Biological characterization of citrus viroid VII

Citrus viroid VII (CVd-VII) was discovered in asymptomatic lemon trees in New South Wales, Australia, in 2015, and has not been recorded in other countries to date. There is a paucity of information known about the biological characteristics of this citrus-infecting apscaviroid. In this study, we investigated the host range, modes of transmission, and interactions of CVd-VII with other viroids. CVd-VII was mechanically transmitted to a range of Citrus species and varieties, causing asymptomatic infections, but a variety of plant species from different botanical families were shown to be non-hosts. CVd-VII was detectable in the seed testa and embryo but grow out tests suggested that it is not seed transmissible. The viroid could be detected in different tissue types from infected lemon and citron plants but at differing titres, which emphasises the importance of sample tissue choice for pathogen detection. The effect of co-infection of CVd-VII with other citrus-infecting viroids was evaluated in a greenhouse trial. Symptoms were more severe in co-inoculated plants, particularly when CVd-VII was co-inoculated with citrus dwarfing viroid and citrus exocortis viroid. This study highlights the potential impact of the viroid and the need to minimise its spread to avoid detrimental effects on citrus tree health and orchard productivity.