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Ultradwarf Bermudagrass: How sensitive is it to PGRs?
February 1, 2004 By: Bert McCarty, Haibo Liu, Patrick McCullough TurfGrass TrendsPressure to consistently produce ideal putting-green quality has superintendents including regular plant growth regulator (PGR) applications in their management programs.
PGRs are compounds that reduce growth by modifying turfgrass hormone synthesis. For fairways, tees and roughs, PGR applications reduce mowing requirements, which minimize labor and equipment operation (Watcshke et al., 1992). PGRs are also used to promote smooth and uniform putting surfaces by reducing diurnal shoot growth fluctuations (McCarty, 2001).
 Figure 1: Dry root weight (g m-2) averaged across three studies for TifEagle bermudagrass treated with seven growth regulators. Different letters indicate a significant difference at P = 0.05 according to Fishers Protected LSD test. Abbreviations: MF, mefluidide; FN, fenarimol; EP, ethephon; MH, maleic hydrazide; TE, trinexapac-ethyl; PB, paclobutrazol; FP, flurprimidol. |
Challenges superintendents face with ultradwarf bermudagrass varieties include maintaining appropriate fertility, managing thatch/mat development and promoting root growth. Dwarf bermudagrass varieties show some sensitivity to herbicide and PGR practices previously acceptable for traditional bermudagrass cultivars.
Hybrid bermudagrass (Cynodon dactylon (L.) Pers. x C. transvaalensis Burtt-Davey) is the warm-season turfgrass used most commonly on putting greens in the warm, humid climatic regions (Beard, 2002). Bermudagrass putting-green quality has long been considered inferior to fine-textured creeping bentgrass (Agrostis palustris Huds.) because of the inabilities of cultivars such as Tifgreen and Tifdwarf to withstand routine mowing heights lower than three-sixteenths to one-quarter inch (Beard, 1973).
Problems also exist from genetic instabilities of these cultivars leading to off-type patches of different color and texture, causing greens to become mosaic and difficult to play (McCarty and Miller, 2002; Beard, 2002).
Dwarf bermudagrass varieties have recently been introduced that tolerate mowing heights of one-eighth inch or closer on a consistent basis (McCarty and Miller, 2002). Dwarf bermudagrass is characterized by fine leaf textures, high shoot densities and low growth habits suitable for close mowing and producing ball roll distances once exclusive to creeping bentgrass greens (McCarty and Miller, 2002; Beard, 2002). However, management programs designed for Tifdwarf and Tifgreen bermudagrass putting greens appear questionable for successful long-term ultradwarf bermudagrass culture.
 TifEagle bermudagrass after initial treatments (right) and repeated treatments (far right) with seven plant growth regulators. (below) TifEagle bermudagrass after 12 weeks treated with paclobutrazol at .125, .25, and .375 pounds per acre at six-week intervals. |
Turfgrass color and quality enhancements are commonly observed from regular PGR use, enhancing tolerances to shade, drought and other physiological stresses (Qian and Engelke, 1999; Jiang and Fry, 1998). Growth inhibitors used in turf are divided into two categories: Type I and Type II. Type I compounds, including maleic hydrazide and mefluidide, inhibit cell division and are used predominantly as seedhead and foliar suppressant inhibitors in low maintenance turf areas. Inconsistent performance, turf injury and root-growth reductions have limited the use of these compounds. Type II growth regulators inhibit gibberellic acid, a hormone necessary for stem elongation, and present less likelihood of leaf burn compared with Type I compounds (McCarty, 2001).
As dwarf bermudagrass varieties continue to expand as putting-green turf, concerns exist for incorporating appropriate PGRs into management programs. Turf discoloration and negative rooting responses from PGRs may limit their potential for routine dwarf bermudagrass maintenance. Currently, no PGRs are labeled for use on ultradwarf bermudagrass. Thus, university research is warranted on these responses.
The objectives of our research were to:
(1) provide initial screening of PGRs on ultradwarf bermudagrass for future field and greenhouse research;
(2) compare labeled PGR rates of traditional bermudagrass and creeping bentgrass greens on ultradwarf bermudagrass; and
(3) examine morphological responses of ultradwarf bermudagrass to growth inhibition.
PGR greenhouse studies
Preliminary greenhouse screening studies were conducted to determine the immediate effects of seven plant growth regulators on clipping yield, visual quality and root mass of TifEagle bermudagrass after six weeks.Growth regulators tested included trinexapac-ethyl (Primo), flurprimidol (Cutless), paclobutrazol (Turf Enhancer, Trimmit), mefluidide (Embark), maleic hydrazide (Royal MH-30), ethephon (Proxy) and fenarimol (Rubigan). Due to the lack of labeled rates for growth regulators on ultradwarf bermudagrass, rates for previous bermudagrass cultivars, bentgrass greens and low rates for higher-mowed hybrid bermudagrass were applied (Figure 1).
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Two applications of each compound were made over a six-week period in three separate studies to TifEagle bermudagrass plugs placed in 5-inch deep pots with 28 square-inch areas. PGR applications for all studies were made with a greenhouse spray cabinet.
Visual quality was unacceptable with repeat fenarimol, flurprimidol and paclobutrazol applications (Pictures 1 and 2). Ethephon, a PGR that induces ethylene, had unacceptable turf quality one week after application.
Total dry weight clippings from six weekly samples were reduced 56 percent from trinexapac-ethyl, 86 percent from paclobutrazol, 88 percent from flurprimidol, 25 percent from mefluidide, 46 percent from maleic hydrazide, and 41 percent from ethephon (data not shown).
Figures 2 & 3. |
After six weeks, root mass was reduced 49 percent by fenarimol and 43 percent by flurprimidol, while all other PGRs had root mass similar to untreated turf (Figure 1). TifEagle bermudagrass treated with trinexapac-ethyl averaged 45 percent more root mass compared to mefludide, paclobutrazol, fenarimol, and fluprimidol (Figure 1). Trinexapac-ethyl was the only compound to reduce clippings and enhance turf quality without negative effects on rooting in three studies.
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