Virulence and inoculum density‐dependent interactions between clubroot resistant canola (Brassica napus) and Plasmodiophora brassicae

Abstract

To mitigate the impact and dissemination of clubroot in western Canada, canola (Brassica napus) producers have relied on clubroot resistance traits. However, in 2013 and 2014, new strains of the clubroot pathogen, Plasmodiophora brassicae, emerged that are virulent on most clubroot‐resistant (CR) canola genotypes. Novel strains of the pathogen were inoculated onto two susceptible canola cultivars, one resistant line and six CR cultivars. Although all cultivars/lines showed a susceptible response to inoculation with the new strains of P. brassicae, the severity of disease reaction, root hair infection rates and the amount of P. brassicae DNA present in each canola genotype varied depending on the strain. In addition, the effect of inoculum density on disease severity and gall formation was recorded for one of these new strains on a universally susceptible Chinese cabbage cultivar and one susceptible and 10 resistant canola genotypes. Although root galls were observed at an inoculum density of 103 spores per mL of soil, clear differentiation of susceptible and resistant reactions among canola cultivars/lines was not observed until the inoculum density reached 105 spores mL−1. At a spore density of 106 spores mL−1 and above, all cultivars/lines developed susceptible reactions, although there was some differentiation in the degree of reaction. This study shows the potential to develop a unique disease profile for emergent clubroot pathotypes and shows a useful range of spore densities at which to study new P. brassicae strains.

Greenhouse Evaluation of Clubroot Resistant-Brassica napus cv. Mendel and Its Efficacy Concerning Virulence and Soil Inoculum Levels of Plasmodiophora brassicae

“…On the other hand, low levels of high virulent isolates have existed within the population of P. brassicae for some years, and the population of high virulent isolates increased because of the selection pressure exerted by extensive use of clubroot-resistant cultivars. Our results confirm previous work [ 33 ] showing that the severity of disease reaction, root hair infection rates, and the amount of P. brassicae DNA present in each canola genotype varied depending on the strain of the pathogen. Similarly, an earlier study demonstrated strong positive correlations between the amount of P. brassicae DNA and the level of root hair infection, where this relationship was stronger for the resistant cultivar than for the susceptible one [ 9 ].…”

“…Mendel, often found not to be durable for a long time, and the overcoming of resistance after a few growing seasons in clubroot-infested fields have been reported in different countries [1,2,18,19,24]. Generally, it was found that increasing inoculum amounts translate into increasing values of both disease incidence and severity [26,33]. However, the influence of the pathotype virulence and the susceptibility of the host cultivar on this relationship has not been intensively investigated.…”

“…They found that, with the increasing inoculum density, clubroot severity increased and plant height and seed yield decreased. Additionally, the effect of inoculum density of one P. brassicae isolate on disease severity and gall formation was also recorded for a new Canadian virulent isolate on a universally susceptible Chinese cabbage cultivar, a susceptible spring oilseed rape cultivar, and 10 resistant canola genotypes [ 33 ]. Although root galls were observed at an inoculum density of 10 3 spores per mL of soil, clear differentiation of susceptible and resistant reactions among spring canola cultivars/lines was not observed until the inoculum density reached 10 5 spores per mL.…”

“…Although root galls were observed at an inoculum density of 10 3 spores per mL of soil, clear differentiation of susceptible and resistant reactions among spring canola cultivars/lines was not observed until the inoculum density reached 10 5 spores per mL. At a spore density of 10 6 spores per mL and above, all spring canola cultivars/lines developed susceptible reactions, though there was some differentiation in the degree of reaction observed [ 33 ].…”

The Use of Genetic and Gene Technologies in Shaping Modern Rapeseed Cultivars (Brassica napus L.)

“…Common diseases in rapeseed reported worldwide includes blackleg disease, caused by Leptosphaeria maculans [ 110 , 111 ] clubroot, caused by Plasmodiopora brassicae [ 112 , 113 ] and Sclerotinia stem rot, caused by Sclerotinia sclerotiorum [ 114 , 115 ]. The availability of genomic data and genetic maps of Brassica species, along with associated bioinformatics tools, allow deeper dissection of the genetic architecture of resistance.…”