Submitted to: Acta Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/14/1998
Publication Date: N/A
Interpretive Summary: Dapple apple viroid (DAVd) is associated with a fruit-blemishing, graft-transmissible disease of apple which occurs in several countries in the Northern hemisphere. Attempts to transmit the viroid to herbaceous hosts have been unsuccessful. Graft inoculation onto woody hosts results in infection and disease symptoms, but it is unclear if DAVd is responsible for symptom formation. In this study, infectious complementary DNA copies of DAVd were delivered to susceptible woody hosts via a bacterial plasmid vector inoculated onto the stems of pear and apple seedlings. Systemic infection resulted from the inoculation, thereby providing the basis for further studies on role of DAVd in fruit symptom development. The results of this study will be of interest to scientists and USDA action agencies, such as APHIS, who are studying and indexing for woody host pathogens by demonstrating that an alternative means of inoculation can be used to determine the role of pathogens in the disease process.
Technical Abstract: Dapple apple viroid (DAVd) is member of the apple scar skin viroid (ASSVd) group of viroids. Attempts to transmit these viroids to herbaceous hosts have been unsuccessful (Osaki et al., 1986); transmission to woody hosts has been primarily by grafting from infected materials. In order to complete Koch's postulates and to show the DAVd is responsible for the systemic infection in infected trees, cloned dimeric copies of DAVd were inserted into a binary vector containing the CaMV35S promoter, which was subsequently transformed into Agrobacterium strain LBA4404. Seedlings of "Gala" apple and Western pear were agroinoculated on their stems with a solution of the Agrobacteria containing the plasmid pBI121:DAVd construct. Seedlings were kept in a growth chamber and monitored for viroid infection by dot and Northern blot hybridization analyses. A few seedlings of pear and apple became systemically infected with DAVd and the viroid progeny were confirmed by nucleotide sequence analysis to be identical to that of the inoculum.