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Host Range and Phylogenetic Relationships of Albugo candida from Cruciferous Hosts in Western Australia, with Special Reference to Brassica juncea

White rust, caused by Albugo candida, is a serious pathogen of Brassica juncea (Indian mustard) worldwide and poses a potential hazard to the presently developing canola-quality B. juncea industry in Australia. Nine isolates of A. candida, representing strains collected from B. juncea, B. rapa, B. oleracea, B. tournefortii, Raphanus raphanistrum, R. sativa, Eruca vesicaria subsp. 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 or 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 pathogenic strain 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 subsp. 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 grain belt and horticultural regions in Australia. The B. oleracea isolate (race 9) was pathogenic to B. juncea ‘Vulcan’ whereas the isolate from B. juncea (race 2V) was not pathogenic on B. oleracea. Similarly, the strain from C. bursa pastoris (race 4) was pathogenic on B. juncea Vulcan but the B. juncea strain was not pathogenic on C. bursa pastoris. In contrast, the strain from R. sativus (race 1) was pathogenic on B. juncea and the B. juncea strain was also pathogenic on R. sativus. Field isolates from B. rapa, B. tournefortii, E. vesicaria subsp. sativa, and S. irio were all nonpathogenic on B. juncea. Isolates from B. juncea and R. raphanistrum were pathogenic on B. napus (FAN 189). For the nine A. candida isolates from W.A., complete rDNA internal transcribed spacer region nucleotide sequence analysis showed a nucleotide identity range of 72.4 to 100% in comparison with previous Australian collections of A. candida and those previously reported in Europe and Asia. The B. tournefortii isolate of A. candida from W.A. formed a distinct clade on its own, with an identity range of 77.4 to 80.5% compared with the other isolates. Isolates from R. raphanistrum and R. sativus from W.A. were least similar to the other isolates, with a nucleotide identity similarity of only 72.4%. Characterization of the races of A. candida in Western Australia adds to the current knowledge regarding the diversity of this pathogen, allows choice of Brassica spp. or cultivars with resistance to races across different regions, and highlights the particular cruciferous weeds involved in pathogen inoculum carryover between successive cruciferous crops, particularly B. juncea crops.