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Turfgrass Spring Blog #2: 
2022 Edition

Herbicide Resistance of Annual Bluegrass in Georgia Lawns

Drs. Patrick McCullough and Clint Waltz, University of Georgia
March 2022
Annual bluegrass (Poa annua) is the most problematic winter weed of lawns in Georgia. It germinates in fall, overwinters in a vegetative state, and resumes active growth in spring. Annual bluegrass typically dies out by May in Georgia, but cool temperatures in spring and regular irrigation may extend survival of populations into early summer. 

Annual bluegrass can be identified in lawns from the bunch-type growth habit that grows into to larger clumps as the plant matures. The seedhead is a key characteristic for annual bluegrass identification in spring. Plants produce a light-colored branched panicle that produces hundreds of viable seed.
Herbicide Resistance
Herbicide resistance develops in annual bluegrass populations when the same herbicide or mode of action has been sprayed repeatedly over years. Plants that survive these applications are able to spread and eventually become the predominant biotype of the population.  This is made worse when a high seed producing species, like annual bluegrass, is the target weed.  This type of selection pressure has shifted annual bluegrass populations to resistant biotypes, particularly in warm-season grasses throughout Georgia.

Resistance could eventually preclude the exclusive use of a single class of herbicides and require a tank-mix partner to achieve control. Segregation of annual bluegrass populations after applications of these herbicides should be monitored to determine if resistant biotypes could be present in a lawn. If resistance is a concern, tank-mixing two herbicides with different modes of action will enhance the potential to control annual bluegrass with herbicide resistance. An example of postemergence control would be applying a sulfonylurea with simazine or glyphosate in dormant bermudagrass. The additional mode of action in tank-mixtures increases the potential to control the resistant biotypes while controlling the susceptible population. 

Cross-resistance can also occur. This is when a weed becomes resistant to multiple herbicide modes of action.  For example, if a population of annual bluegrass is not controlled with a tank-mix combination of simazine and glyphosate, two different modes of action.  
Dinitroaniline (DNA) herbicides are widely used for preemergence control of annual bluegrass in turf. The DNA herbicides include pendimethalin (Pendulum, others), prodiamine (Barricade, others), and oryzalin (Surflan). These herbicides are relatively inexpensive, broad spectrum, and safe to use on established turfgrass species. However, the exclusive use of DNA herbicides over the years has led to the spread of resistant biotypes in Georgia. Dithiopyr (Dimension) is a pyridine herbicide that inhibits mitosis at a different site of action than the DNAs. Annual bluegrass with resistance to DNA herbicides has also shown resistance to Dimension. Root development of resistant plants are unaffected by these herbicides and can establish in treated areas.

Turf managers have several alternatives to the DNA herbicides for preemergence control of annual bluegrass. Ronstar (oxadiazon) and Bensumec (bensulide) offer different modes of action than the DNAs for preemergence control of annual bluegrass. However, Ronstar is labeled for nonresidential turf and Bensumec can only be used in residential lawns. 

More specific to warm-season lawns, Specticle (indaziflam) is another alternative to DNA herbicides. It has a different mode of action than the DNA herbicides for pre- and early postemergence control of annual bluegrass. Specticle should be applied in September in North Georgia or October in South Georgia and only used on established turfgrasses under optimal growing conditions. Specticle has some risk for turf injury on sandy soils with low organic matter and turfgrass that is stressed. Adjusting the rate or making split applications can reduce potential turfgrass injury from these treatments.

Ethofumesate (Prograss, PoaConstrictor, others) and mesotrione (Tenacity) are other preemergence options for controlling annual bluegrass in residential and non-residential turf. Two fall applications can control annual bluegrass seedlings and provide residual control during periods of peak germination. Both ethofumesate and mesotrione are poor postemergence herbicides for spring applications of mature annual bluegrass.

Pronamide (Kerb) may be used for pre- and postemergence control of annual bluegrass in non-residential warm-season lawns. Kerb must be absorbed by roots. It is critical that applications received rainfall or irrigation within 24 hours to maximize efficacy. Like the DNA herbicides, Kerb is a mitotic inhibitor, but it has a different site of action. UGA research has found that Kerb provides effective preemergence control of annual bluegrass with resistance to DNA-herbicides and dithiopyr. A concern with pronamide is the potential for lateral movement to susceptible (cool-season) turfgrasses. Applicators should avoid treatments on slopes or to saturated soils if cool-season grasses are adjacent to targeted areas. 

Because of widespread use for postemergence control, annual bluegrass resistance to the sulfonylurea and triazine class of herbicides is concerning for Georgia lawns. Flazasulfuron (Katana), foramsulfuron (Revolver), trifloxysufluron (Monument), and the combination product Tribute Total (thiencarbazone + foramsulfuron + halosulfuron) are examples of sulfonylureas. The triazine herbicides, atrazine (Aatrex, others) and simazine (Princep, others), have been extensively used in centipedegrass and St. Augustinegrass lawns for decades. 

Amicarbazone (Xonerate) provides selective postemergence control of annual bluegrass. Applications must be made in early spring to maximize selectivity in established tall fescue. Xonerate should be applied once temperatures have reached approximately 60 to 65 degrees (F) during the daytime in early March. Sequential applications at 14- to 21-day intervals have shown effective control of established annual bluegrass in Georgia. 

As with all pesticides, read and follow label directions for proper use and to minimize the potential for resistance. Incorporating other modes of action in sequential programs can delay the onset of resistance and cross-resistance. Most labels have the herbicide Group Number on the front page to identify the mode of action. Turf managers should understand herbicide classifications when selecting products for annual bluegrass control. However, costs, efficacy, and turfgrass injury potential may be significant limitations to rotating modes of action in many turfgrass species.