RESEARCH ON CANDIDATUS PHYTOPLASMA MALI TRANSMISSION BY INSECT VECTORS IN TRENTINO

Since 1996, a thorough research has been carried out in Trentino (Northern Italy) in order to identify the insect vectors of Candidatus Phytoplasma mali. Phloem-sucking insects like leafhoppers and psyllids have been investigated for their role in phytoplasma transmission. Among the psyllid species found on apple trees, Cacopsylla melanoneura and Cacopsylla picta were the most abundant. Since 1998, transmission trials have been carried out with C. picta, C. melanoneura and with the leafhopper Empoasca vitis. No transmission of Candidatus Phytoplasma mali occurred with Empoasca vitis. The transmission efficiency of C. melanoneura was very low. Only 0,36% of the test plants showed symptoms and were tested Phytoplasma-positive by ELISA. C. picta was the most efficient vector and transmitted the Candidatus Phytoplasma mali repeatedly. Of the test plants, 4,1% became infected in transmission trials with C. picta. Based on these results a continuous survey of the psyllid species present on cultivated apple trees in Trentino was carried out. The life cycle of both vector species on apple was described and different control strategies were tested. C. melanoneura completes one generation per year and is present in the apple orchards from end of January to middle of May - middle of June. Also, C. picta performs only one generation per year and remains in the apple orchards from end of March–beginning of April until July. The natural transmission period in the orchards was investigated by the exposure of bait plants in four different orchards from 4th February 2002 to 3rd October 2002 in bi-weekly or monthly intervals. Infected test plants were found in the period between 11th June and 24th June and between 24th June and 3rd October. Thus, the migration period of the new generation of C. picta seems to be the period with the highest risk for AP-transmission in the orchards.