Brittle Root Disease of Horseradish: Evidence for an Etiological Role ofSpiroplasma citri

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Abstract
The symptoms of brittle root (BR) disease of horseradish include foliar chlorosis, stunting and a discoloration in the phloem ring of affected plant roots. The disease has resulted in severe crop losses in Illinosis [USA] horseradish in certain years since it was first reported in 1936. Dienes'' staining of hand-cut sections of BR and nonbrittle root (NBR) horseradish roots revealed irregularly distributed blue-stained cells in the phloem of BR but not of NBR plants. Spiroplasmas were cultured in liquid media from all BR plants examined but from none of the NBR plants. Spiroplasmas were also isolated from Circulifer tenellus reared in captivity and allowed to feed on BR plants, and from horseradish plants initially free of BR symptoms to which infective leafhoppers had been given inoculation access. All spiroplasma isolates tested (including those from field-collected BR plants, from infective leafhoppers were allowed inoculation access) were indistinguishable from S. citri by serological growth inhibition and organism deformation tests, and by SDS[sodium dodecyl sulfate]-polyacrylamide gel electrophoresis. These results established for the 1st time a consistent association of a pathogenic agent with plants having BR symptoms and its absence from symptomless plants. C. tenellus injected with horseradish isolates of S. citri grown in pure culture transmitted the spiroplasma to horseradish test plants, which subsequently developed symptoms of BR. S. citri was reisolated from these plants. These results indicate an important etiological role for S. citri in BR disease of horseradish.