. In the field, applications of simazine at rates of 140 to 9000 g ha⁻¹ controlled R annual bluegrass £3% by 212 d after treatment. Applications of indaziflam (35, 52.5, and 70 g ha⁻¹), oxadiazon (3350 g ha⁻¹), prodiamine + sulfentrazone (844 + 416 g ha⁻¹), and pendimethalin + dimethenamid-P (2240 +1686 g ha⁻¹) controlled this R phenotype 98 to 100% and reduced plant counts similarly. Our findings confirm the presence of annual bluegrass in a zoysiagrass sod production field that is resistant to PSII-inhibiting herbicides via target site mutation of the D1 protein. However, rotation to preemergence herbicides with alternative mechanisms of action to simazine can effectively control this phenotype. This marks the first reported case of herbicide-resistant annual bluegrass in a sod production system. Management of this phenotype is critically important considering that harvesting and shipment of sod could potentially disseminate plants with target site mutations, conferring resistance to PSII-inhibiting herbicides over a wide geographical area.