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 some weeks (quick decline of pear) to a few years (Fiore et al., 2019; Marcone et al., 2010; Marcone & Rao 2019; Seemüller et al., 1986, 2018). This indicates that the M. domesticaCa. Phytoplasma mali´-system is more adapted and balanced in their interaction than P. persicaCa. P. prunorum´ and P. communisCa. P. pyri´ pointing to a higher tolerance and survival rate for phytoplasma-infected apple. We collected an extensive set of data (summarized in Table 3) to elucidate anatomical and physiological responses of each plant species to a phytoplasma infection, supporting a co-evolutionary impact.