Introduction
Growth regulators for turfgrasses have been in use since the mid-1900’s and have become widely used on golf course putting greens, fairways, and tees. As of 2024 only two major hormonal pathways are being regulated by products labeled for turf, gibberellins (Primo, Trimmit, Anuew, etc.) and ethylene (Proxy). However, auxins are group of hormones that are involved in numerous stress responses and their regulation could improve turfrgass tolerance to a variety of stresses. Synthetic auxins are applied as herbicides (2,4-D, dicamba, etc.) and inhibitors of auxin transport can also regulate growth and stress responses. Chlorflurenol, sold as Maintain CF 125, is an auxin transport inhibitor and one of the oldest commercially available growth regulators.
This product was used to regulate growth of roadside turf in the 1960’s and 70’s, but was used considerably less as gibberellin inhibitors were released into the market. In 2023, researchers from Rutgers University showed that chlorflurenol increased creeping bentgrass tolerance to heat, and becuase auxins are highly involved in tillering, we were interested to see if chlorflurenol increased tolerance to shade. Still, we had no clue how greens or fairway height creeping bentgrass would tolerate chlorflurenol and if it would interact with other standard growth regulators like trinexapac-ethyl. In 2024, we established field experiments in Wooster and in Columbus to see if chlorflurenol could be tolerated by finely cut creeping bentgrass. An experiment was also established on naturally shaded tall fescue to see if chlorflurenol improved shade tolerance.
Materials and Methods
The experiment in Columbus was carried out on a creeping bentgrass putting green (007 XL) and a creeping bentgrass fairway, both maintained with best management practices. Applications of chlorflurenol (Maintain CF 125) and trinexapac-ethyl (T-Nex) were made on 6/28/24, 7/11/24, 7/26/24, and 8/11/24. Chlorflurenol was applied at 10 or 100 micromolar (Figure 1). These two different concentrations are 24x and 240x reduced concentrations relative to the label rate (0.50 gallon per acre). Trinexapac-ethyl was applied at 0.125 fl. oz. / M on the putting green and at 0.250 fl. oz. / M on the fairway. On July 26th photoselective filters were placed on fairway plots to simulate shade from trees to examine if chlorflurenol improves turfrgass growth under shade.
Figure 1: Treatments and rates used
Current results
All experiments we are conducting using chlorflurenol currently indicate that this auxin transport inhibitor is safe to use at low concentrations on low-cut creeping bentgrass and on high-cut tall fescue.
On the other hand, the label rate of Maintain CF 125 and treatments at 1,000 micomolar have shown damage to high-cut tall fescue. The turf recovered, but these higher rate treatments led to severe wilting (though water was not limiting). Thought once recovered, plots treated with these rates performed very well.
The 10 and 100 micromolar rates result in lighter creeping bentgrass color on greens and fairways. Current results also indicate that Poa annua is sensitive to chlorflurenol mixed with trinexapac-ethyl, though long-term control of Poa annua is still unknown. Phytotoxicity is especially noticeable on fairway plots with higher trinexapac-ethyl rates with 10 micromolar chlorflurenol.
Figure 2: Turfrgass quality and NDVI results from Columbus plots.
Figure 3: Light box images of putting green plots taken on 7/26/24.
Figure 4:Light box images of fairway green plots taken on 7/26/24.
While not significant, 100 micromolar chlorflurenol with trinexapac-ethyl (0.125 or 0.250 rates) improves turfrgass quality compared to trinexapac-ethyl alone in both Wooster and Columbus plots (Figure 3 and 4). This combination also seems to suppress growth more and longer compared to trinexapac-ethyl alone, especially on fairways with the higher rate of trinexapac-ethyl (Figure 4). The higher rate of chlorflurenol with trinexapac-ethyl also results in a darker green color compared to trinexapac-ethyl alone, very noticeable in Wooster (Figures 5 and 6).
Figure 5: Drone photo of putting green plots in Wooster on 8/11/24.
Figure 6: Drone photo of fairway plots in Wooster on 8/11/24.
The results of this research is promising. Chlorflurenol is safe at very low rates on putting greens and fairways, and results show that this auxin transport inhibitor can increase color and quality. This plant growth regulator may further suppress growth when combined with a gibberellin inhibitor, and initial results do show that chlorflurenol improves growth of turfgrasses growing under shade. Further research is needed on different rates, combinations with other pesticides and growth regulators, and other auxin transport inhibitors that are formulated as pesticides such as NPA.