Abstract
Abstract does not appear. First page follows. Summary Five species of aphids, tested in lots of 20, were demonstrated to be vectors of mild-mosaic virus of annual stock; these are: Bur clover or cowpea aphid, Aphis medicaginis Koch Cabbage aphid, Brevicoryne brassicae Linnaeus Artichoke aphid, Myzus braggi (Gillette) Green peach aphid, Myzus persicae (Sulzer) Turnip or false cabbage aphid, Rhopalosiphum pseudobrassicae (Davis) Of these, only the turnip aphid breeds on annual stock plants under natural conditions. It causes pale-green circular areas around the mouth-part punctures, and, when abundant, dwarfing and yellowing or blanching of the flowers. The turnip aphid failed to transmit the virus to nine varieties of healthy cauliflower. Several infections were obtained with single turnip and green peach aphids fasted for 2 hours, fed ½, 1, 2, 5, or 10 minutes on leaves from infected stock plants, and then transferred to healthy plants, 1 aphid per plant. No infections were obtained with several hundred turnip, green peach, and cabbage aphids tested singly without fasting and with longer periods on infected stock plants. In tests on retention of the virus, turnip aphids, fasted for 30 minutes, then fed singly 5 or 10 minutes on mild-mosaic-infected annual stock and 5 or 10 minutes on 5 or 6 successive healthy stock plants, produced infections only in the first healthy plant. Lots of 20 turnip aphids had lost their infectivity by the fourth day after transfer from an infected to a previously healthy stock plant. If allowed to remain on the plant from 7 to 13 days, however, the aphids were able to recover the virus from the plant they had inoculated; this was long before symptoms appeared. The incubation period of the disease in the original inoculated plants varied from 16 to 22 days.
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