VIEW ARTICLE

Etiology

Detection of Viroids in Dwarfed Orange Trees by Transmission to Chrysanthemum. Mark W. Schwinghamer, Visiting Scientist, New South Wales Department of Agriculture, P.M.B. 10, Rydalmere, N.S.W. 2116, Australia, Present address: New South Wales Department of Agriculture, Agricultural Research Centre, Tamworth, N.S.W. 2340, Australia; Patricia Broadbent, Senior Research Scientist, New South Wales Department of Agriculture, P.M.B. 10, Rydalmere, N.S.W. 2116, Australia. Phytopathology 77:210-215. Accepted for publication 26 May 1986. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-210.

The viroid indicator Chrysanthemum morifolium was inoculated with four mild isolates of graft-transmissible dwarfing agents (M-isolates), which previously gave mild leaf-curling reactions in citron (Citrus medica) but negative results in other tests for viroids. One isolate failed to induce any symptoms in chrysanthemum; there was no evidence of viroid infection by hybridization with a ³²P-labeled complementary RNA probe specific for citrus exocortis viroid (CEV) or by polyacrylamide gel electrophesis (PAGE). Each of the other three isolates gave rise to two types of symptom: a severe reaction characteristic of CEV and a mild reaction the same as that reported for viroids other than CEV. The mild reaction consisted of large chlorotic leaf spots without the leaf distortion and stunting caused by CEV. Both types of reaction were observed during a second passage in chrysanthemum where plants with mild symptoms were used as the source of inoculum. Extracts of plants showing severe reactions gave strong hybridization with the CEV probe and an intense band with the same mobility as CEV as shown by PAGE. These plants clearly contained CEV. Plants with mild reactions gave weak hybridization with the CEV-probe and no band by PAGE. Therefore, the agent(s) responsible for mild reactions was either a mild form of CEV, which can be extracted only in extremely small amounts, or a type of viroid distinct from CEV. The results suggest that the three M-isolates that give symptoms on chrysanthemum are either mixtures of the mild viroid and CEV in which the latter is suppressed, or strains of the mild viroid that, when extracted and inoculated onto chrysanthemum, can mutate to give the typical (severe) form of CEV. The results, though inconclusive for the one M-isolate that failed to give symptoms, support our previous evidence that viroids are associated with dwarfing.