Pythium root rot is one of the most common diseases of commercially grown poinsettia and P. aphanidermatum is the predominant species causing pythium root rot in North Carolina (Lookabaugh et al., 2015). Under favorable environmental conditions, P. aphanidermatum causes stunting, root rot, wilting, defoliation, and in severe cases, plant death. Traditionally, pythium root rot has been managed primarily through a combination of strict sanitation practices and preventative fungicide applications, but long production seasons and common irrigation practices make this disease difficult to control. Unfortunately, only a few fungicides are efficacious against P. aphanidermatum and resistance to mefenoxam, the primary active ingredient used to control pythium root rot, is common in Pythium populations (Lookabaugh et al., 2015; Moorman and Kim, 2004; Moorman et al., 2002). To meet these challenges, growers need additional tools to implement a more integrated approach to disease management.
Host resistance is among the most important tools for integrated disease management, but disease resistance has not been widely used in floricultural crops, including poinsettia (Daughtrey and Benson, 2005). Commercial poinsettia cultivars have not been systematically evaluated for their response to pythium root rot, nor has the genetic basis of resistance been explored in university research (Trejo et al., 2012). Complete resistance to pythium root rot is unknown in cultivated poinsettia and generally is lacking in other ornamental species (Múnera and Hausbeck 2015). Resistance to pythium root rots is partial; i.e., quantitatively expressed, in geranium [Pelargonium ×hortorum (Chagnon and Belanger, 1991)], common bean [Phaseolus vulgaris (Lucas and Griffiths, 2004)], soybean [Glycine max (Bates et al., 2008)], and wheat [Triticum aestivum (Higginbotham et al., 2004)]. Caladium (Caladium ×hortulanum) is one of the few floriculture species evaluated for resistance to pythium root rot; 7 of 42 cultivars expressed partial resistance when inoculated with Pythium myriotylum (Deng et al., 2005a, 2005b).
Complete resistance or immunity often is found in wild relatives of cultivated plants, which are used to develop resistant cultivars through hybridization and selection. Among many examples, closely related wild species are sources of resistance to Fusarium wilt in tomato [Solanum lycopersicum (Gonzalez-Cendales et al., 2016)], rusts in small grains (Miedaner and Korzun, 2012), and late blight resistance in potato [Solanum tuberosum (Fry, 2008)]. It is not known if wild and cultivated relatives of poinsettia are sources of resistance to pythium root rot, or if interspecific hybridization can offer increased resistance to pythium root rot. However, poinsettia breeding programs recently have incorporated interspecific hybrids of poinsettia and the uncultivated dogwood poinsettia (Euphorbia cornastra) or scarlet plume (Euphorbia fulgens) into commercial production lines (Ecke et al., 2004). These cultivars have been introduced primarily for their novel growth habits, which feature numerous, small, flat bracts. As with other poinsettia cultivars, information about their resistance or susceptibility to pythium root rot is lacking.
Growers and breeders could benefit from information about the response of poinsettia cultivars to pythium root rot to avoid cultivars that are highly susceptible and to integrate partially resistant cultivars into their disease management programs. The objective of this research was to evaluate commercially available poinsettia cultivars and hybrids for resistance to pythium root rot caused by P. aphanidermatum.
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