Pathogenic behaviour of Albungo candida on Brassica juncea and mechanisms of host resistance

[Truncated abstract] White rust caused by Albugo candida, is a serious pathogen of Brassica juncea (Indian mustard) and one posing a potential hazard to the presently developing canolaquality B. juncea industry in Australia. This PhD project was aimed at identifying the races/pathotypes of A. candida, its pathogenic behaviour on B. juncea to better understand its pathogenic potentials and to explore the mechanisms of resistance to the pathogen by B. juncea. Nine isolates of Albugo candida, representing strains collected from Brassica juncea, B. rapa, B. oleracea, B. tournefortii, Raphanus raphanistrum, R. sativa, Eruca vesicaria ssp. sativa, Capsella bursa-pastoris and Sisymbrium irio from different locations in Western Australia (W.A.) were tested on cruciferous host differentials to characterize their pathogenicity. In particular, these studies were aimed to determine the hazard to the newly emerging B. juncea industry in Australia from races/pathotypes of A. candida present. Pathogenicity tests with appropriate differentials demonstrated the presence in W.A. of a unique strain from B. rapa that did not show characteristics of either Race 7A or 7V and clearly is a distinct new pathotype within Race 7. Different strains collected from W.A. differed in their host range, with the strains from B. tournefortii and S. irio being highly host specific, failing to be pathogenic on any other differentials. B. tournefortii was host to a strain attacking B. juncea and E. vesicaria ssp. sativa. The strain from R. raphanistrum showed a relatively wide host range among the differentials tested. B. tournefortii, C. bursa-pastoris, R. raphanistrum and S. irio are common weeds within grainbelt and/or horticultural regions in Australia. The B. oleracea isolate (Race 9) was pathogenic to B. juncea cv. Vulcan whereas the isolate from B. juncea (Race 2V) was not pathogenic on B. oleracea.