4 minute read
Feature / In The Weeds
Herbicide Resistant Sedge and The Future of University Turfgrass Programs
By J. Scott McElroy, PhD Professor, Auburn University
Herbicide Resistant Sedges. I hate to be the bearer of bad news, but the next big herbicide resistance problem is going to be sedges – and not just one. Probably all of them.
The two major weeds that have caused the greatest problem relative to herbicide resistance to date are annual bluegrass and goosegrass. In 2010 Harold Walker and I were the first to identify sulfonylurea herbicide resistant populations of Poa annua. Before that, resistance to triazines had been identified throughout the southeast, initially in Mississippi by Dr. Eual Coats. Resistance to preemergence herbicides to prodiamine, pendimethalin, and dithiopyr were identified in North Carolina and by me, in Tennessee. Since all of this, there are populations that have evolved resistance to glyphosate (Roundup) and likely to indaziflam (Specticle).
Not to toot my own horn, but my group at Auburn are the principal leaders when it comes to herbicide resistant goosegrass research. We were the first to report oxadiazon (Ronstar), foramsulfuron (Revolver) and sulfentrazone (Dismiss) resistance. Resistance to dinitroaniline preemergence herbicides and metribuzin (Sencor) were previously reported. Because of this resistance and due to the loss of diclofop (Illoxan), there are some turfgrass situations in which there is basically no herbicide available.
Sedges have four main herbicide groups that are effective for control:
• ALS inhibitors,
• sulfentrazone (Dismiss),
• traditional preemergence herbicides indaziflam (Specticle)
• oxadiazon (Ronstar);
and secondary preemergence herbicides dimethenamid (Tower) and S-metolachlor (Pennant Magnum).
ALS inhibitors include sulfonylureas such as trifloxysulfuron (Monument), flazasulfuron (Katana), halosulfuron (Sedgehammer), imazosulfuron (Celero), and others; and also, imazaquin (Sceptre, formerly known as Image.)
I should include MSMA, but its use is so limited that it is not seen as a viable option for all turfgrass situations. Certainly, if you have access and it is labeled for your situation, using it is not a bad idea. Basagran (bentazon) is also available, and although control is limited, it still is an option.
Patrick McCullough, formerly an extension specialist at the University of Georgia, was the first to see the developing problem of herbicide resistant sedges. McCullough identified annual sedge and green kyllinga resistance to ALS inhibiting herbicides. Working with weed scientists who work in rowcrops, we have also identified ALS-inhibitor resistant yellow nutsedge, small-flower umbrella sedge, and rice flat sedge in Georgia and Arkansas. (Research papers we have collaborated on are listed at the end).
ALS inhibitors are a key set of herbicides for postemergence sedge control. The only other post herbicide in turfgrass is Dismiss and preemergence herbicides are still available. Here is the problem: First, from a postemergence standpoint, this puts all the pressure on Dismiss. From what we have seen with Poa and goosegrass, when the control selection shifts to only one active ingredient, it is not long until that herbicide is facing resistance as well. Preemergence herbicides are problematic because many of the sedges are perennial and pre herbicides are not timed properly to coincide with sedge emergence. That is why sedges are typically targeted for post applications rather than pre.
What is the plan moving forward? First, if you are dealing with an annual sedge or kyllinga problem, make sure you are timing those preemergence applications properly. This means some follow-up pre-application in June or July. Virtually every other weed has its window of germination in the spring months, but some sedges – namely cock’s comb kyllinga and annual sedge – germinate later in the June time frame. Second, if you don’t have resistance, it is probably a good idea to start mixing or rotating different modes of action. Since there are basically only two post herbicides, it requires mixing sulfentrazone with ALS inhibitors, or rotating them from year to year. If you have had success with Basagran (bentazon), then consider integrating it as well.
