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Brittle Root Disease of Horseradish: Evidence for an Etiological Role of Spiroplasma citri. Jacqueline Fletcher, Department of Plant Pathology, University of Illinois, Urbana 61801; Gerald A. Schultz(2), R. E. Davis(3), Cathy E. Eastman(4), and Robert M. Goodman(5). (2)(4)Section of Economic Entomology, Illinois Natural History Survey, Urbana; (3)Plant Virology Laboratory, PPI, SEA, USDA, Beltsville, MD 20705; (5)Department of Plant Pathology, University of Illinois, Urbana 61801. Phytopathology 71:1073-1080. Accepted for publication 4 March 1981. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1981. DOI: 10.1094/Phyto-71-1073.

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 Illinois horseradish in certain years since it was first reported in 1936, but no pathogenic agent has been conclusively implicated in the etiology of the disease until now. 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 C. tenellus and from the plants on which infective leafhoppers were allowed inoculation access) were indistinguishable from Spiroplasma citri by serological growth inhibition and organism deformation tests, and by SDS-polyacrylamide gel electrophoresis. These results established for the first 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.

Additional keywords: Armoracia rusticana, mycoplasmalike organism.