Surveying Potential Vectors of Apple Proliferation Phytoplasma: Faunistic Analysis and Infection Status of Selected Auchenorrhyncha Species

SIMPLE SUMMARY: Apple proliferation disease is one of most important threats for European apple cultivation. It is caused by the bacterium ’ Candidatus Phytoplasma mali’ which is transmitted by the psyllids Cacopsylla picta and Cacopsylla melanoneura in South Tyrol. Since no curative treatment is available against the disease, uprooting of infected trees and phytosanitary measures against the known insect vectors are the only strategies to prevent a spread of the pathogen. Interestingly, densities of the known vector insects and disease manifestation in do not always correlate, indicating that other insect vectors might be involved in the transmissive cycle. To elucidate if other insects are involved in ’ Ca. Phytoplasma mali’ transmission, a faunistic characterization of the Auchenorrhyncha community of the agroecosystem “apple orchard” was performed and potential vectoring insect species were PCR-analyzed to unravel if they carry the pathogen. Several Auchenorrhyncha species were identified that have never been described before in the territory of South Tyrol, but ’ Ca. Phytoplasma mali’ was detected only sporadically in species other than the known vectors. The results provide valuable insights into the Auchenorrhyncha species composition in apple orchards, confirm the prevalent role of the known vectors in the region and are thus an important basis for phytosanitary decision-making. ABSTRACT: Apple proliferation (AP) is one of the economically most important diseases in European apple cultivation. The disease is caused by the cell-wall-less bacterium ’ Candidatus Phytoplasma mali’, which is transmitted by Cacopsylla picta (Foerster) and Cacopsylla melanoneura (Foerster) (Hemiptera: Psylloidea). In South Tyrol (Italy), severe outbreaks were documented since the 1990s. Infestation rates of AP do not always correlate with the population densities of the confirmed vectors, implying the presence of other, so far unknown, hemipterian vectors. By elucidating the species community of Auchenorrhyncha (Insecta: Hemiptera) at a regional scale, more than 31,000 specimens were captured in South Tyrolean apple orchards. The occurrence of 95 species was confirmed, whereas fourteen species are new records for this territory. Based on the faunistical data, more than 3600 individuals out of 25 species were analyzed using quantitative PCR to assess the presence of AP phytoplasma. The pathogen was sporadically detected in some individuals of different species, for example in Stictocephala bisonia Kopp and Yonk (Hemiptera: Membracidae). However, the concentration of phytoplasma was much lower than in infected C. picta and C. melanoneura captured in the same region, confirming the role of the latter mentioned psyllids as the main insect vectors of AP- phytoplasma in South Tyrol.