Yellow‐net virus of sugar beet. I. Transmission and some properties

A virus found in sugar beet in England causes similar symptoms to the Californian virus called yellow‐net (YNV) by Sylvester (1948). It persists in the feeding vectors for at least 7 days. The minimum feeding time needed for aphids to acquire virus from infected leaves was 6 hr. and the minimum needed to acquire and transmit was 12 hr., but ability to transmit was greatly increased by increasing feeding time up to 2 days on infected and 24 hr. on healthy plants. These properties agree with those of the American virus. Experiments using different aphid numbers feeding for constant times suggested that susceptibility of sugar beet to infection varies and this may account for some of the delay in infecting healthy plants.

YNV was accompanied in beet by a second virus, yellow‐net mild yellows (YNMYV). YNMYV did not infect Nicotiana clevelandi Gray or N. bidlovia, which are susceptible to YNV, but from which YNV could not be recovered by aphids. Neither virus is transmissible by inoculation with sap from infected plants. It is suggested that YNMYV is a ‘carrier’ virus, without which YNV cannot be transmitted. Possibly it combines with YNV to form an aphid‐transmissible combination, but it may merely help YNV to invade sugar beet.

YNMYV causes similar symptoms and has similar properties and host range to other viruses that cause mild yellowing diseases of sugar beet, including avirulent strains of beet yellows virus (BYV), Irish mild yellows (IMSV) and ‘41 yellows’ (41 YV). No such virus has been reported associated with YNV in America, and possibly it is peculiar to the British isolate.