Necrotic yellows: A newly recognized virus disease of lettuce

Abstract

A previously undescribed virus disease of lettuce, for which the name lettuce necrotic yellows is proposed, occurs in epiphytotic proportions in Victoria and, to a similar or lesser extent, in parts of Queensland, New South Wales, and South Australia. Infected plants are extremely chlorotic and have a flattened appearance. They exhibit varying degrees of flaccidity and necrosis, and the mortality rate may be high. Chronically affected survivors have small, slightly distorted, but otherwise normal heart leaves. The tomato spotted wilt virus, which causes an almost identical disease in lettuce, lacks the "recovery" phase. The virus was sap-transmissible from infected lettuce or sowthistle (Sonchus oleraceus L.) to several indicator species, but not to lettuce or sowthistle. Lettuce has been infected on a few occasions with inoculum from Nicotiana glutinosa L. There is no evidence that the virus is transmitted through lettuce or sowthistle seed. The virus was inactivated in N. glutinosa sap between 52 and 54°C, was relatively short-lived in vitro (1–8 hr), and its dilution end-point was close to 10-2. Symptoms are described on known experimental hosts, including N. glutinosa, petunia, and spinach, that are regarded as the best differential indicators. Several lettuce-infesting insects—thrips (Thrips tabaci Lind. and Frankliniella schultzei (Tryb.)), leafhoppers (Orosius argentatus Evans), and aphids (Myzus persicae (Sulz.) and Macrosiphum euphorbiae (Thos.))—failed to transmit the virus from infected lettuce or sowthistle. Transmission was achieved with the aphid Hyperomyzus lactucae (L.) bred on infected sowthistle, the only known source in Victoria of both the vector and the virus. The virus persisted through a moult of the vector, and thus the mode of transmission is of the circulative type. H. lactucae has not previously been recognized as a vector of a "circulative" virus. It is considered unlikely that lettuce necrotic yellows virus is indigenous to Australia, because sowthistle, the only known natural host of both the virus and the vector, is an introduced species.