Infection and development ofPlasmodiophora brassicaein resistant and susceptible canola cultivars

Abstract

Commercial cultivars of canola with resistance to clubroot (Plasmodiophora brassicae) have been registered recently in Canada. However, little is known about how and when resistance is expressed. Time series assessments of root hair infection and cortical infection were made in four cultivars that differed in reaction to two pathotypes, P3 and P6 (Williams' system), of P. brassicae. The cultivars were '45H29' (resistant), 'InVigor 5030' (moderately resistant), '46A76' (susceptible) and '45H21' (susceptible to P3, resistant to P6). For assessment of root hair infection (RHI), seedlings were harvested at 4, 8 and 12 days after inoculation (DAI). For assessment of cortical colonization (% root area occupied by the pathogen), plants were harvested at 16, 22 and 28 DAI. In compatible interactions (susceptible cultivar × virulent pathotype), RHI developed quickly and was slower in intermediate and incompatible interactions. The maximum RHI for both compatible and incompatible interactions was 65-70%; maximum RHI was slightly but significantly lower (59%) in the intermediate interaction type. At 28 DAI, cortical colonization was high in '46A76' (P3 = 45%, P6 = 35%), intermediate in 'InVigor 5030' (P3 = 23%, P6 = 16%), and no colonization (0%) was observed for either pathotype in '45H29'. In '45H21', cortical colonization by P3 was high (35%), but inoculation with P6 resulted in no colonization. In the compatible interactions, cortical colonization by P3 was consistently higher than by P6. There were small differences in the pattern of RHI associated with resistance, but resistance was most clearly expressed during secondary infection and development.Résumé: Des cultivars commerciaux de canola, résistants à la hernie (Plasmodiophora brassicae), ont été enregistrés récemment au Canada. Toutefois, on ne sait pas vraiment comment et quand s'exprime la résistance. L'évaluation sur séries temporelles de l'infection de radicelles et de l'infection corticale a été effectuée chez quatre cultivars qui réagissaient différemment aux pathotypes P3 et P6 (système de William) de P. brassicae. Les cultivars étaient le '45H29' (résistant), le 'InVigor 5030' (moyennement résistant), le '46A76' (réceptif) et le '45H21' (réceptif à l'égard de P3 et résistant à P6). Pour évaluer l'infection des radicelles (RHI), les semis ont été récoltés à 4, 8 et 12 jours après l'inoculation (JAI). Pour évaluer la colonisation corticale (pourcentage de la surface des racines envahie par l'agent pathogène), les plants ont été récoltés à 16, 22 et 28 JAI. Sur le plan des interactions compatibles (cultivar réceptif × pathotype virulent), la RHI s'est développée rapidement, mais plus lentement au cours des interactions intermédiaires et incompatibles. Le taux maximum de RHI pour les interactions compatibles et incompatibles était de 65 à 70 %. Quant aux interactions de type intermédiaire, le taux maximum était légèrement, mais significativement plus bas (59 %). À 28 JAI, le taux de colonisation corticale chez '46A76' était intense (P3 = 45 %, P6 = 35 %), moye...

A comparison of clubroot development and management on canola and Brassica vegetables

“…Studies on canola grown at 25/20 • C day/night produced a similar result (Deora et al, 2012). Root hair infection was 68% at 14 DAI, and there was 38% cortical infection at 28 DAI.…”

“…The experience with resistance to clubroot across a range of Brassica crops (discussed previously) indicates that resistance based on a single gene is unlikely to be highly durable, especially given that inoculum pressure is high (often 10 7 or more resting spores per g of dry soil in heavily infested fields), cropping rotations are often short (1 year between canola crops), the affected acreage is extensive, and the pathogen populations in the field appear to be genetically heterogeneous (Cao et al, 2009). A recent trial demonstrated that there were no substantial differences in resistance response among these clubroot-resistant canola cultivars to the pathotypes of P. brassicae prevalent in Canada (Deora et al, 2012(Deora et al, , 2013a. This indicates that all of these lines may carry resistance genes from a similar source.…”

“…In radish, moderate resistance delayed the development of secondary infection (Kroll et al, 1983). In canola, the infected area within the cortex was reduced and pathogen development was slower in a moderately resistant cultivar than in susceptible cultivars (Deora et al, 2012). This indicates that the mechanism of resistance in moderately resistant cultivars is different from that in resistant cultivars.…”

“…For example, clubroot was most severe in soils with low organic matter in a survey of oilseed rape (B. napus) crops growing on mineral soil in northern Europe (Wallenhammar, 1996), but there are reports of high levels of clubroot on soils with high organic matter (60-70%) on radish (Thuma et al, 1983), Shanghai pak choy (McDonald & Westerveld, 2008) and other Brassica vegetables (Adhikari et al, 2012) and canola (Deora et al, 2012). In a trial where cabbage was grown on mineral (Fox sandy loam, 2-3% organic matter) and muck (74% organic matter) soil sites in the same season and region, clubroot was severe on both soil types (Saude et al, 2012).…”

“…Most Canadian canola cultivars are resistant to pathotype 6 (M.R. McDonald, unpublished), which is predominant in Ontario, but are highly susceptible to pathotype 3, which is the dominant pathotype in Alberta (Strelkov et al, 2006;Deora et al, 2012). The canola cultivars 'Mendel' and 'Tosca', which were released in Europe in 2000, carry pathotype-specific clubroot resistance.…”