EVALUATING TRANSMISSION RISKS OF POSPIVIROIDS

During the past decade, pospiviroids have been repeatedly reported in many ornamentals and crops all over Europe. The commercial trade of latently pospiviroid-infected plants all over the world implies major phytosanitary risks. To what extent insects, naturally occurring weeds and viruses play a role in the transmission of these pathogens was not well known. In order to gain a better insight into pospiviroid epidemiology and transmission risks, various experiments were organized for different types of interactions. To study interactions between pospiviroids and insects, fluorescence in-situ hybridization (FISH) experiments were conducted to localize and quantify viroids in/on insects. In aphids (Myzus persicae) that were fed on pospiviroid-infected plants, viroid signals were observed in the aphid’s stylet and digestive system, but not in the embryos. In subsequent transmission experiments, M. persicae was not able to transmit the viroid to healthy host plants. Also insects from other functional groups were tested as potential vectors: i.e. Bombus terrestris (pollinators) and Macrolophus pygmaeus (biological control agents). The percentage of each insect species that tested positive for pospiviroids in a specific q(RT)-PCR was 100% for M. persicae, 4% B. terrestris, and 0% for M. pygmaeus. The transmission rate of pospiviroids was 0% for M. persicae and M. pygmaeus, and 2.6% for bumblebees. To examine the risk of pospiviroid spreading from weeds to economically important crops, a survey in commercial greenhouses, a contact-experiment and an inoculation-experiment were organized. The results from these experiments show that only 5% of the inoculated weed species are potential hosts for viroids, and that most of the pospiviroid “wild” hosts are species belonging to the families of the Solanaceae and the Asteraceae. Lastly, the interaction of viroids with other plants viruses was investigated, and more specifically the phenomenon of “transencapsidation”, where viroids are incorporated within a viral particle. While previous reports indicate that virus-assisted transmission of pospiviroids between plants could happen, this has not been observed during our experiments. Finally, using phylogenetic analyses, we investigated the origin of more than 30 Belgian pospiviroid isolates and found that the country of origin, and the type of host plant, are important clustering factors. The results from this comprehensive study will be broadly discussed in the context of EU phytosanitary measures for pospiviroids.