Apricot yellows associated with 'Candidatus Phytoplasma phoenicium' in Iran

Almond witches' broom associated with 'Candidatus Phytoplasma phoenicium' is an economically important disease of almond in Iran and Lebanon. During surveys of almond witches' broom in 2012-2015, an apricot yellows disease was observed in Fars Province of Iran. The characteristic symptoms of the disease were leaf yellowing, inward leaf curl, scorch of leaf margins, shortened internodes, production of rosettes at the tips of the branches, and decline, stunting, and death of affected trees. Healthy bitter almond and apricot seedlings, grafted with shoots from symptomatic trees, exhibited phytoplasma-type symptoms. A 16S rDNA fragment of 1,250 bp was amplified by nested-PCR from affected trees and grafted seedlings. Nucleotide sequence identity, presence of species-specific signature sequences, and phylogenetic analysis of 16S rDNA allowed the assignation of the phytoplasma strains identified to the 'Ca. P. phoenicium'. In vitro and in silico RFLP analyses of the amplified fragment allowed affiliation of the apricot yellows phytoplasma to a molecular variant in the subgroup 16SrIX-B. Within the population strains identified in this and previous studies, 16 genetic lineages were determined within 16S rDNA nucleotide sequences by the combination of 19 single nucleotide polymorphisms. The apricot yellows phytoplasma strains belong to a unique genetic lineage distinguished by the presence of three lineage-specific SNPs. This first report of 'Ca. P. phoenicium' in association with apricot yellows in Iran opens new perspectives on the epidemiology of almond witches' broom, suggesting possible adaptation of the phytoplasma to other fruit tree species.