Long-term survival of Pseudomonas syringae pv. actinidiae in asymptomatic micropropagated plants of Actinidia deliciosa

In recent years, in Italy the spread of bacterial canker of kiwifruit caused by Pseudomonas syringae pv. actinidiae (Psa) is creating serious problems, especially as regards the early detection of the pathogen in the micropropagated plant material from which originate most of the plants that are planted in new orchards. Indeed, the use of micropropagation techniques overcomes the seasonal limitations encountered with cuttings or grafting and it is favoured when a large number of plants is required in a short time. The possibility to assess innovative nursery techniques allowing a reliable and fast detection of Psa in asymptomatic micropropagation material is essential to prevent the further spread of the pathogen which can remain latent for a long time at very low concentrations. To evaluate the Psa survival in micropropagated material, over three years ago the apical part of micropropagated shoots of Actinidia deliciosa cv. Hayward were artificially inoculated with a virulent Psa gfp-expressing/Rif-resistant strain (Psa::gfp) and used as explants for the production of kiwifruit plantlets. In the following 10 months, 7 generations of shoots were obtained in which the presence of Psa::gfp was confirmed. After the multiplication and rooting phase, the plantlets were transferred to the greenhouse for re-establishment. The microbiological and molecular analyses of the plants three years after the inoculation of the shoot tips showed that Psa::gfp was able to survive endophytically without inducing the disease symptoms. The development of a method for detection of Psa at low inoculum dose in asymptomatic nursery material has been developed.