Transmission of Potato Leaf Roll

Abstract

IN his letter in NATURE of July 19, p. 96, Dr. Kenneth Smith appears to be under the impression that I ‘deplore’ the importance attached by virus workers to Myzus persicæ as a vector of potato leaf roll. The importance of this insect in this respect was not called into question in my letter in NATURE of June 28, but rather “the growing tendency … to regard the relation of M. persicæ to leaf roll transmission as specific and unique”. Dr. Smith reminds me that in May 1929 he expressed the opinion that M. persicæ is probably not the only carrier of leaf roll. In his latest publication, however, (Jour. Min. Agric., June 1930, p. 227) he emphasises the similarity between leaf roll transmission by M. persicæ and other plant virus diseases in which it is said that one, and only one, insect is capable of carrying the virus. M. persicæ, he states, has a marked affinity for several potato viruses, especially for the leaf roll virus, whilst “other insects such as capsid bugs, leaf-hoppers, and the remaining species of aphides, have failed to transmit the diseases”. Here is evidence of the growing tendency referred to in the mind of at least one virus worker. Although proof is lacking, I think it quite probable that M. persicæ bears a more subtle relationship to leaf roll transmission than that of a mere mechanical agent, but this relationship, whatever it may be, would appear to be shared to some extent by M. circumflexus. So far from minimising the importance of M. persicæ in leaf roll transmission, in my letter I attributed little or no importance to M. circumflexus as an active agent in spreading virus diseases.