Potato leafroll virus (PLRV) (genus Polerovirus, family Luteoviridae) is of great economic importance. It has a very narrow host range consisting of only 21 known species, 14 of which are Solanaceae. Ten aphid species transmit PLRV in a persistent manner, but the green peach aphid (Myzus persicae Sulzer) is by far the most important vector and the only important vector in the Columbia Basin of the Pacific Northwest (3). Solanum sarrachoides (Sendtner) may be especially important in the epidemiology of PLRV because it is a predominant weed in potatoes (S. tuberosum L.), other annual crops, and waste areas of the Columbia Basin (1), and herbicides available for potato are relatively ineffective against this weed (2). Since our research began in 1974, we have routinely observed many S. sarrachoides plants throughout the Columbia Basin that acquired an abnormal erect stiff appearance in middle to late summer. Their leaflets curled upward at the margins and expressed interveinal chlorosis and necrosis and a reddened or purple color on abaxial surfaces. The presence of PLRV in symptomatic plants was routinely detected by double-antibody sandwich enzyme-linked immunosorbent assay and by density gradient sedimentation and light absorbance properties of the purified virus when we evaluated the host as a source of purified PLRV in 1980. A systematic study to satisfy the requirements of Koch's postulates to prove that PLRV causes the disease in S. sarrachoides was conducted in 1998. The virus produced typical PLRV symptoms on three young Physalis floridana, Datura stramonium, and potato plants when transmitted by the green peach aphid from each of four symptomatic S. sarrachoides collected from widely spaced potato fields in the Columbia Basin and from purified virus preparations from these plants. Back-transmission from each of these hosts and from purified preparations to S. sarrachoides plants observed the same symptoms. The green peach aphid readily colonized S. sarrachoides in the field, and populations on this host were typically higher than on adjacent potato plants in potato fields. The concentration of PLRV was approximately equal in infected S. sarrachoides and P. floridana plants, as was the efficiency of virus transmission from these hosts by individual green peach aphids. The aphids used in transmission efficacy assays were all the same clone and were reared on the host species assayed. The incidence of PLRV infection among S. sarrachoides in potato fields was typically higher than in the potato crop. Only two other summer annual hosts of PLRV, Amaranthus caudatus and S. nigrum, occur sporadically in potato fields of the Columbia Basin (1). The virus rarely infected these species. To our knowledge, this is the first report that S. sarrachoides is highly susceptible to PLRV and may play an important role in PLRV epidemiology in the Columbia Basin.
References: (1) A. G. Ogg, Jr. and B. S. Rogers. Rev. Weed Sci. 4:25, 1989. (2) L. S. Quakenbush and R. N. Anderson. Weed Sci. 33:386, 1985. (3) P. E. Thomas et al. Plant Dis. 81:1311, 1997.