SYSTEMIC MIGRATION OF PSEUDOMONAS-SYRINGAE PV AVELLANAE IN TWIGS AND YOUNG TREES OF HAZELNUT AND SYMPTOM DEVELOPMENT

At the beginning of October 1992 and 1993, healthy adult and young tre es of hazelnut, cultivars Tonda Gentile Romana and Nocchione, were ino culated with a Pseudomonas syringae pv. avellanae strain through leaf scars located midway from the tip of the twig. In adult trees, the sys temic migration and the population level of the pathogen were monitore d during autumn and winter by means of weekly to monthly isolation and countings. In young plants, the possible displacement of the bacteriu m was checked in May by sampling different sites of the plant. At each sampling, the presence of internal and external symptoms was carefull y assessed. During the 2-year study, the results on migration and symp tom development were consistent between the cultivars. Until the end o f February, P.s. pv. avellanae colonized for approximately 45 mm, only the trait of the twig below the site of inoculation. At that time the population level ranged, in the farthest point reached by the pathoge n, from 4.1 to 5.6 = 10(4) cfu/3 mm of twig-portion. During March the pathogen was recovered from the tip of the twig. The first sign of dis ease was noticed in November as brown discolouration of the epidermis near the site of inoculation as well as internal necrosis of the vascu lar tissue. In young trees, P.s. pv. avellanae from the twig reached t he branches, collar and roots from where it also colonized the branch which did not carry an inoculated leaf scar. Extensive twig dieback wa s noticed during April, whereas in June, most of the adult trees inocu lated in autumn with approximately 25 000 cells of the pathogen, rando mly distributed in 25 sites of the crown, wilted completely. The role of leaf scars as a favourite site of penetration exploited by the path ogen as well as phytosanitary measures are discussed.