Dahlia (Dahlia ×hybrida) is a common annual garden plant that provides a variety of color and interest to the landscape and is an important part of the modern floriculture and landscape industries. In the past several years, potted annual dahlias have gained popularity due to their versatility in planters and landscape plantings (Dolce, 2020; Schoellhorn, 2015). Dahlia was named Plant of the Year in 2019 by the National Garden Bureau (2019), further highlighting public interest in this species. Increased production of vegetatively propagated dahlia for use as annual landscape plants and novel cultivar development has led growers to a new and challenging problem: dahlia decline. Dahlia decline occurs during dahlia production near market date and has been reported in different dahlia series and breeding lines. Reported dahlia decline symptoms include plants exhibiting reduced rooting quality, graying foliage, foliage wilting, and plant death. Growers have indicated that plant symptoms affect significant numbers of plants (i.e., whole benches of crops) and near market stage in production (P.A. Hammer, personal communication). Dahlia decline thus far, to our knowledge, has only been reported during greenhouse production and associated with plants produced from vegetative cuttings. To date, no definitive cause or causes have been identified. One hypothesized explanation for dahlia decline is root damage induced by supraoptimal RZTs (P.A. Hammer, personal communication). When plant root zones are exposed to extreme ranges outside normal growing conditions, roots can be damaged, which can alter nutrient and water uptake, and ultimately affect plant growth and development, and in extreme cases, can result in plant death (Dodd et al., 2000; Ingram et al., 2015; Nambuthiri et al., 2015).
Other studies on supraoptimal RZTs have been reported in nursery crops such as foster holly (Ilex ×attenuata) and lacebark elm (Ulmus parvifolia) (Martin et al., 1989; Yeager et al., 1991), southern magnolia [Magnolia grandiflora (Martin et al., 1991)], and vegetable crops including cucumber [Cucumis sativus (Du and Tachibana, 1994)], pepper [Capsicum annuum (Dodd et al., 2000)], potato (Solanum tuberosum), sweetpotato (Ipomoea batatas), and cassava (Manihot esculenta) (Sattelmacher et al., 1990). Studies on supraoptimal RZT effects on potted floriculture crops are limited. Information on suboptimal RZT modification for floriculture crop growth has been published on snapdragon [Antirrhinum majus (Hood and Mills, 1994; Wai and Newman, 1992)] and poinsettia [Euphorbia pulcherrima (Olberg and Lopez, 2016)]. However, these studies did not cover the supraoptimal RZT aspect of plant growth. Hood and Mills (1994) found that as RZTs increased, snapdragons improved their nutrient uptake and growth. Wai and Newman (1992) reported that time to flower for cut flower snapdragon was reduced when RZTs increased even when air temperatures were cool. Although neither of these snapdragon studies discussed supraoptimal RZTs, they demonstrated the impact of RZT on growth and development of floriculture crops.
In an effort to characterize and understand the potential mechanisms involved in dahlia decline, we investigated supraoptimal RZT effects on several commercially available dahlia cultivars across different hybridized dahlia series over 2 years (2019 and 2020).
Beckerman, J. 2011
Daughtrey, M.L. & Benson, M.D. 2005 Principles of plant health management for ornamental plants Annu. Rev. Phytopathol. 43 141 169 doi: https://doi.org/10.1146/annurev.phyto.43.040204.140007
Dodd, I.C., He, J., Turnbull, C.G.N., Lee, S.K. & Critchley, C. 2000 The influence of supra-optimal root-zone temperatures on growth and stomatal conductance in Capsicum annuum L J. Expt. Bot. 51 343 239 248 doi: https://doi.org/10.1093/jexbot/51.343.239
Dolce, J. 2020 https://gpnmag.com/2020/01/dahlias-every-gardeners-delight/
Du, Y.C. & Tachibana, S. 1994 Effect of supraoptimal root temperature on the growth, root respiration and sugar content of cucumber plants Scientia Hort. 58 4 289 301 doi: https://doi.org/10.1016/0304-4238(94)90099-X
Hood, T.M. & Mills, H.A. 1994 Root-zone temperature affects nutrient uptake and growth of snapdragon J. Plant Nutr. 17 2–3 279 291 doi: https://doi.org/10.1080/01904169409364727
Ingram, D.L., Ruter, J.M. & Martin, C.A. 2015 Review: Characterization and impact of supraoptimal root-zone temperatures in container-grown plants HortScience 50 4 530 539 doi: https://doi.org/10.21273/HORTSCI.50.4.530
Martin, C.A., Ingram, D.L. & Nell, T.A. 1989 Supraoptimal root-zone temperature alters growth and photosynthesis of holly and elm J. Arboric. 15 11 272 276
Martin, C.A., Ingram, D.L. & Jenks, M.A. 1991 Response of southern magnolia to supraoptimal root-zone temperatures J. Therm. Biol. 16 5 281 285 doi: https://doi.org/10.1016/0306-4565(91)90018-W
Moorman, G.W. & Daughtrey, M.L. 2002 https://gpnmag.com/article/dont-expect-pythium-root-rot-always-act-same/
Nambuthiri, S., Fulcher, A., Koeser, A.K., Geneve, R. & Niu, G. 2015 Moving toward sustainability with alternative containers for greenhouse and nursery crop production: A review and research update HortTechnology 25 1 8 16 doi: https://doi.org/10.21273/HORTTECH.25.1.8
National Garden Bureau 2019 https://ngb.org/year-of-the-dahlia/
Olberg, M.W. & Lopez, R.G. 2016 Growth and development of poinsettia (Euphorbia pulcherrima) finished under reduced air temperature and bench-top root-zone heating Scientia Hort. 210 10 197 204 doi: https://doi.org/10.1016/j.scienta.2016.07.017
Pappu, H.R., Wyatt, S.D. & Druffel, K.L. 2005 Dahlia mosaic virus: Molecular detection and distribution in dahlia in the United States HortScience 40 3 697 699 doi: https://doi.org/10.21273/HORTSCI.40.3.697
Sattelmacher, B., Marschner, H. & Kühne, R. 1990 Effects of the temperature of the rooting zone on the growth and development of roots of potato (Solanum tuberosum) Ann. Bot. 65 1 27 36 doi: https://doi.org/10.1093/oxfordjournals.aob.a087903
Wai, K.S. & Newman, S.E. 1992 Air root-zone temperatures influence growth and flowering of snapdragons HortScience 27 7 796 798 doi: https://doi.org/10.21273/HORTSCI.27.7.796
Yeager, T.H., Harrison, R.H. & Ingram, D.L. 1991 ‘Rotundifolia’ holly growth and nitrogen accumulation influenced by supraoptimal root-zone temperatures HortScience 26 11 1387 1388 doi: https://doi.org/10.21273/HORTSCI.26.11.1387