APPLE PROLIFERATION SUSCEPTIBILITY AND SENSITIVENESS IN OLD APPLE-TREES VARIETIES NATURALLY AND ARTIFICIALLY INFECTED

An apple-tree collection orchard was established in Enemonzo (North-East Italy) in 1992 with 83 accessions of old local varieties. Every accession is represented by 3 plants, and the orchard has been grown according to organic methods. At the end of the ‘90s a severe epidemic of apple proliferation (AP) occurred in the orchard. Diagnosis by PCR confirmed the presence of ‘Candidatus Phytoplasma mali’. A high percentage of plants, with or without AP symptoms on canopy, resulted infected in the roots. During the period 1998–2005, the annual rate of symptomatic plants ranged from 14,6% to 32,9%. Recovery phenomena were often observed. Every year symptoms expression and their severity were recorded for all the plants; symptoms severity was evaluated using a scale ranging from 0 to 3. Depending on the genotype, during the 8 years of monitoring the annual expected probability of an apple tree to exhibit AP symptoms ranged from 0 to 66,6%. The mean degree of symptoms severity during the years in which symptoms were present, ranged from 0 to 2,12. For eleven accessions, not one of the three plants showed symptoms during the observation period. During May 2005, buds of the above mentioned accessions were grafted on AP symptomatic plants cv. Florina. At the end of October, after symptoms observation, leaf samples were collected from the grafted buds and from nearly symptomatic Florina leaves. A semiquantitative ELISA was carried out in order to compare phytoplasma concentrations in the samples. Results showed a high range of phytoplasmas concentration in the tested samples. Concentration of phytoplasmas in the tissues seemed to be more related to the intensity of symptoms than to the different genotypes. Only for the old variety “Blancje di Cattino” all tested samples gave a lower concentration of phytoplasmas if compared to the reference Florina plants. Real–time PCR was also carried out in order to confirm results obtained by semiquantitative ELISA.