Demethylation inhibitor fungicides [also known as sterol inhibitor fungicides, belonging to the Fungicide Resistance Action Committee (FRAC) Code Group #3 (FRAC, 2012)] are widely used on turfgrasses throughout the United States. These fungicides are labeled for use on golf courses, athletic fields, sod farms, commercial properties, and on home lawns. The success of this group of materials stems from its relatively broad spectrum of activity and relatively low mammalian toxicity (Kamrin, 1997). In addition, this group of fungicides is relatively old and has been in use in turf for at least four decades (Kane and Smiley, 1983). Active ingredients belonging to this class move acropetally and all those registered for use in turf management belong to the triazole subgroup with the exception of fenarimol, which is a pyrimidine (FRAC, 2012). In addition, resistance development to this class of fungicide is generally slow, occurring quantitatively, and this group is considered to be a low to moderate risk for resistance development (FRAC, 2012). Active ingredients that are currently registered for use in the United States include: fenarimol, myclobutanil, metconazole, propiconazole, tebuconazole, triadimefon, and triticonazole (fenarimol use is voluntarily cancelled on 31 July 2013 but end users will be allowed to continue previously labeled applications until supplies are exhausted).
Unfortunately, DMI fungicides have long been known to cause unintentional PGR effects when applied to foliage or applied as a soil drench that is ultimately translocated upward to plant crowns and leaves. On turfgrasses and other plants, the most noticeable effect of DMI fungicide use is often a darkening of foliar color (Buchenauer and Grossman, 1977; Kane and Smiley, 1983; Reicher and Throssell, 1997). Additional effects include delayed senescence and a general reduction in growth, often accompanied by a reduction in turfgrass quality. Some formulations of these fungicides can also cause phytotoxicity to turfgrasses when applied in high heat, at rates above label, on warm-season grasses, or on annual bluegrass (Poa annua L.) greens; DMI fungicides also affect the gibberellic acid synthesis pathway in plants (Bigelow et al., 1995; Buchenauer and Grossman, 1977; Kane and Smiley, 1983; McCullough et al., 2006; Pennypacker et al., 1982; Reicher and Throssell, 1997). The DMI-related products developed and used specifically as PGRs (i.e., gibberellic acid inhibitors) have conversely shown low levels of fungicidal affects in turfgrasses (Burpee et al., 1996). Because of the potential negative impacts of DMI fungicides, end users often rotate in other fungicide classes such as the quinone outside inhibitors or various contact or penetrant fungicides (N.A. Mitkowski, personal observation), particularly during summer months. This is especially true when managers are also regularly using PGRs such as trinexapac-ethyl, paclobutrazol, or flurprimidol for fear of compounded PGR effects on cool-season putting greens.
Research regarding plant growth regulator effects of DMI fungicides on turfgrass is generally consistent but there have been some conflicting studies when trials are undertaken in the field. Although a number of studies have demonstrated increased clipping weights on turf using propiconazole, studies have shown decreases in clipping weights using other DMI fungicides (Kane and Smiley, 1983; McCullough et al., 2006; Pennypacker et al., 1982; Reicher and Throssell, 1997; Xunzhong and Schmidt, 2000). Kane and Smiley (1983) reported that variable field responses have resulted with studies using triazole fungicides on Poa pratensis L. despite consistent results in growth chamber experiments. The researchers suggested that the complex nature of the field environment (variable temperatures, precipitation, drought stress, soil types, etc.) was likely to be responsible for these results. Pennypacker et al. (1982) found that temperature and precipitation have a significant impact on observed PGR-induced effects of DMI fungicides, supporting Kane and Smiley’s (1983) assertion.
Despite the complex interactions inherent in field based trials, it is important to understand potential unwanted effects that any fungicide may have on high-maintenance turf grown in different regional climates. In the Northeast in particular, few negative effects from DMI fungicides have been observed on creeping bentgrass greens. In the last decade, a number of new DMI fungicides have been registered for turfgrass use, often being touted as “safer” than older DMI active ingredients (causing fewer plant growth regulator effects) (N.A. Mitkowski, personal observation). This study was conducted to compare the effects of different DMI fungicides on clipping weight and rooting depth of established creeping bentgrass (Agrostis stolonifera L.) greens.
Bigelow, C.A., Schmidt, R.E. & Chalmers, D.R. 1995 Creeping bentgrass putting green turf as influenced by trinexapac-ethyl and propiconazole application. Amer. Soc. Agron. Annu. Mtg. Abstr. p. 150
Burpee, L.L., Green, D.E. & Stephens, S.L. 1996 Interactive effects of plant growth regulators and fungicides on epidemics of dollar spot in creeping bentgrass Plant Dis. 80 1245 1250
Fidanza, M.A., Wetzel, H.C. III, Agnew, M.L. & Kaminski, J.E. 2006 Evaluation of fungicide and plant growth regulator tank-mix programs on dollar spot severity of creeping bentgrass Crop Prot. 25 1032 1038
Fungicide Resistance Action Committee 2012 FRAC code list: Fungicides sorted by mode of action (including FRAC Code numbering). Fungicide Resistance Action Committee. 14 June 2013. <http://www.frac.info/publication/anhang/FRAC-Code-List2011-final.pdf>
Kaminski, J.E. 2009 Phytotoxicity to Poa annua following repeated application of Trinity, Banner Maxx and experimental fungicides, p. 33–35. In: Guillard, K. (ed.). 2008 Turfgrass research report of the University of Connecticut. UCONN, Storrs, CT
Kamrin, M. 1997 Pesticide profiles: Toxicity, environmental impact and fate. Lewis Publishers, CRC Press, New York, NY
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Pennypacker, B.W., Sanders, P.L., Gregory, L.V., Gilbride, E.P. & Cole, H. Jr 1982 Influence of triadimefon on the foliar growth and flowering of annual bluegrass Can. J. Plant Pathol. 4 259 262
Xunzhong, Z. & Schmidt, R.E. 2000 Application of trinexapac-ethyl and propiconazole enhances superoxide dismutase and photochemical activity in creeping bentgrass (Agrostis stoloniferous var. palustris) J. Amer. Soc. Hort. Sci. 125 47 51