Response of Prunus species to graft-inoculation by two Iranian strains of almond witches'-broom phytoplasma

Almond witches'-broom (AlmWB) is a serious phytoplasma disease of stone fruits; thus far reported from Iran and Lebanon. In the present study the host range of two Iranian AlmWB phytoplasma strains, known as PEAB and AlKI, were examined through graft-inoculation assay. Two-year-old stone fruit seedlings of almond (three cultivars), peach, plum, Myrobalan plum, apricot and sweet cherry were separately graft-inoculated by scions obtained from symptomatic almond seedlings infected by PEAB or AlKI phytoplasma strains. The presence of phytoplasma in the inoculated seedlings, and the type and development of symptoms were evaluated and recorded monthly over one year. Results showed that both phytoplasma strains could be transmitted to almond ('Pooya', 'Mamaei' and 'Talkhe' cultivars), peach and plum seedlings. The disease symptoms were more obvious in almond cultivars. AlmWB phytoplasma strains could not be detected in the inoculated sweet cherry and the Myrobalan plum seedlings. On the other hand, infection of apricot seedlings could only be detected in seedlings inoculated with the PEAB strain. The homologous of the AYWB SAP11 effector protein, known to play a crucial role in phytoplasma infection and symptom expression, was sequence-analyzed in both PEAB and AlKI strains. The SAP11 nucleotide sequence of AlKI strain was identical to that of the SA213 AlmWB phytoplasma strain from Lebanon. Analysis showed high SAP11 sequence diversity between AlmWB phytoplasmas and phytoplasma strains of other 16Sr groups. Despite poor sequence conservation, the structural features and the motifs of SAP11 protein were conserved between AlmWB and distantly related phytoplasma strains; speculating a similar function for this gene in diverse phytoplasmas.

Automated summary

The host range of two Iranian AlmWB phytoplasma strains, PEAB and AlKI, were examined through graft-inoculation assay. Both strains could infect almond, peach and plum seedlings, with more obvious symptoms in almond cultivars. The strains could not be detected in sweet cherry and Myrobalan plum seedlings. In addition, infection of apricot seedlings could only be detected in those inoculated with the PEAB strain.