Further studies on the almond witches’ broom in Lebanon
As reported in EPPO RS 2001/094, a new phytoplasma disease (EPPO Alert List) is causing severe losses in almond production in Lebanon. Further studies were done on the identification and phylogenetic relationships of the phytoplasma associated with this new disease. During the last decade, the disease has lead to the rapid decline of almond trees in 3 major almond-producing regions: in the north (cazas of Koura, Zgharta, Tripoli, Akkar), south (Saida, Nabatiye, Jezzine) and the Bekaa region (Zahle). Symptoms include early flowering, stunted growth, leaf rosetting, dieback, off-season growth, proliferation of slender shoots, and witches’ brooms arising mainly from the trunk and roots. Fruit yield is greatly reduced. Fruits are small, dark with shrivelled seeds. Affected trees decline rapidly and die within 3-4 years, So far, thousands of almond trees have been killed by the disease in the three affected regions in Lebanon (ranging from coastal parts to altitudes of 1000 m). As during surveys, the most characteristic symptom found was the proliferation of shoots at several points on the main trunk, the name ‘almond witches’ broom’ was given to this disease. PCR tests showed that phytoplasmas occurred in all almond trees showing disease symptoms. RFLP and phylogenetic analyses of PCR-amplified 16S rDNA showed that almond witches’ broom occurring in Lebanon is associated with a new phytoplasma (belonging to a new subgroup (16Sr IX-B) within the pigeon pea witches’ broom phytoplasma group). This new phytoplasma is distantly related to other stone fruit phytoplasmas. The rapid spread of the disease which affects young and old trees, both in well-managed and completely neglected orchards, suggest the involvement of an aerial vector. Studies are being done to identify possible vectors by trapping leafhopper species living in weeds or crops growing in the vicinity of diseased almond trees. It is also noted that a preliminary survey has indicated that the same phytoplasma could cause diseases in some nectarine and peach seedlings showing shoot proliferations in a field adjacent to an infected almond orchard. It is concluded that almond witches’ broom represents a very serious threat to almond production in Lebanon, and elsewhere. The need for international action to eradicate or prevent any further spread is underlined, as well as the need for improved almond certification and stricter quarantine measures.
Sources
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Abou-Jawdah, Y.; Karakashian, A.; Sobh, H.; Martini, M.; Lee, I.M. (2002) An epidemic of almond witches’-broom in Lebanon: classification and phylogenetic relationships of the associated phytoplasma.
Plant Disease, 86(5), 477-484.
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