Seasonal colonization pattern of European stone fruit yellows phytoplasmas in different Prunus species detected by specific PCR

The seasonal variation in the colonization of two Japanese plum trees by European stone fruit yellows (ESFY) phytoplasmas was followed by polymerase chain reaction (PCR) detection for 2 years. Samples were obtained monthly from nine above-ground sampling sites and one root. The colonization of the trees was systemic from July until leaf fall. The ESFY phytoplasmas were also detected in off-season grown leaves during winter until March. In contrast, almost no phytoplasmas could be detected in normally grown leaves in April and May. Similar results have been obtained for European plum cultivars using 4',6-diamidino-2-phenylindole (DAPI) staining and PCR and for apricot cultivars tested by PCR. A possible explanation of this phenomenon is discussed. The root system of the corresponding Prunus marianna GF 8-1 rootstocks remained infected throughout the year and the phytoplasmas were equally distributed within the roots as determined when the trees were uprooted. In vitro culture was used to demonstrate that ESFY phytoplasmas detected by PCR in winter in aerial parts of the tree were viable. Nine ESFY-diseased shoot cultures were obtained at four different time points during winter. Sampling protocols for ESFY phytoplasma detection by PCR in routine diagnosis are discussed.