CLASSIFICATION OF PATHOGENICITY OF PLASMODIOPHORA BRASSICAE FIELD ISOLATES IN JAPAN BASED ON RESISTANCE OF F1 CULTIVARS OF CHINESE CABBAGE (BRASSICA RAPA L.) TO CLUBROOT

Current Status and Challenges in Identifying Disease Resistance Genes in Brassica napus

Brassica napus is an economically important crop across different continents including temperate and subtropical regions in Europe, Canada, South Asia, China and Australia. Its widespread cultivation also brings setbacks as it plays host to fungal, oomycete and chytrid pathogens that can lead to serious yield loss. For sustainable crop production, identification of resistance (R) genes in B. napus has become of critical importance. In this review, we discuss four key pathogens affecting Brassica crops: Clubroot (Plasmodiophora brassicae), Blackleg (Leptosphaeria maculans and L. biglobosa), Sclerotinia Stem Rot (Sclerotinia sclerotiorum), and Downy Mildew (Hyaloperonospora parasitica). We first review current studies covering prevalence of these pathogens on Brassica crops and highlight the R genes and QTL that have been identified from Brassica species against these pathogens. Insights into the relationships between the pathogen and its Brassica host, the unique host resistance mechanisms and how these affect resistance outcomes is also presented. We discuss challenges in identification and deployment of R genes in B. napus in relation to highly specific genetic interactions between host subpopulations and pathogen pathotypes and emphasize the need for common or shared techniques and research materials or tighter collaboration between researchers to reconcile the inconsistencies in the research outcomes. Using current genomics tools, we provide examples of how characterization and cloning of R genes in B. napus can be carried out more effectively. Lastly, we put forward strategies to breed resistant cultivars through introgressions supported by genomic approaches and suggest prospects that can be implemented in the future for a better, pathogen-resistant B. napus.

Resistance to Plasmodiophora brassicae in Brassica rapa and Brassica juncea genotypes From China

Clubroot disease, caused by Plasmodiophora brassicae Woronin, has become a major problem in cruciferous crops worldwide. Chinese cabbage (Brassica rapa), pak choi (B. rapa), and mustard (B. juncea) are important vegetable crops in China. Development of clubroot-resistant cultivars of these crops is urgently needed. In this study, 71 B. rapa and B. juncea genotypes from China, including cultivars and inbred lines, were evaluated for resistance to three P. brassicae pathotypes. A significant interaction was observed between the P. brassicae pathotypes and the Brassica genotypes. Pathotype 3, as defined on the differentials of Williams, exhibited the weakest virulence on all plant material. By contrast, pathotypes 5 and 6 were both highly pathogenic on most of the tested genotypes. In all, 10 of the 14 Chinese cabbage cultivars were resistant to all three pathotypes, while 4 were resistant only to a specific pathotype. Seven of eight progenies obtained from the selfing of Chinese cabbage cultivars were resistant to pathotype 3 but most were susceptible to pathotypes 5 and 6. Most inbred lines of Chinese cabbage and all inbred lines of pak choi and mustard were susceptible to all three pathotypes but their susceptibility was lower to pathotype 3 than to pathotypes 5 and 6.