Discussion
Although the phytoplasmas ‘Candidatus Phytoplasma mali´, ‘Ca. P. prunorum´ and ‘Ca. P. pyri´ belong to the same 16SrX group, their pathogenicity is quite different in their respective host plants Malus domestica (apple), Prunus persica(peach) and Pyrus communis (pear). Apple trees can survive a phytoplasma infection for decades, whereas phytoplasma-infected peach and pear trees often die after a few years and sometimes even after a few weeks (quick decline of pear) (Fiore et al., 2019; Marcone et al., 2010; Marcone & Rao 2019; Seemüller et al., 1986, 2018). This indicates a higher tolerance resulting in better survival rate for phytoplasma-infected apple compared with pear and peach; however, the underlying reason is not known yet. To address this open question, we collected a comprehensive dataset (summarized in Table 3) covering anatomical and physiological responses of each plant species to its particular phytoplasma infection, supporting a co-evolutionary impact. Based on these results we are discussing several implications thereof in the following paragraphs.