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Characterization of a Zucchini Yellow Mosaic Virus Isolate with a Deficient Helper Component. H. Lecoq, directeur de recherches, INRA, Station de Pathologie Végétale, B.P. 94, 84143 Montfavet-Cédex, France; D. Bourdin(2), B. Raccah(3), E. Hiebert(4), and D. E. Purcifull(5). (2)attaché scientifique contractuel, INRA, Station de Pathologie Végétale, B.P. 94, 84143 Montfavet-Cédex, France; (3)virologist, Department of Virology, A.R.O., The Volcani Center, Bet Dagan, Israel; (4)(5)professors, Department of Plant Pathology, University of Florida, Gainesville 32611. Phytopathology 81:1087-1091. Accepted for publication 11 February 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-1087.

Two isolates of zucchini yellow mosaic virus (ZYMV) were found to be either highly (HAT) or poorly (PAT) aphid-transmissible. The inefficient transmission of ZYMV-PAT was independent of the virus source plant or aphid species used. The ZYMV-PAT and ZYMV-HAT, from which the former was derived, did not differ in their host range or serological properties. Purified virions of either ZYMV-HAT or ZYMV-PAT were readily transmitted by aphids when crude or concentrated helper component (HC) from ZYMV-HAT was provided, but were not transmitted when mixed with crude or concentrated HC from ZYMV-PAT. It is concluded that ZYMV-PAT HC is responsible for the defect in aphid transmissibility of this isolate. However a component serologically related to HC was detected both in ZYMV-PAT-infected plants or in ZYMV-PAT RNA in vitro translation products, indicating that an ineffective form of HC is produced by ZYMV-PAT. Furthermore, in in vitro acquisition experiments conducted with ZYMV-PAT and a non-aphid-transmissible isolate (ZYMV-NAT) deficient in the coat protein, high levels of transmission were observed only in the combination containing purified virions of ZYMV-PAT and HC from ZYMV-NAT. This provided additional proof for the deficiencies in the HC for ZYMV-PAT and in the coat protein for ZYMV-NAT.