Infection Of Almond Trees In Lebanon By A New Phytoplasma Disease

Elia Choueiri⁽¹⁾ Eric Verdin⁽²⁾ Jean L. Danet⁽²⁾ Fouad Jreijiri⁽¹⁾ Souheir El Zammar⁽¹⁾ Ibrahim El Harfouch⁽¹⁾ Salah D. Issa⁽¹⁾ Pascal Salard⁽²⁾ Joseph M. Bové⁽²⁾ and Monique Garnier^(2)
(1)Lebanese Agricultural Research Institute, Tal Amara, Rayak, Zahle, PO Box 287, Lebanon
E-mail: echoueiri@lari.gov.lb
⁽²⁾Institut de Biologie Végétale Moléculaire, INRA et Université de Bordeaux 2, BP-33883, Villenave d'Ornon Cedex, France

Abstract

Almond (Prunus amygdalus) has been cultivated in many regions in Lebanon since a long time especially in the Bekaa valley, in the north and south of Lebanon. In October 1999, during a survey conducted to evaluate the sanitary condition of stone fruits in Lebanon, symptoms of leaf yellowing, shoot proliferation and dieback were noticed on almond trees in the Bekaa region. In spring 2000, almond growers faced a problem of severe almond trees decline. Samples (cuttings and leaves) were collected and tested serologically for the presence of PNRSV, ApMV, PDV, ACLSV, CLRV, PPV, ToRSV and SLRV, but none was detected. The most prevalent symptom noticed on diseased trees was witches'-broom. Since such symptoms are often associated with phytoplasma diseases, samples were analyzed by PCR at the Molecular Laboratory of INRA-Bordeaux using universal primers for amplification of phytoplasma ribosomal RNA genes. DNA was extracted from the leaf midveins and/or bark phloem tissue from symptomatic and symptomless trees as well as from healthy trees. PCR resulted in amplification of an expected 1.8 kbp rDNA fragment from all symptomatic samples but not from the healthy or symptomless samples. The amplified DNA was further analyzed by RFLP for characterization. The restriction profiles were different from those of other phytoplasmas specially those infecting almond trees in Western Europe. Sequence analysis of the amplified DNA showed the almond phytoplasma to be a member of the pigeon pea witches' broom group (PPWB). The most severely affected areas were Raskifa and Dair Amar in the north of Lebanon. Trees died within two years after first symptoms appearance. The rapid spread of the disease and its phytolasma etiology suggests the presence of an insect vector. Epidemiological studies of the disease are being carried out to determine the vector in order to reduce the incidence of the disease and adopt an integrated control. This is the first report of a phytoplasma infection in Lebanon and the first report for a PPWB group phytoplasmas in almond trees