Feature

Carotenoid Inhibiting Herbicides Increasing in Use Despite Injury Concerns

By Scott McElroy, Professor, Auburn University

Gracie Cotter, Graduate Research Assistant, Auburn University

Debra Smitherman, Graduate Research Assistant, Auburn University

Carotenoid-inhibiting herbicides are valuable in turfgrass management. There are two carotenoid inhibitors available – Tenacity (mesotrione) and Pylex (topramezone). Tenacity has both pre and postemergence control of crabgrasses and some broadleaf weeds. When used in multiple application strategies they can control goosegrass and Poa annua as well. Pylex has become well known for control of goosegrass and also has activity on crabgrasses and miscellaneous broadleaf weeds.

On the face of it, these two herbicides seem ideal for use in turfgrass management; however, as anyone who has used them knows, it can be challenging to balance out the injury and the desired level of control. Both herbicides can cause injury to some, if not all, desirable turfgrasses they are labeled on. The injury presents as an intense white color, or as it is commonly called, “bleaching”. Even though tolerant turfgrasses may present with white or bleached symptoms, these symptoms can quickly abate, and turfgrass can actually be more intensely green post-recovery.

I (McElroy) began working on mesotrione for turfgrass uses in 2004 when I started as an assistant professor and extension specialist at the University of Tennessee. Jay McCurdy came on as a graduate student soon thereafter and worked on mesotrione use as well. Our research identified safety on cool-season grasses like tall fescue, Kentucky bluegrass, heat-tolerant bluegrasses, fine fescue, and centipedegrass. Mesotrione could even be used on seeded establishment of these grasses. Other warm-season grasses like bermudagrass, St. Augustinegrass, and zoysiagrass were more problematic. Mesotrione would not kill these desirable turfgrasses, but the bleaching was largely deemed unacceptable, limiting its broader use. And in case you did not know, mesotrione will control creeping bentgrass.

Topramezone testing began in the mid-2010s with similar trial work being conducted to determine turfgrass tolerance and weeds controlled. Early on, researchers could not decide how topramezone should be used. Pylex at higher rates (> 1.5 fl oz/a) combined with Turflon Ester (triclopyr) could control bermudagrass. At lower rates (< 0.5 fl oz/a) it could control goosegrass in bermudagrass. Such utility has its pros and cons. It is definitely a pro that a herbicide can be used for multiple things, but slight miscalculations in the rate might cause problems.

My lab has continued research on Pylex use in summer 2025. Pylex plus Drive has controlled smooth crabgrass to a nearperfect level; however, injury was “unacceptable” approximately 7 to 14 days after treatment (Figure 1). See the final comment below on what is “unacceptable.” Pylex alone also controlled goosegrass nearly 100% when applied to 3-4 tiller goosegrass. Injury at this location was more intense, which we attribute to dry conditions, that can cause increased bleaching. Bermudagrass recovered completely by 17 days at both locations.

Figure 1. Smooth crabgrass control with Pylex and Drive 28 days after treatment.

Figure 2. Goosegrass control with Pylex as observed 35 days after treatment. Near 100% bleaching symptoms occurred on the bermudagrass 5 to 15 days after treatment but complete recovery occurred at 17 days after treatment.

Strategic Use is the Key

Regardless of these problems, interest in the topramezonebased product Pylex has increased, especially for goosegrass control. Pylex is highly active on goosegrass with control achieved at rates of 0.25 to 0.5 fl oz/a. Creeping bentgrass seems generally more tolerant than bermudagrass but that has not stopped use on bermudagrass.

Regardless of the bleaching injury with Tenacity and Pylex, the use of these herbicides is likely going to have to increase. There is simply too much herbicide resistance developing to not use these herbicides. However, I don’t want to lose this technology like we have lost other herbicide technologies. We must use these judiciously. First, tank-mix if possible. For goosegrass control in bermudagrass mixtures of Revolver, Celsius, or other herbicides with goosegrass activity may be an option. These herbicides are showing some safening effect as well, but safening can mean antagonism on weeds so follow up applications can be beneficial. Second, make sure you are using a preemergence herbicide to reduce the number of treated plants. Anderson’s goosgrass/crabgrass, other herbicides with oxadiazon, the granular flumioxazin Stayguard, standard pre herbicides like Barricade, Dimension, and Pendulum applied with Tower or Pennant Magnum are all options depending on the turfgrass scenario. Follow up with a carotenoid inhibitors or other postemergence herbicides to control any escapes.

Finally, we use the word “unacceptable” when it comes to injury. These herbicides are going to cause a bleaching-type effect to some desirable turfgrasses. However, we deem these as acceptable or unacceptable based on what we like or don’t like to see. Tolerant turfgrasses do recover, so to them the injury is acceptable; they simply are white, which some people do not like.