Clubroot, caused by the soilborne protist Plasmodiophora brassicae Woronin, has long been recognized as an important disease of Brassica vegetable crops worldwide, and most Brassica spp. are highly susceptible. Management of clubroot is a challenge where Brassica crops are grown frequently because resting spores can survive for many years and are readily disseminated through movement of infested soil (Karling, 1968; Wallenhammar, 1996).
Development and proliferation of P. brassicae in susceptible hosts cause disorganized growth (hyperplasia and hypertrophy) of the root tissues that disrupts uptake and transport of water and nutrients (Mithen and Magrath, 1992). Plants become wilted and stunted, and yield can be reduced when symptoms are severe. Clubroot is estimated to cause 10% to 15% loss in Brassica crops throughout the world (Dixon, 2006).
Management strategies to reduce clubroot severity include extended crop rotation with nonhosts (Wallenhammar, 1996), liming (Dobson and Gabrielson, 1983; Murakami et al., 2002; Myers and Campbell, 1985), manipulation of seeding date to avoid warm periods critical for infection and symptom development (Adhikari, 2010; Gossen et al., 2011; McDonald and Westerveld, 2008), and application of fungicides (Mitani et al., 2003; Suzuki et al., 1995) and biofungicides (Narisawa et al., 2000; Peng et al., 2011). However, each of these approaches has limitations, so they need to be used together as components of an integrated management system.
Clubroot-resistant cultivars of several Brassica vegetable crops have been developed in recent years. For example, genes for clubroot resistance in European fodder turnips (B. rapa L.) have been used to develop clubroot-resistant Chinese cabbage (Hirai, 2006), and resistant cultivars of cabbage have been released by Syngenta (Donald and Porter, 2009). However, the resistance of most of these cultivars has not proven to be durable because the pathogen is genetically diverse (Williams, 1966), and most sources of resistance are pathotype-specific (Diederichsen et al., 2009).
Clubroot has recently become a major issue for canola (B. napus L.) producers in Canada because of its rapid spread since it was first reported on canola in western Canada in 2003 (Tewari et al., 2005). The first clubroot-resistant canola cultivar registered in Canada, ‘45H29’, was released by Pioneer Hi-Bred for the 2010 growing season. Resistance in this line is effective against pathotypes 3 and 5 (Cao et al., 2009) and to pathotype 6 (unpublished), but the durability of this resistance is not yet known.
A range of studies on clubroot has been conducted using Arabidopsis as a model crop, but for certain studies, it would be useful to assess a model crop in the genus Brassica with an architecture and pattern of development that was more similar to commercial crops. Plants in the RCBC, also known as Wisconsin Fast Plants, were developed by selecting and crossing early-flowering lines of Brassica spp. The result is a collection of lines with a short life cycle (36–48 d) and small stature (Williams and Hill, 1986). Their small size and rapid development make them excellent candidates for use as model crops in studies of clubroot. Screening selected RCBC lines against P. brassicae could identify lines that could be used for research where space is limited (e.g., under controlled conditions) and results from these lines could be obtained more quickly than with conventional crop cultivars. Similarly, the small stature and rapid development of several Asian Brassica vegetable crops such as Shanghai pak choy [Brassica rapa L. subsp. Chinensis (Rupr.) var. communis Tsen and Lee] and Chinese flowering cabbage [B. rapa subsp. Chinensis (Rupr.) var. utilis Tsen and Lee] might also make them suitable as model crops for studies of clubroot.
The main objective of this study was to identify the reaction of selected lines of several crops to the predominant pathotype of P. brassicae in Ontario (Cao et al., 2009; Reyes et al., 1974) [pathotype 6, as classified on the differential set of Williams (Williams, 1966)] or ECD 16/0/14 based on the European Clubroot Differential set (Buczacki et al., 1975). In addition, we sought to identify RCBC lines that might have a role as model crops for vegetables, canola, and other commercial crops in future studies of clubroot and to identify clubroot-resistant and susceptible Brassica lines that could be used in the wide range of host-pathogen interaction studies.
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