Annual bluegrass control: Herbicide selection and resistance management

Annual bluegrass (Poa annua L.) is the most problematic winter weed in golf course turf. Plants have a light green color, coarse leaf texture and unsightly seedheads that reduce turf aesthetics (Figure 1). Annual bluegrass also has poor tolerances to heat, disease and other stresses that compromise turf quality.

Seeds germinate in late summer or early fall once soil temperatures drop below 70 degrees. Annual bluegrass matures in fall and overwinters in a vegetative state. Plants resume active growth in spring and can produce hundreds of viable seeds. Annual bluegrass is competitive with turfgrasses and may lead to significant stand thinning by early summer.

Plants may survive in regions with favorable growing conditions if they are irrigated and pests are adequately controlled. Superintendents can reduce populations through several cultural practices that limit annual bluegrass growth. However, cultural management often does not provide acceptable control, and herbicides are needed.

Pre-emergence: Alternatives to dinitroaniline

Pre-emergence herbicides applied in late summer or fall may prevent annual bluegrass establishment from seed. Dinitroaniline (DNA) herbicides are widely used for pre-emergence control of grassy weeds in turf. The DNA herbicides include pendimethalin (Pendulum, others), prodiamine (Barricade, others) and oryzalin (Surflan). These herbicides are absorbed from the soil by weed seedlings immediately after germination. Susceptible biotypes of annual bluegrass are controlled through mitosis inhibition in roots and shoots.

The DNA herbicides are relatively cheap, broad spectrum and safe for most established turfgrass species. However, exclusive use of DNA herbicides over the years has led to resistant biotypes on many golf courses. Dithiopyr (Dimension) is a pyridine herbicide that has a similar mode of action to the DNAs. Annual bluegrass with resistance to DNA herbicides is not generally controlled by dithiopyr, so selecting herbicides with an alternative mode of action may be critical for successful pre-emergence control.

Resistance develops from selection pressure by repeated use of the same herbicide or mode of action over years. Many biotypes of annual bluegrass may be present in a population. Genetic differences among biotypes contribute to levels of susceptibility to herbicides. The most common mechanisms for resistance are through an altered target site or enhanced metabolism. Other resistance mechanisms may include reduced absorption, herbicide sequestration or overproduction of the target-site enzyme. The resistant biotypes continue to reproduce and spread seed in these areas as susceptible biotypes are controlled by a particular herbicide over years. This type of selection pressure shifts an annual bluegrass population from susceptible to resistant biotypes over time.

Turfgrass managers have several options for delaying the development of resistance to DNA herbicides. Indaziflam (Specticle) is a cellulose biosynthesis inhibitor that provides pre-emergence and early post-emergence control of annual bluegrass in warm-season turfgrasses. Indaziflam is not labeled for cool-season turfgrasses and may cause significant injury to susceptible species. Indaziflam may also cause unacceptable injury to turfgrasses grown in stressful conditions such as shade or traffic stress. Therefore, only use indaziflam on established turfgrasses under optimal growing conditions. There also is increased risk of turfgrass injury from use on sandy soils with low organic matter. Adjusting the application rate or making split applications reduces potential turfgrass injury from treatments.

Oxadiazon (Ronstar, others) is a chlorophyll synthesis inhibitor that effectively controls annual bluegrass in fall. Oxadiazon doesn’t generally inhibit turfgrass rooting because of the herbicidal activity in shoots of susceptible plants. This is advantageous for golf courses with a history of disease (such as spring dead spot and winterkill), or if grasses need to be sodded or sprigged in treated areas. Oxadiazon controls DNA-resistant annual bluegrass, goosegrass and weeds with resistance to other modes of action.

Superintendents should use only the granular oxadiazon formulations on actively growing turfgrass. Use sprayable formulations on dormant warm-season grasses because applications during active growth cause significant turf injury.

