Phytoplasmas of apricot and Japanese plum.

Apricot (Prunus armeniaca L.) and Japanese plum (Prunus salicina L. spp) are the cultivated fruit trees species most sensible to phytoplasmas (Goidanich et al., 1980). The most commonly detected phytoplasma associated with these two species is the European stone fruit yellows (ESFY or 16SrX-B) phytoplasma, recently named “Candidatus Phytoplasma prunorum”, (Seemüller and Schneider, 2004); despite a 16r RNA gene sequence that shows more than 97,5 % similarity to those of other “Candidatus phytoplasma” mainly detected in fruit trees such as pear and apple, this phytoplasma is characterized by distinctive biological, phytopathological and genetic properties, therefore it was possible to describe it a as a separate Candidatus species (IRPCM, 2004). Previous molecular studies on ESFY phytoplasma demonstrated similarity or identity of phytoplasmas inside the Apple proliferation 16S ribosomal group (16SrX). In particular the ESFY phytoplasmas in apricot, Japanese plum, European plum, and in flowering cherry and almond in European cultivations were shown to be indistinguishable at the molecular level (Seemüller et al., 1998). The chromosome map of a German ESFY strain was realized extracting phytoplasma DNA from Nicotiana glutinosa and/or from some tomato cultivars, after transmission of the pathogen to these species by dodder bridges (Marcone and Seemüller, 2001). In other studies the virulence variability of ESFY phytoplasma strains was observed in field infected plants, mainly apricots, and the presence of avirulent or low virulent strains was proposed together with the idea of presence of non-specific phytoplasmas which could attenuate virulent ESFY phytoplasma strains (Seemüller, 1999). Other phytoplasmas were reported to infect apricot or Japanese plum: in Spain a molecularly distinguishable 16S ribosomal subgroup of aster yellows (16SrI-F, Lee et al., 1998a) was transmitted to periwinkle from apricots, in Italy 16SrIII-B and/or 16SrX-B phytoplasmas were dodder transmitted to periwinkle from symptomatic plums (Carraro et al., 1992). Recently, 16SrX-B phytoplasmas were identified from both symptomatic and asymptomatic Japanese plum, in areas where the presence of these phytoplasmas is very strong: testing 40 samples from 11 Japanese plum varieties, collected in June, from symptomatic plants, about 23% showed positive results in the first nested PCR reaction and 92% in the second, performed with 16SrX group specific primers 16Sr(X)F1/R1. When the same plants were retested in October, 39 out of the 40 samples were positive in direct PCR. However testing 57 asymptomatic Japanese plums, about 20% of the samples was positive to ESFY presence, moreover in other symptomatic samples phytoplasmas belonging to groups 16SrXII-A, 16SrI-B and 16rIII-B were identified (Paltrinieri et al., 2004).