Controlling a Prodiamine-Resistant Annual Bluegrass Biotype With Indaziflam.

Annual bluegrass (Poa annua L.) is a problematic grassy weed of golf course, athletic fields, and home lawn turf. Prodiamine, a mitotic inhibiting herbicide, is registered for preemergence (PRE) control of annual bluegrass in warm- and cool-season turf.  A population of annual bluegrass not controlled by prodiamine at 1120 g ha^-1 was identified on a golf course in Alcoa, TN in 2012. A whole-plant hydroponics bioassay was used to screen this biotype (PR) for prodiamine resistance compared to a known susceptible population (SS).  Multi-tiller (i.e., > 4 tillers) PR and SS annual bluegrass plants were established in hydroponic culture and exposed to prodiamine at 0, 0.001, 0.01, 0.10, 1.0, and 10.0 mM. Exposure to prodiamine at 0.001 mM reduced the root length of the SS biotype to 26% of the non-treated check (i.e., 0.000 mM prodiamine) but had no effect on the PR biotype.  When exposed to 10 mM prodiamine, root length of the PR biotype was reduced to 24% of the non-treated check compared to 9% for the SS biotype. I₅₀ values for the PR and SS biotypes were 0.04 and 2.8 x 10^-6 mM, respectively.  In field trials, prodiamine at 560, 840, 1120, and 1400 g ha^-1 only controlled the PR biotype 0 to 22%. PRE applications of the cellulose biosynthesis inhibitor indaziflam at 35, 52.5, and 70 g ha^-1 controlled this PR biotype 70 to 97%. This marks the second instance of annual bluegrass developing resistance to prodiamine in Tennessee during the past five years. Future research should evaluate indaziflam efficacy for control of other prodiamine-resistant biotypes of annual bluegrass as well as annual bluegrass biotypes resistant to herbicidal inhibitors of 5-enolpyruvylshikimic acid-3-phosphate synthase, acetolactate synthase, and photosystem II.