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SUMMARYPotato 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 them 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.

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SUMMARY Beet mild yellowing virus (BMW) was reversibly precipitated at temperatures below about 5°C and this property was used as a final step in a purification procedure which yielded about 1 mg virus/kg tissue. Purified virus was infective and had an A200/A280 ratio of about 1–8. BMW particles were isometric with a diameter of 26 nm, sedimented at 116 S, had a buoyant density in caesium chloride of 1.42 g/cm3 and a coat protein mol. wt of 25 400. An antiserum to BMW had a titre in immunodiffusion tests of 1/256 and was used in immunodiffusion tests, immunospecific electron microscopy (ISEM) and enzyme‐linked immunosorbent assay to demonstrate a close serological relationship between BMW and beet western yellows virus. BMW was readily detected by ISEM in plants and also in aphid vectors after treatment of aphid extracts with a chloroform:butanol mixture.