Hema Kasinathan Thesis May 1 2012.pdf - Atrium - University of ...

1.2.1 PathotypesHonig (1931) was the first researcher to suspect the existence of strains ofP. brassicae (cited in Karling 1968). Later, the occurrence of physiologic strains inP. brassicae was confirmed using turnip varieties (Walker 1942). In the literature onclubroot, the terms race and pathotype have often been used interchangeably. However, arace is “a genetically and often geographically distinct mating group within a species”(Agrios 2005). Neither the population of P. brassicae nor the differential hosts used toassess the differences in the pathogen population are genetically uniform. Therefore,based on the concept of Parlevliet (1985), the term race has been replaced by the termpathotype (Buczacki et al. 1975, Strelkov et al. 2006).Pathotypes are classified based on their virulence on selected hosts. A simplesystem for classifying P. brassicae into distinct pathotypes using differential hosts wasdeveloped by Williams (1966), based on the percentage of infection on selected Brassicacrops. The reaction of 124 isolates from around the world was assessed on four Brassicahosts, and nine pathotypes were identified. Pathotypes 4, 5, 8 and 9 were identified forthe first time (Williams 1966). In Canada, four pathotypes of P. brassicae (1, 2, 4 and 6)were identified based on the Williams’ differential system (Ayers 1972).The European Clubroot Differential set (ECD) is another system for separatingpathotypes. This classification system employs a scoring technique to classify pathotypesof P. brassicae, based on the response of selected five cultivars each of B. campestris, B.napus, B. oleracea (Buczacki et al. 1975). In Japan, clubroot-resistant (CR) F1 hybridcultivars and B. rapa lines (five CR F1 cultivars of Chinese cabbage, two CR accessionsof fodder turnip and Chinese cabbage parental line No. 4) were used to classify field4