Experiments in a glasshouse have shown that the most favourable conditions for transmission of cabbage black ringspot virus by Myzus persicae (Sulz.) to turnip plants with two foliage leaves were as follows: an acquisition access period of five minutes; a test-feeding period of not less than two hours; the use of nine viruliferous aphids per test plant; the use of aphids from uncrowded colonies; the use of the third leaf from the base of an infected turnip (5-leaf stage) as virus source; and inoculation of the younget leaf of a test plant. Darkening plants for 24 or 48 h did not increase susceptibility to aphid inoculation, but increased it to mechanical inoculation in the summer. When the youngest leaf was inoculated, the age of the test plants (10-25 days after germination) did not influence transmission by aphid or mechanical inoculation.

Transmission of cabbage black ringspot virus to turnip plants by Myzus persicae (Sulz.) was reduced by foliar sprays of five parts per million gibberellic acid together with 0·2% ammonium nitrate, but not by each applied separately. The reduction occurred at 10°C but not 15, 20 or 25°C, and at a light intensity of 10 000 lux but not at 5 000 lux. The response was usually only evident towards the end of the normal range of the incubation periods, indicating that protection was probably only achieved when relatively small amounts of inoculum were introduced. The reduced transmission could not be related to aphid behaviour but changes were found in the free proline content and fine surface structure of treated leaves, and it is suggested that the results are to be interpreted in terms of changes in host cell metabolism.