Natural and experimental transmission of Candidatus Phytoplasma mali by root bridges

Apple proliferation was detected in trees located adjacent in the same row or 'in patches' in affected orchards. Natural root grafts in neighbouring trees were found during uprooting and/or excavations in nurseries and orchards. This indicated the spread of the disease by root bridges. In medium-aged and old apple orchards in Trentino (Northern Italy) this phenomenon seems to play an important role in the disease spread. Trials applying a systemic herbicide in slanting cuts in the bark of infected trees caused herbicide symptoms in neighbouring trees presumably by compound translocation through a root bridge. Furthermore, these trees showed subsequently expression of typical apple proliferation symptoms. Another trial was then conducted under controlled conditions in order to verify the possible transmission of Candidatus Phytoplasma mali by natural or driven root bridges. Sixty healthy one-year-old apple seedlings were put in groups of two plants in 12 litres pots. The roots of the two plants were inserted in a plastic hose (1 cm diameter and 5 cm length) to create a permanent contact. One of the seedlings was then experimentally graft-inoculated with Candidatus Phytoplasma mali. Two years after inoculation of the donor plants 3 out of 25 healthy receptor plants showed clear symptoms of the disease. Specific PCR and immunofluorescence diagnosis confirmed the presence of Candidatus Phytoplasma mali in these three plants. Examining the roots, spontaneous anastomosis was found in two cases outside the plastic hoses and in one case in the "induced contact" area. Histological observations confirmed a tissue connection of sieve tube elements and phytoplasmas could be detected in these sieve tubes by immunofluorescence studies. Thus, Candidatus Phytoplasma mali transmission by natural root bridges could be experimentally demonstrated for the first time.