Turfgrass growth regulators (TGRs) are used to reduce the mowing requirement and/or seedhead formation of tall fescue plants along airport runways, fence rows, and highways, and in cemeteries, golf course roughs, and industrial parks (Bingham, 1986; Welterlen, 1988). Some TGRs inhibit cell division in meristematic regions of the plant after being absorbed by aerial shoots; others are absorbed primarily by crown tissue and roots and may inhibit the formation of gibberellic acid. Turfgrass growth regulators are categorized according to mode of action into one of four classes: A, B, C, or D (Christians, 2001):
Trinexapac-ethyl (ethyl (RS)-4-cyclopropyl(hydroxy)methylene-3,5-dioxocyclohexanecarboxylate; Primo, Novartis, Greensboro, NC), a Class A TGR, interferes with gibberellic acid production very late in the biosynthetic pathway (Kaufmann, 1994).
Class B TGRs inhibit gibberellic acid production early in the biosynthetic pathway. A limited gibberellic acid level often results in little plant cell elongation. For example, Flurprimidol [(RS)-2-methyl-1-pyrimidin-5-yl-1-(4-trifluoromethoxyphenyl)propan-1-ol; Cutless, Dow Elanco, Indianapolis, IN] is a Class B TGR; since plants respond unevenly following application, Cutless is not recommended for use in tall fescue turf.
Maleic hydrazide (1,2-dihydropyridazine-3,6-dione; Drexel Retard Liquid Growth Regulator, Drexel Chemical Co., Memphis, TN) was one of the first Class C TGRs used on turfs. Similarly, mefluidide [5′-(1,1,1-trifluoromethanesulfonamido)acet-2′,4′-xylidide; Embark, 3M Company, St. Paul, MN] is an older Class C TGR. Class C TGRs inhibit cell division quickly after their absorption by aerial shoots. Often, they are temporarily phytotoxic, and suppress both seedhead formation and vertical growth rate (Spak et al., 1993). Type C TGRs usually are applied within 2 wk before flowering. Both Retard and Embark are labeled for use in tall fescue turf.
The herbicides chlorsulfuron [1-(2-chlorophenyl)sulfonyl-3-(4-methoxy-6-methyl-1,3,5-triazin-2-yl)urea; Corsair, Nufarm Americas, Burr Ridge, IL; Telar, DuPont, Wilmington, DE], metsulfuron methyl [methyl 2-(4-methoxy-6-methyl-1,3,5-triazin-2-ylcarbamoylsulfamoyl)benzoate; Blade, PBI/Gordon, Kansas City, MO; Manor, Nufarm], and glyphosate [N-(phosphonomethyl)glycine], are examples of Class D TGRs. At low rates of application these products produce a growth-regulating response (Gover et al., 1994). Chlorsulfuron, metsulfuron methyl, and glyphosate injure tall fescue turf severely.
Ethephon (2-chloroethylphosphonic acid; Proxy, Bayer, Research Triangle Park, NC), a TGR that does not fit into the existing classes, causes an increase in ethylene that reduces the rate of plant growth (McCullough et al., 2005). Proxy is labeled for use in tall fescue turf.
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