An obligate fungus Albugo candida (Pers. ex L6v.) Ktze. (race unidentified) was successfully grown on host callus tissues of Brassicajuncea cv. Varuna. Of the various type of diseased explants used, young (green) hypertrophied inflorescence axis bearing non-erumpent zoosporangial blisters allowed the fungus to multiply asexually over the host calli on modified MS-medium (Murashige and Skoog, 1962). The dual cultures were maintained up to 6--8 subcultures without loss of viability of zoosporangia on MS-medium supplemented with 10.0 mg L-~ IBA, 0.05 mg L -l kinetin, 25.0 mg L -l AA, 1.0 mg L -1 biotin, 1.0 mg L -l thiamine-HCl and 1.0 g L i casein hydrolysate. The fungus grew only on the callus cells and not axenically on the medium. Pathogenicity test and histopathology of cultures proved the existence of the viable fungus in vitro.

Maximum staghead formation was obtained in 26‐day‐old (growth stage [GS] 3.1) Brassica juncea plants by inoculating differentiating flower buds with a zoospore suspension of Albugo candida race 2 V; exposing apical meristem tissues by opening the flower buds with forceps proved more conducive to staghead formation. Inoculation of 35‐ and 45‐day‐old plants (GSs 4.1 and 5.0, respectively) produced fewer hypertrophies, mainly in isolated flowers. Inoculation of 7‐ and 13‐day‐old plants (GSs 1.0 and 2.1, respectively) did not produce any hypertrophied flowers, but did result in the production of hypertrophied branches at the first node on the main stem. In general, hypertrophies were initiated more readily under greenhouse conditions than in the growth chamber. Other Brassica hosts inoculated with A. candida race 2 V or 7 V at GS 31 showed similar rankings for staghead formation and leaf infection. The technique should prove useful in screening breeding lines for disease resistance, particularly staghead formation, the most damaging phase as far as yield loss is concerned.

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