Annual bluegrass biotypes resistant to the dinitroaniline herbicides have been identified in Tennessee, North Carolina, and Georgia. The dinitroaniline herbicide class incudes prodiamine (Barricade, others), pendimethalin (Pendulum, others) , oryzalin (Surflan, others), and trifluralin (Treflan, others), key turfgrass and ornamental herbicides. Annual bluegrass resistant to glyphosate (Roundup, many others) has been identified in Tennessee and Missouri.

Biotypes of Poa annua resistant to the sulfonylurea herb• icides have been documented in Tennessee, Alabama, and Virginia. The sulfonylurea herbicide class includes foramsulfuron (Revolver), trifloxysulfuron (Monument}, flazasulfuron (Katana), rimsulfuron, metsulfuron (MSM, Manor, others), and rimsulfuron + metsulfuron (Negate),

chemicals used to selectively control cool-season grasses in bermudagrass and certain other warm-season turfgrasses.

We have verified a biotype of annual bluegrass from a golf course that is resistant to the commonly used sulfonylurea herbicides in bermudagrass. The golf course had been using Monument yearly to control annual bluegrass but after years of use had noted reduced control. In this case, one would have to look at preemergence applications of chemicals like Barricade, Specticle or simazine, or use postemergence treatments of glyphosate or Kerb (pronamide).

Recently, annual bluegrass resistant to early postemergence applications of Specticle (indaziflam) was detected at several locations in the southern U.S. Of extra concern in that research was that one of those annual bluegrass biotypes that tolerated early postemergence applications of Specticle also tolerated postemergence applications of flumioxazin, forarnsulfuron, glyphosate, metribuzin, pronamide, and simazine. So that biotype could tolerate six different herbicide modes of action. How can one control such a biotype?

Recently, annual bluegrass biotypes have been identified in Georgia that exhibit differential susceptibility to protoporphyrinogen oxidase (PPO) inhibitors, which includes Ronstar (oxadiazon) and flumioxazin (SureGuard, others). Whether it is due to genetic variability in this species or some other reason, clearly annual bluegrass is prone to developing herbicide resistance.

Other weeds besides annual bluegrass have developed resistance to certain herbicides.

Goosegrass has developed resistance to Ronstar in Virginia and to the dinitroaniline herbicides in Tennessee. Lawn burweed in New Zealand and buckhorn plantain in Indiana has developed resistance to the synthetic auxins, so the problem is not limited to grassy weeds. Compressed sedge in Alabama has developed resistance to key sedge herbicides, including halosulfuron (Sedgehammer, Prosedge). Large crabgrass and smooth crabgrass have developed resistance to the postemergence grass herbicides, which includes Acclaim Extra (fenoxaprop),as well as sethoxydim (Segment} and fluazifop (Fusilade, Ornamec).

A concern I have is if crabgrass species in Virginia develop resistance to quinclorac (sold by itself under the name Drive, and as a component of combination products such as Q4 Plus, among others.) Quinclorac is the primary herbicide used for postemergence crabgrass control in turf situations and is one of the few postemergence options in bermudagrass. One of the studies we have been con• ducting at the research station is evaluating alternative herbicides to quinclorac for postemergence crabgrass control in bermudagrass. There are more alternatives to quinclorac for postemergence crabgrass control in cool season turf.