Importance of psyllids’ life stage in the epidemiology of apple proliferation phytoplasma

Phytoplasmas, microorganisms associated with severe plant diseases, are obligate parasites transmitted by phloem-feeding insects. Cacopsylla melanoneura and Cacopsylla picta are involved in the transmission of ‘Candidatus Phytoplasma mali,’ the etiological agent of apple proliferation (AP) disease. Research conducted in different geographic regions showed different transmission efficiencies for the two psyllids. In this study, acquisition and transmission trials were carried out to investigate the role of different life stages of these vectors in the epidemiology of AP after a sudden outbreak in northeastern Italy. Both species resulted able to acquire ‘Ca. P. mali,’ with higher infection rates recorded in C. picta. F1 generations showed a higher acquisition ability compared to adults in both species. ‘Ca. P. mali’ transmission was successful: Up to 1.5% of test plants were infected in trials with C. melanoneura and up to 10.2% in trials with C. picta. Overwintered adults of C. melanoneura showed a lower vectoring ability compared to C. picta. F1 nymphs and F1 adults, developed on infected plants, resulted in being competent vectors. Data on phytoplasma acquisition suggest a different relationship of ‘Ca. P. mali’ with the two species, evidencing a stronger affinity with C. picta. Moreover, taking into account the different factors influencing AP transmission, the probability of infection is mainly influenced by the presence of F1 nymphs and the phytoplasma load. In conclusion, this study evidences that C. picta is the most effective AP vector in the studied area, even if C. melanoneura can represent a potential risk in the presence of high inoculum sources.