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.