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Induction of Systemic Resistance to Albugo candida in Brassica juncea by Pre- or Coinoculation with an Incompatible Isolate

In an investigation of the interaction between two isolates of Albugo candida that were compatible (CO) and incompatible (IN) on a Brassica juncea accession, the IN isolate induced both local and systemic protection of cotyledons and true leaves against the CO isolate. The extent of the protection was proportional to the zoosporangia concentration used in the inducing (IN) inoculation. Protection was greatest locally on cotyledons and least on true leaves (the most remote tissue from the point of the inducing inoculation). Protection occurred when the two isolates were inoculated together but was greater when the interval between the IN and CO isolate inoculations was longer. The IN isolate induced only slight protection when it was inoculated after the CO isolate. No induced susceptibility to the IN isolate occurred with any treatment. There was some evidence of competition between CO and IN zoospores for infection sites (stomata). The occurrence of systemic protection and changes detected in phenylalanine ammonia lyase and total soluble peroxidase activities in inoculated cotyledons, particularly after the inducing (IN) inoculation, suggested that host-mediated factors also may be involved in the interaction between the two isolates.