A reliable method for Pseudomonas syringae pv. actinidiae detection in kiwifruit explants during in vitro propagation

According to recent statistics, in Italy a significant spread of the bacterial canker of Actinidia caused by Pseudomonas syringae pv. actinidiae (Psa) is recorded with peaks of 70 and 100%, and lower percentages ranging around 50%. The control of the Psa is a priority especially considering the hazard represented by the endophytic bacterial presence in nursery material. To evaluate the risk that could be associated with micropropagated shoots when using asymptomatic mother plants, Psa survival was studied in micro-propagated shoots of A. deliciosa ‘Hayward’ inoculated with different concentrations of a virulent Psa gfp-expressing/rifampicin-resistant strain (Psagfp-Rifres). Microbiological and molecular analysis were carried out at each in vitro transfer and on rooted plantlets in greenhouse. The Psagfp-Rifres reisolation on selective media confirmed by PCR analysis and by the ability to induce symptoms in Actinidia and HR in tobacco plants, was achieved in both the micropropagated materials and in the plants analyzed in toto after their re-establishment. A reliable and efficient method for Psa detection in nursery material is proposed to verify the presence Psa infection before symptom development.