Evaluation of PVdI transmission through top working infected trees

The aim of this project was to determine if rootstocks from infected trees can be a source of PlVd-I infection if top-worked. Although graft transmission has been demonstrated when infected scion material was bud grafted to healthy trees, the reverse has not been demonstrated.

Two African delight orchards on a commercial farm were identified for this study. Additional trees from another orchard were also identified as healthy budwood donors. All symptomatic trees tested positive for PlVd-I and healthy trees tested negative. Symptomatic trees were cut off below the graft union. As soon as sufficient new growth from the rootstock was available, healthy buds were grafted on the rootstock’s new growth. The successful grafts’ new growth was sampled later in the season and tested for PlVd-I.

Test sites and trees were identified and validated for the grafting trial. Different obstacles were encountered due to the experimental site being in a commercial orchard, however sufficient results were obtained. In only one of the successful grafts, PlVd-I was not detectable in the new growth.

This study showed conclusively that topworking PlVd-I infected material is not a viable option. New trees will be infected and PlVd-I is not only graft transmitted between different scions, but also between scion and rootstock

This study showed conclusively that topworking PlVd-I infected material is not a viable option. The majority of new trees will be PlVd-I positive in the first season.