The vector of apple proliferation behaves more adapted to
phytoplasma infection than the other vectors
By analysing VOCs (volatile organic components) emitted by the leaves of
apple trees, it was shown that ‘Ca . P. mali’ changed the odour of
infected trees compared to healthy ones by inducing the production of
the sesquiterpene β-caryophyllene (Mayer et al. , 2008a,b). The
main vector of ‘Ca . P. mali’, the apple psyllid Cacopsylla
picta , reproduces on apple and overwinters on conifers. The adults of
the new generation (emigrants) are attracted by β-caryophyllene and
lured to infected apple trees (Mayer et al. , 2008b), before
migrating to their overwintering host. This behaviour increases the
number of psyllids, which are able to acquire ‘Ca . P. mali’. By
returning in early spring to apple trees, they prefer healthy apple
trees for oviposition, in order to avoid detrimental effects of the
phytoplasma on the offspring development (Mayer et al., 2011). As they
need to feed before oviposition, they transmit the phytoplasma to
healthy apples. This is a perfect balanced transmitting system, which
improves the spread of the phytoplasma without negatively impact the
vector. A similar adaptation of C. pruni , the migrating vector of
‘Ca . P. prunorum’ is not
known. This species did not distinguish its host plants by odour but
phloem constitution (Gallinger et al., 2019; Gallinger & Gross,
2020). As the development on P. persica infected by ‘Ca .
P. prunorum’ had no detrimental effects on the vector (Gallinger &
Gross, 2020), there is no selection pressure on distinguishing between
infected and uninfected host plants as observed in apple (Mayer et al.,
2011). The two main vectors of ‘Ca . P. pyri’, C. pyri andC. pyricola do not migrate between different host plant species,
therefore volatile signals might be less important in host choice of
this psyllid species (Jarausch et al. , 2019a).