Clubroot of crucifers, caused by Plasmodiophora brassicae, is emerging as an important disease of canola (Brassica napus) in Alberta, Canada. Populations of the pathogen often consist of a mixture of different pathotypes. Therefore, a simple and efficient method to isolate single resting spores of P. brassicae was developed, based on serial dilution of spore suspensions. The virulence of 24 single-spore isolates, representing five populations of the pathogen from Alberta, Ontario, and British Columbia, was characterized on the differentials of Williams and Somé et al. Symptoms were rated 6 weeks after inoculation and Fisher's least significant difference (P < 0.05) was used to differentiate resistant from susceptible host reactions. The pathotype composition of P. brassicae in Canada appeared more diverse when single-spore isolates were examined rather than populations of the pathogen. In Alberta, at least three and possibly four pathotypes were identified among the 14 isolates tested, whereas a maximum of only two pathotypes had been reported previously when populations of the pathogen were examined. Pathotype 3 or P2, as classified on the differentials of Williams and Somé et al., respectively, was found to be predominant in the province. The occurrence of other pathotypes at lower frequencies suggests that caution should be used in any breeding strategy, because rare pathotypes of P. brassicae may quickly become predominant if susceptible host genotypes are continuously grown.
A field survey for clubroot of crucifers, caused by Plasmodiophora brassicae, was conducted in the regions surrounding Edmonton, Alberta, Canada, in 2005. The presence of clubroot was confirmed in 41 of the 112 canola (Brassica napus) fields surveyed. These P. brassicae-infested fields were located in Sturgeon, Strathcona, Leduc and Wetaskiwin counties, as well as in a rural area of northeast Edmonton. Infected roots were also received from an infested field in Flagstaff County, southeast of Edmonton. Although there was a significant negative correlation between index of disease and soil pH, the occurrence of clubroot was not restricted to fields with acidic soils. Populations of the pathogen were selected from 10 fields and used in pathotype classification on the differential hosts of Williams, Some´et al. and the European Clubroot Differential (ECD) set. Kruskal-Wallis analysis indicated no significant differences in the virulence of the 10 populations tested, suggesting that they are relatively homogenous. If a disease index of 50% was regarded as the cut-off between a resistant and a susceptible reaction, then all P. brassicae populations tested were classified as ECD -⁄15⁄12 on the hosts of the ECD set, or as pathotypes 3 or P2 on the differentials of Williams or Some´et al. respectively. However, it may be difficult to detect rare or infrequent pathotypes when field populations of the pathogen are used for characterization.
Pseudomonas syringae pv. actinidiae (Psa) causes kiwifruit bacterial canker (KBC), with a severe infection of the kiwifruit plant resulting in heavy economic losses. Little is known regarding the biodiversity and genetic variation of populations of Psa in China. A collection of 269 strains of Psa were identified from 300 isolates obtained from eight sampling sites in five provinces in China. The profiles of 50 strains of Psa and one strain of P. syringae pv. actinidifoliorum (Psaf) were characterized by Rep-, IS50-P, and RAPD-PCR. Discriminant analysis of principal coordinates (DAPC), principal component analysis (PCA), and hierarchical cluster analysis were used to analyze the combined fingerprints of the different PCR assays. The results revealed that all isolates belonged to the Psa3 group, that strains of Psa from China have broad genetic variability that was related to source geographic region, and that Chinese strains can be readily differentiated from strains from France, but are very similar to those from Italy. Multilocus sequence typing (MLST) of 24 representative isolates using the concatenated sequences of five housekeeping genes (cts, gapA, gyrB, pfk, and rpoD) demonstrated that strain Jzhy2 from China formed an independent clade compared to the other biovars, which possessed the hopH1 effector gene, but lacked the hopA1 effector gene. A constellation analysis based on the presence or absence of the four loci coding for phytotoxins and a cluster analysis based on the eleven effector genes showed that strains from China formed two distinct clades. All of the strains, including K3 isolated in 1997 from Jeju, Korea, lacked the cfl gene coding for coronatine. In contrast, the tox-argK gene cluster coding for phaseolotoxin was detected in K3 and in the Biovar1 strains (K3, Kw30, and Psa92), and produced a false positive amplicon for the hopAM1-like gene in this study. To date, only one biovar (Biovar3) is represented by the strains of Psa from China, despite China being the center of origin for the kiwifruit.
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