Hypo- and hyper-virulence in apricot trees infected by European stone fruit yellows

Abstract

An apricot orchard, located in an area of north eastern Italy under serious pressure from European stone fruit yellows (ESFY) infection, has been monitored since the year it was planted (1990). During this time, most of the trees displayed symptoms or were shown by PCR analyses to be infected. Two groups of apricot trees were particularly interesting: some trees were asymptomatically infected while others recovered from the symptoms but not from the pathogen. In order to isolate those strains of the phytoplasma characterised by varying virulence, each of the two groups was used as mother plants and propagated. The new trees were used to constitute experimental orchards, where they were observed for the presence of symptoms and in part were tested by PCR, starting in 2003. The results obtained confirmed the presence of strains of the pathogen characterised by varying virulence. The strains originally present in infected apricot trees which recovered from the symptoms of ESFY were seen to be hypovirulent; none of the propagated infected trees ever showed symptoms of the disease. Surprisingly, the strains present in asymptomatic apricot mother plants were hypervirulent and the propagated trees always displayed severe symptoms. In the propagated trees, the transmission of the pathogen was higher in the hypervirulent strains than in the hypovirulent ones. A graft transmission trial carried out in the greenhouse using some of the identified hypo- and hypervirulent strains, confirmed the results obtained in open field. Real time PCR analyses showed that in the trees infected by hypovirulent strains the colonisation of the pathogen was lower than in those infected by the hypervirulent strains. It is possible to affirm that the hypovirulent strains were present in those mother plants which had originally recovered. The research will continue with the aim of verifying the possibility of cross protection among the identified hypo- and hypervirulent strains.

Keywords: ‘Candidatus Phytoplasma prunorum’, real-time PCR, Prunus