Confirming Resistance to Prodiamine and Glyphosate in a Single Annual Bluegrass (Poa annua L.) Biotype from Tennessee

Annual bluegrass (Poa annua L.; POAAN) is a cool-season weed that commonly infests warm-season turfgrasses during winter dormancy. In spring 2012, poor POAAN control (<50%) was reported on golf course roughs in Alcoa, TN (35.75 °N, -83.88 °W) following treatment with a tank mixture of prodiamine (1120 g ha^-1[hectare]) and glyphosate (840 g ae ha^-1) during bermudagrass dormancy. The objective of this research was to determine if this POAAN biotype was resistant to prodiamine and glyphosate.

Using mature plants from the field, 81 of the 100 selections were not controlled by glyphosate and prodiamine; 96 of the 100 selections were not controlled by glyphosate, while 84 were unaffected by prodiamine. Only a single plant sampled was susceptible to both herbicides.

Seed was harvested from surviving plants and used to confirm resistance in progeny. Percent control of the resistant and susceptible biotypes varied in response to increasing rates of prodiamine and glyphosate (P < 0.0001). Approximately 8.5 times more glyphosate (1940 g ha^-1) was required to reduce dry biomass of the resistant population by 50% compared to a known glyphosate susceptible population (228.9 g ha^-1). Moreover, glyphosate susceptible plants accumulated 50% more shikimic acid (898 mg kg^-1[kilogram]) 6 days after treatment than those resistant to glyphosate (394 mg kg- ¹).

Results of this research indicate that the POAAN biotype at Lambert Acres Golf Course has evolved resistance to both prodiamine and glyphosate. This marks the second occurrence of POAAN evolving multiple-resistance in managed turf and the first instance of a POAAN population evolving resistance to both PRE and POST herbicides.