Root Grafting and Mechanical Transmission of Citrus Exocortis Viroid Within a Citrus Budwood Multiplication Block

Abstract

In 1968 a closely planted budwood multiplication block was established at the Agricultural Research Centre, Dareton using fully indexed clones. In 1985, during routine re-indexing of 860 mother trees, some lemon trees were found infected with citrus exocortis viroid (CEV). In 1970, an imported Monroe Lisbon clone had been planted in the block. This Lisbon clone was later found to be infected with CEV, and was removed in 1973. No hedging or pruning took place before its removal. As the oldest infections occurred immediately adjacent to those Monroe Lisbon trees (within 2 m), it is possible that transmission by root grafting occurred. Later contamination of other lemon trees, further removed from the original infections, was probably by mechanical transmission of CEV on secateurs and hedging equipment. Biological indexing on citron 60-13 or Arizona 861 and hybridisation of CEV cDNA probes to RNA dot blots have been used to re-index all trees. The results were generally similar, but some specific discrepancies occurred. The role of root grafting in transmission of CEV was studied by excavating root systems.