Studies on the epidemiology of almond witches’ broom (AlmWB) phytoplasma, spread in South Lebanon, vector transmission and potential alternative hosts -

Abstract

Almond witches’ broom (AlmWB) phytoplasma appeared in North Lebanon in the early nineteen nineties. Molecular diagnosis showed that the phytoplasma belongs to the pigeon pea 16S rDNA group IX, and it was named ‘Candidatus Phytoplasma phoenicium’. AlmWB phytoplasma caused death to more than 100,000 almond trees in addition to a large number of nectarine and peach trees. This work was conducted in order to study the epidemiology of AlmWB phytoplasma disease, mainly extent of spread, potential insect vectors and alternative hosts in order to find and develop efficient strategies to contain the spread of this disease. PCR tests performed on almond, peach and nectarine samples confirmed that AlmWB is widely spread in South Lebanon and that two out of 64 samples collected from nurseries were also infected. A map for spread of AlmWB was drawn. For the second objective, PCR tests confirmed the presence of phytoplasma belonging to 16S rDNA group IX in the following wild plants: Pistacia palaestina, Rhamnus punctata, Bryonia sp., Amaranthus, and Quercus. However, preliminary PCR testing with AlmWB specific primers gave a negative result. Further tests are required to prove if these weeds or other weeds may serve as alternative hosts for AlmWB. Malaise and yellow sticky traps were placed in AlmWB infested orchards; out of 58 leafhopper samples collected, 18 samples belonging to five genera were positive by PCR using AlmWB semi-specific primers, namely Asymmetrasca decedens, Blacanocerus ramallaphicus, Megophthalmus scabripennis, Psammotettix gr. provincialis and, Thamnotettix wittmeri. Leafhoppers (Asymmetrasca decedens) and plant hoppers (Cixiidae) were collected from AlmWB infested trees and allowed to feed on healthy seedlings. A total of 16-34 seedlings inoculated by A. decedens showed atypical symptoms. PCR tests confirmed the presence of AlmWB phytoplasma in symptomatic seedlings as well as in the salivary glands of A. decedens providing a strong indication that this insect is a vector of AlmWB. Se