The soilborne pathogen clubroot is a significant pest of brassica crop-producing regions worldwide (Dixon 2006, 2009). The pathogen causes root swelling (galls or clubs) that disrupt the vascular tissue, and impair water and nutrient uptake. Infected plants may be stunted, wilt even when soil moisture is adequate, appear discolored, or when severe, die. This can result in significant economic loss because of reduced crop yield and product quality (Dixon, 2009; Strelkov et al., 2005).
Eradicating the clubroot pathogen from an infested field is unlikely because clubroot produces thick-walled resting spores, some of which may remain viable for 17 years or more in the absence of a host (Wallenhammar, 1996). However, most spores die off quickly. Wallenhammar (1996) estimated the half-life of clubroot spores to be ≈4 years, although recent work suggests that the half-life may be less (Peng et al., 2015). Because there is a direct relationship between spore concentration and disease severity (Hwang et al., 2011; Tsushima et al., 2010), a long enough rotation out of brassicas can reduce disease severity and economic loss. However, predicting rotation lengths that will minimize crop loss is difficult because of the complex interactions among spore concentration, soil physical and chemical properties, and environmental conditions on disease development (Gossen et al., 2014). In heavily infested fields, rotations out of brassicas for a minimum of 5 years may be required to minimize economic losses (Peng et al., 2015).
Clubroot is a problem for Oregon farmers. In 2012, a group of 37 conventional and organic vegetable growers who grow brassicas were surveyed about their experience with clubroot. Of the 19 who responded, all indicated that clubroot was an important issue, with 58% saying that it was very important (unpublished data). Many smaller-acreage diversified vegetable farms do not have sufficient acreage to implement a rotation out of brassicas of 5 years or more that is necessary to reduce clubroot severity. Nor do they have rotational crops as profitable as brassicas. Clubroot has had less economic impact on larger farms in Oregon that grow brassicas for processing. Larger farms often have more crop rotation options, and they can avoid infested fields by moving brassica production to uninfected leased fields.
A large body of research has explored strategies to minimize crop loss from clubroot. This work has been compiled in multiple review papers (Donald and Porter, 2009; Hwang et al., 2014). As discussed in these review papers, many strategies are not consistently effective, do not provide sufficient disease suppression, require changes to farming practices, are too expensive, or are not permitted by organic certifiers. Of all the management strategies studied, growing resistant cultivars is an attractive option. Resistant cultivars require no change in farming practices or equipment, provide greater flexibility as to when and where to plant, and may be far less expensive than other options. Furthermore, resistant cultivars produce far fewer clubs than susceptible cultivars (Adhikari et al., 2012; Saude et al., 2012; Sharma et al., 2013), which reduces the number of spores being added to the soil compared with susceptible cultivars (Hwang et al., 2012). Thus, resistant cultivars have the potential to reduce future disease incidence and severity.
Brassica vegetable crops have been developed with durable resistance to clubroot, and seed companies have released clubroot-resistant cultivars of broccoli, cauliflower, cabbage, brussels sprouts, and napa cabbage (Diederichsen et al., 2009; Piao et al., 2009). In field and greenhouse trials (Adhikari et al., 2012; Saude et al., 2012; Sharma et al., 2013) cultivars have been evaluated for resistance to pathotypes 2, 3, 5, and 6 as designated by the Willimans clubroot differential set (Williams, 1966), but not to pathotype 7, which has been identified as predominant on the west coast of the United States (Dobson et al., 1983). The resistance genes that have been incorporated into brassica crops may not be effective against all clubroot pathotypes (Diederichsen et al., 2009). Therefore, it is important that cultivars with purported clubroot resistance are evaluated against the pathotypes present in each brassica production region.
The goal of this research was to provide farmers with information about brassica cultivars that have demonstrated resistance to the clubroot pathotypes present in western Oregon. To meet this goal, we had two objectives: 1) to identify the pathotypes present in western Oregon using the ECD set (Warwick Crop Center, University of Warwick Wellesbourne, United Kindom), and 2) to evaluate clubroot resistance of commercially available cultivars with purported resistance to the dominant pathotypes identified by the ECD set.
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