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).
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