Incidence and control of thrips-borne tomato spotted wilt virus in Dahlia in the Netherlands

In this study on tomato spotted wilt virus (TSWV) in dahlia various factors involved in the control are dealt with. The overall incidence was in 1992: ca. 15%, 1993: ca. 5%, and 1994: ca. 2%. The virus occurred irrespective of shape and colour of flowers, while substantial differences in susceptibility of cultivars were evident. The regional virus rate per holding precluded the overall effect of long-distance virus spread and of abundant virus sources in herbaceous hosts and other ornamental bulb crops. The symptoms were only occasionally clearly recognisable but mostly did not appear at all, or were temporarily masked. Yield of cuttings per tuber was reduced by up to 20%. Virus detection in leaves was erroneous, but reliable if tubers were tested late during storage. Virus transmission by mechanical deflowering of plants by mowing occurred infrequently or not at all. The thrips-vectored transmission under glass proved circumstantially most unlikely. In the open field the thrips plankton in the air and the adult thrips in leaf and flower material yielded twelve species. Franklinietta occidentalis was not found. Thrips tabaci was assumed and F. intonsa was found to be the virus vector in the field. The numbers of thrips in flowers strongly decreased with sprays of deltamethrin and acefate which were more effective than dimethoate. The control of TSWV-spread was most effective with weekly sprays of deltamethrin (83% reduction of spread), and less with dimethoate (reduction 46%). The impact of various control factors, e.g., symptoms, detection by ELISA in tubers, the fairly rapid propagation via cuttings, and the virus spread by virus vector species and curtailment by insecticide sprays, preferably selectively during the field season, is indicated.