Plant material and phytoplasma inoculation
Apple trees (Malus domestica ) cv. ‘Gala Royal’ were grown on clonal rootstock cv. ‘M9’ (non-infected control, n=14), pear trees (Pyrus communis L.) cv. ‘Williams Christ’ were grown on cv. ’Kirchensaller Mostbirne’ rootstocks (non-infected control, n=5), and peach trees (Prunus persica (L.) cv. ‘South Haven’ were grown on peach seedlings cv. ‘Montclar’ (non-infected control, n=4). Additional apple, pear and peach plants were inoculated by grafting of two buds from trees infected with the respective phytoplasmas. Apple trees were infected with a virulent accession (3/6, n=13) in 2017 (Seemüller et al., 2010, 2011, 2013). Pear trees were infected with ’Ca. P. pyri’ (PD-W, n=5) in 2012. This strain causes mild symptoms but no quick decline (Seemüller et al., 1986), peach trees were infected with ’Ca. P. prunorum’ (ESFYQ06, n=4) in 2017. Experiments with pear and peach trees were conducted in 2018. Apple trees were investigated in 2019. All plants were grown under natural conditions in an insect safe environment.