Phytoplasmas, Aceria bezzii and drought in declining European hackberry (Celtis australis L.)

Growth abnormalities and decline of hackberry trees have been reported throughout the Mediterranean growth area of the species for over 50 years. Recently, the presence of phytoplasmas belonging to aster yellows and elm yellows groups was detected by molecular techniques in both declining and asymptomatic hackberries. Symptoms of decline appear to be influenced by the amount of rainfall during early spring. In Italy hackberry trees are also infested by the eriophide mite Aceria bezzii (Corti) which kills most of the main, fertile buds causing delayed sprouting of adventitious, sterile buds and loss of fruit production. These three factors seem to interact in determining the sanitary status of this Ulmacea. The widespread phytoplasma presence seems to insidiously weaken trees, which clearly show stress symptoms in drought years, whereas mite infestation acts beneficiously on tree health by reducing the number of sprouting buds and fruit-set, and delaying burst of adventitious buds.