Phytoplasmas affect the vascular morphology of apple trees more than peach and pear trees.
Although apple trees are known to survive for decades with a phytoplasma infection (Seemüller et al., 2018), many significant declines were found on the level of leaf (width, length, midrib), tissue (vascular bundle, phloem and xylem) and cell (sieve elements) sizes (Figures 1 and 2). In contrast, pear and peach trees showed less significant differences between healthy and phytoplasma-infected leaves, if at all, we found significant increases for leaf size and midrib ratio for peach and leaf width for pear (Table 3). That is surprising as one might expect that plants with a higher tolerance and survival rate would show a lower rate of symptoms than plants having demonstrably a higher mortality (Marcone & Rao 2019). Additionally, the recovery phenomenon, describing the remission of symptoms and the disappearance of phytoplasmas in the crown, was observed for both apple and apricot trees (Prunus armeniaca ), but not pear trees (Carraro et al., 2004; Musetti et al., 2013). Hence, the morphological and physiological changes can be considered representing the ability to handle a phytoplasma infection and might be the result of a selective adaptation.