Fuchs, M., Department of Plant Pathology, Cornell University, New York State Agric. Exp. Station, Geneva, NY 14456, United States, INRA, Unite de Recherche Vigne et Vin, Laboratoire de Virologie, 28 rue de Herrlisheim, 68021 Colmar, France
Gal-On, A., Department of Virology, Agricultural Research Organization, The Volcani Center, P.O. Box 6, Bet Dagan 50-250, Israel
Raccah, B., Department of Virology, Agricultural Research Organization, The Volcani Center, P.O. Box 6, Bet Dagan 50-250, Israel
Gonsalves, D., Department of Virology, Agricultural Research Organization, The Volcani Center, P.O. Box 6, Bet Dagan 50-250, Israel

Spread of the aphid nontransmissible Zucchini yellow mosaic virus (ZYMV) strain MV was monitored over two consecutive years in field plots of nontransgenic and transgenic squash expressing the coat protein (CP) gene of the aphid transmissible strain FL of Watermelon mosaic virus (WMV). The experimental approach was to mechanically inoculate plants with ZYMV strain MV and to assess subsequent transmissions, assumed to be vectored by aphids, of this strain to nonmechanically inoculated plants. Strain MV was distinguished from other ZYMV isolates by a threonine at position 10 of the CP or by a distinct electrophoretic pattern of a Nla IV-digested genomic cDNA fragment generated by RT-PCR. ZYMV strain MV was not detected in fields of nontransgenic plants, but was apparently aphid transmitted to 77 of 3,700 plants (2%) in transgenic fields. Despite the availability of numerous test plants and conditions of high disease pressure but low selection pressure, an epidemic of ZYMV strain MV did not develop in fields of transgenic plants. In contrast, the aphid transmissible ZYMV strain NY was aphid-transmitted to 99% (446/450) of transgenic plants under similar conditions. The relevance of these results in assessing environmental risks of transgenic plants expressing CP transgenes is discussed.