Citrus viroid indexing Citrus trees on Poncirus trifoliata rootstock show diverse performance, manifested as variable tree size, shape and appearance of the bud union and stock. These symptoms range in severity from a reticulum of surface cracks on the shoulder below the bud union to hard, persistent bark scaling. There is no evidence that they are transmitted by insects or in citrus seed, but they can be spread from infected to neighbouring trees within an orchard by root grafting. Transmission in nurseries is mainly through use of infected budwood and contaminated budding knives and digging tools, while in orchards, blades of clippers, secateurs and implements such as mechanical hedgers can spread the disease. Fortunately, because of the limited ways in which they can be transmitted, the spread of CEV and CV are easily prevented through the use of clean budwood and simple hygiene techniques.
Viroids are the smallest and structurally best characterised infectious agents presently known.
The viroids in citrus, as is the case with other non-herbaceous crop species such as grapevines and plums, are present in such minute amounts, that it is almost impossible to isolate the viroids directly from field grown trees. Loading plant extracts onto polyacrylamide gels for electrophoretic determination of viroid status. All superior clonal selections were subjected to biological indexing and sPAGE in order to ascertain the viroid status of these trees. The results of viroid indexing using sPAGE are summarised in Table 1. The stringent criteria followed during the selection of superior clones of these citrus cultivars has resulted in all these cultivars testing negative to all the known citrus viroids.