Bensulide (Betasan, Bensumec, others) also is an alternative mode of action to the DNA herbicides. Make applications alone or in combination with oxadiazon to enhance the spectrum of weeds controlled. Pre-packaged combinations or tank mixtures of two pre-emergence herbicides with different modes of action enhance the potential to control biotypes of annual bluegrass with suspected resistance.

Bensulide, ethofumesate (Prograss) and oxadiazon offer alternative modes of action to DNA herbicides in most cool-season turfgrasses for pre-emergence control of annual bluegrass. Ethofumesate inhibits lipid biosynthesis in susceptible species. Sequential applications in fall provide pre- or early post-emergence control of annual bluegrass.

Superintendents also may use ethofumesate during establishment of perennial ryegrass and tall fescue to control seedling annual bluegrass. Most warm-season grasses such as bermudagrass are susceptible to ethofumesate injury, and discoloration is a risk if it’s used during fall overseeding. Mesotrione (Tenacity) is a carotenoid synthesis inhibitor that may control annual bluegrass during establishment of tall fescue or Kentucky bluegrass from seed. Applications at seeding provide a competitive growth advantage of tolerant turfgrasses over annual bluegrass during establishment.

Post-emergence: Alternatives to sulfonylureas and triazines

Herbicides may control annual bluegrass after emergence in many turfgrass species. Triazines, sulfonylureas and several other post-emergence herbicides that control annual bluegrass are used in warm-season turfgrasses only. Superintendents may use amicarbazone (Xonerate), ethofumesate (Prograss) and bispyribac-sodium (Velocity) in several cool-season turfgrasses for post-emergence control of annual bluegrass. In the Southern United States, late fall applications of these herbicides provide more consistent control than spring timings. This is because most post-emergence herbicides are more efficacious on annual bluegrass prior to seedhead development or when daytime temperatures are consistently 50 degrees F or higher.

Atrazine (Aatrex, others) and simazine (Princep, others) are used in warm-season grasses in fall for early post-emergence control of annual bluegrass. These herbicides inhibit photosynthesis at Photosystem II (PSII) in susceptible weeds, and provide about six weeks of residual control, depending on application rate. Acetolactate synthase (ALS)-inhibitors are widely used in golf course management for annual bluegrass control in tolerant turfgrasses. These herbicides include bispyribac-sodium (Velocity), flazasulfuron (Katana), foramsulfuron (Revolver), rimsulfuron (TranXit) and trifloxysufluron (Monument). Tribute Total is a combination product that contains multiple ALS-inhibitors, including foramsulfuron, that provide post-emergence control of
annual bluegrass in bermudagrass and zoysiagrass.

Resistance to ALS and PSII-inhibitors has increased exponentially in turfgrass. These herbicides have been overused, especially in warm-season turfgrasses, and this has shifted many annual bluegrass populations to resistant biotypes. Figure 2 shows the tolerance of a resistant biotype to an ALS-inhibitor, trifloxysulfuron, compared to a susceptible biotype at three weeks after treatment. In warm-season grasses, pronamide (Kerb) is used for annual bluegrass control as an alternative to these herbicides. Pronamide is a mitotic inhibitor that must be absorbed by roots. A concern with pronamide is the potential for lateral movement to susceptible (cool-season) turfgrasses. Avoid treatments on slopes or to saturated soils if cool-season grasses are adjacent to targeted areas. Efficacy of pronamide on annual bluegrass also is slow, and treatments may take six to eight weeks to obtain acceptable control.

Flumioxazin (Sureguard) is another alternative to ALS and PSII-inhibitors for annual bluegrass control. Flumioxazin is a chlorophyll synthesis (PPO) inhibitor — similar to oxadiazon — that is labeled for use only on dormant bermudagrass. Applications in late fall provide early post-emergence control of annual bluegrass plus residual pre-emergence control in winter. Turfgrass injury is a major limitation to flumioxazin use. The herbicide must be applied to dormant bermudagrass to avoid discoloration during green-up. Late fall applications of flumioxazin to bermudagrass have induced early winter dormancy. These flumioxazin effects may be beneficial for improving turfgrass aesthetics without inhibiting bermudagrass green-up in spring.

