Transmission of ‘Candidatus Phytoplasma mali ’ by root bridges under natural and experimental conditions

Transmission of ‘Candidatus Phytoplasma mali ’ could be demonstrated under experimental as well as under natural conditions in apple orchards in Trentino (Northern Italy). Experimental transmission by root bridges was obtained in a trial with 30 groups of two apple seedlings potted together. One seedling of each group was inoculated by grafting with ‘Ca. P. mali ’ in 2003 and the plants were kept under insect proof conditions. In 2006, three years after inoculation, 4 out of remaining 25 healthy test plants showed typical apple proliferation symptoms and tested positive for ‘Ca. P. mali ’ by specific PCR and immunofluorescence analysis. In all four cases the formation of root bridges between inoculum plant and receptor plant could be detected. Natural transmission by root bridges was studied in an 11-year’s old apple orchard by treating cut apple proliferation-infected trees with the herbicide glyphosate in autumn 2005. In spring 2006, neighbouring trees showed not only herbicide symptoms suggesting root bridges but also typical apple proliferation symptoms indicating phytoplasma transmission. ‘Ca. P. mali ’ was detected by specific PCR in 57 % of trees with herbicide symptoms. Key words: apple proliferation, epidemiology, immunofluorescence, anastomosis, root nodule