1986 Volume 52 Issue 1 Pages 59-68
Phoma sp., Erwinia and Pseudomonas soft rot bacteria, and a coryneform bacterium were isolated from rhizomes of wasabi (Eutrema wasabi Maxim.) with the internal black rot syndrome. These pathogens were also isolated in fibrous roots with black necrotic steles which are commonly found on the rhizomes with or without the internal black rot syndrome. The isolation frequencies of these pathogens were significantly different depending on various factors such as growing seasons, localities, or mountain streams in which the wasabi fields are laid. The cultures of Phoma sp. were identified as P. wasabiae Yokogi, the causal agent of black leg disease, on the basis of morphological and pathological characteristics. Soft rot bacteria were isolated in association with saprophytic bacteria at a significantly low level from the tissues of rhizomes and roots which seldom exhibited soft rot symptoms. They were sometimes isolated in association with Phoma wasabiae. Soft rot bacteria showed distinct seasonal alternation, erwinias being prevalent in summer and pseudomonads in winter. Soft rot erwinias and pseudomonads were isolated from water flowing on crop beds or in drainage canal. Soft rot pseudomonads were also isolated from water flowing out of a spring. A coryneform bacterium showed a very high population in the diseased tissues so that it grew singly on the isolation plates. In some rhizomes and fibrous roots with the same syndrome, no microorganisms but some saprophytic bacteria were isolated, suggesting that the disorder may also be caused by factors other than microbial infection.