SUMMARY

Potato aucuba mosaic virus and potato virus C were transmitted by the aphid Myzuspersicae, not only from plants also infected with a helper virus, but also from plants infected with em alone, provided the aphids fed first on plants infected with the helper virus. Several different viruses acted as helpers but all are in the potato virus Y group. Helper viruses differed in the efficiency with which they aided potato aucuba mosaic virus and potato virus C, and some potato aucuba mosaic virus isolates were transmitted more frequently than others. With potato virus Y as helper, up to 3o % of the aphids transmitted potato aucuba mosaic virus. Aphids were usually fed for brief periods on plants infected with the helper virus but aphids fed for 2 days also transmitted potato aucuba mosaic virus readily. Starving the aphids for I to 2 hr between their acquisition feeds on plants infected with helper and aided virus decreased but did not eliminate transmission. The helper virus need not be infective; potato aucuba mosaic virus and potato virus C were transmitted as frequently when transmission of the helper virus was prevented by exposing the source leaf to u.v. radiation as when it was not. Virus was not transmitted by aphids fed through artificial membranes on extracts of leaves infected with potato virus Y, potato aucuba mosaic virus or a mixture of the two. However, potato aucuba mosaic virus was transmitted from extracts by aphids fed through membranes when the aphids had previously fed on a potato virus Y-infected leaf. Similarly, potato virus Y was transmitted from leaf extracts by aphids fed through membranes when the aphids had previously fed on a potato virus Y-infected leaf that had been irradiated with u.v. to prevent transmission of potato virus Y from this source. Possible mechanisms for the transmission of the helper and aided viruses are discussed