In cool-season turfgrasses, etho-fumesate (Prograss) may provide an alternative mode of action for controlling annual bluegrass with suspected resistance to DNA herbicides, ALS-inhibitors and PSII-inhibitors (triazines). Apply sequential ethofumesate applications in late fall at a three-week to four-week interval for best results. However, applications in spring often provide erratic levels of annual bluegrass control. Ethofumesate applications after overseeding perennial ryegrass in fall also may injure bermudagrass prior to dormancy.

Amicarbazone (Xonerate) provides post-emergence control of annual bluegrass in all major warm- and cool-season turfgrasses. Amicarbazone’s mode of action is the inhibition of PSII, similar to the triazines. Amicarbazone doesn’t control annual bluegrass biotypes resistant to atrazine or simazine, which is a major efficacy limitation. However, amicarbazone may be effective for annual bluegrass control in turfgrasses that don’t have a history of triazine herbicide use or if resistance is suspected from other modes of action.

Superintendents in the southern United States use amicarbazone on bermudagrass or zoysiagrass collars because of the limited injury potential from drift onto creeping bentgrass golf greens. Seashore paspalum also is tolerant to amicarbazone applications, and may provide an effective tool for post-emergence annual bluegrass control.

Selective annual bluegrass control options in cool-season turfgrasses are limited. Ethofumesate controls established annual bluegrass in several cool-season species, St. Augustinegrass and dormant bermudagrass. Low rates of ethofumesate applied sequentially are helpful in seashore paspalum for controlling seedling annual bluegrass in fall.

Course managers may treat dormant bermudagrass with non-selective herbicides such as glyphosate (Roundup, others), glufosinate (Finale) or diquat (Reward). These herbicides provide alternative modes of action to many other herbicides for annual bluegrass control. However, only spray at peak dormancy when no green turfgrass foliage is present. Glyphosate-resistant annual bluegrass has been confirmed on golf courses in several states in the southern United States. Tank-mixing glyphosate with simazine or another herbicide with a different mode of action enhances the potential for controlling populations with suspected resistance.

Resistance management programs

Annual bluegrass resistance to DNA herbicides, ALS-inhibitors, triazines and glyphosate are concerning in golf course turfgrass. The repeated use of these herbicides over years exacerbates the spread of resistant biotypes. Superintendents need to develop management programs that delay the establishment of herbicide-resistant annual bluegrass. As alternatives to DNA herbicides, consider using indaziflam (Specticle) or oxadiazon (Ronstar) for pre-emergence control of annual bluegrass in warm-season turfgrasses. Consider ethofumesate (Prograss) or oxadiazon in rotation with DNA herbicides for cool-season turfgrasses in fall. Alternative herbicides to the DNAs may have significant limitations for use such as cost, turfgrass injury and restrictions on labeled areas. Selecting the appropriate pre-emergence herbicide may be critical for controlling annual bluegrass and minimizing the spread of resistant biotypes.

Tank-mixing two herbicides with different modes of action will enhance the potential to control annual bluegrass with herbicide resistance. For example, a superintendent who needs to control annual bluegrass with suspected resistance to ALS-inhibitors in bermudagrass could apply a sulfonylurea (ALS-inhibitor) with simazine (PSII inhibitor) or glyphosate (EPSP synthase inhibitor). The additional mode of action in tank mixtures increases the potential of controlling resistant biotypes while controlling the susceptible population. Incorporating other modes of action in sequential programs delays the onset of resistance to multiple modes of action as well. Most labels have the herbicide Group Number on the front page to identify the mode of action. Table 1 lists the Group Number, mode of action and chemical family for herbicides discussed for annual bluegrass control. Superintendents should have an appreciation for the herbicide mode of action when selecting products for annual bluegrass control. However, costs, efficacy and turfgrass injury potential may be significant challenges or limitations to rotating modes of action in many turfgrass species.