Evidence of nonspecific transmission of California aster-yellows virus by leafhoppers

Abstract

does not appear. First page follows. Introduction It may be necessary to define two terms before proceeding with this paper. Specificity is the transmission of a particular virus by one or two closely related species of vectors. “When a virus can be transmitted only by one or two closely related insects and cannot be transmitted artificially (except by grafting) the insect transmission is considered obligatory” (Leach, 1940).3 Formerly it was assumed that a specific relation existed between each plant virus and its aphid vector. Among the viruses transmitted by aphids, specificity is now regarded as exceptional rather than common. In the opinion of (Smith (1937)), a rather strong point in favor of a specific relationship between leafhopper and virus lies in the fact that certain plant viruses can be transmitted by only one species. Heretofore it has been assumed that the aster-yellows virus could be disseminated only by the aster leafhopper, Macrosteles divisus (Uhl.) = [Cicadula sexnotota (Foll.)], and by no other means except grafting and budding of the plant. According to (Kunkel (1926)), the intimate and specific relation existing between the aster-yellows virus and its insect vector is important evidence that the causative entity is biological rather than chemical. (Leach (1940)) emphasizes the biological, obligatory, and highly specific transmission of viruses by leafhoppers. There may be some viruses which can be transmitted by only one species of leafhopper. During the past thirty years many species of leafhoppers collected in sugar-beet fields were tested by the author for the transmission of the curlytop virus, but up to the present time no other vector is known except the beet leafhopper, Eutettix tenellus (Baker). According to (DeLong (1942)), the North American fauna of leafhoppers, north of Mexico, consists of 2,000 species belonging to about 150 genera. No other species of the genus Eutettix has been tested up to the present time. It is naturally assumed that closely related species in the same genus should be tested; on the other hand, species in genera belonging to different subfamilies may be vectors. The sugar beet is an introduced plant, and it may be possible that if leafhoppers are collected on native host plants of the virus, other vectors may be discovered. As will be discussed with aster yellows in this paper, host plants highly susceptible to the virus should be used instead of the sugar beet. Herewith is presented evidence that the transmission of the aster-yellows virus by six species of phlepsid leafhoppers is not specific. The characters, distribution, and food plants of these six vectors are discussed in a companion paper (DeLong and Severin, 1945). It is evident in the following review of literature that the prevailing concept has in general favored specificity.