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Phytoplasmas (formerly mycoplasma-like organisms, MLOs) are pleomorphic, cell wall-less bacteria of the class Mollicutes that exist as obligate plant pathogens.

Recoginition

Phyllody and flower malformation appear usually in April/May and are easy to recognize in the field. Over the season, farmers can observe shoot proliferation or light green leaf development, but they normally do not associate such symptoms with a disease that cannot be controlled by using pesticides, and frequently treat the phytoplasma-infected trees with ineffective pesticides.

Related invasive species

  • Candidatus Phytoplasma phoenicium

Related Farm Practice

  • Development
  • Organisms
  • Production
Impact

Phytoplasmas are wall-less parasitic bacteria living exclusively in plant phloem as consequence of transmission by sap-sucking insect vectors (Lee et al., 2000);they have been associated with several hundred plant diseases. ‘ Candidatus Phytoplasma phoenicium’ (CaPphoe), subgroup 16SrIX-B, is the aetiological agent of almond witches’-broom (AlmWB), a severe disease affecting almond, peach and nectarine trees in Lebanon and Iran. The first epidemics of AlmWB occurred in almond trees in Lebanon in the early 1990s and in Iran in 1995. In Lebanon, the disease rapidly spread from coastal to high mountainous areas, killing almost 150,000 trees over a period of 15 years. CaPphoe was first added to the EPPO Alert List in 2001 and removed from the list in 2006. The more recent rapid spread of CaPphoe in peach and nectarine orchards and in other plant hosts, along with the identification of efficient insect vectors, increased the alarm about the risk it poses for stone fruit production in the Middle East and in all the countries of the Mediterranean basin. Thus it was re-inserted in the EPPO Alert List in 2015.

Has Cabi datasheet ID
118193
Symptons

The most characteristic symptoms caused by AlmWB on almond trees are (i) shoot proliferation on the main trunk with the appearance of a witches’-broom, (ii) the perpendicular development of many auxiliary buds on the branches, with smaller and yellowish leaves, and (iii) the general decline of the tree with final dieback. A total loss of production happens 1-2 years after the initial appearance of the symptoms (Abou-Jawdah et al., 2002).
In the case of peach and nectarine trees, the first symptom observed is early flowering (15 to 20 days earlier than normal), followed by the earlier development of all the buds of the infected branches. In addition, phyllody at the flowering period and serrate, slim, light green leaves and witches’-brooms developing several months later from the trunk and the crown of the trees are observed (Abou-Jawdah et al., 2009). Even if the presence of witches’-broom is more common in almond trees than in peach/nectarine, the most important difference between peach/nectarine and almond symptoms is the development, in peach/nectarine trees, of phyllodies, never recorded on almond (Molino Lova et al., 2011).

Hosts

All almond (Prunus dulcis) varieties in the almond growing areas of Iran and Lebanon have been affected by AlmWB disease, but some varieties (e.g. Alwani and Awja in Lebanon and Sangi in Iran) are highly susceptible and develop severe witches’-brooms, leading to rapid death of the tree, while other varieties (e.g. Kachabi) are less affected (Verdin et al., 2003;Choueiri et al., 2001). Grafting experiments and molecular analyses indicate that AlmWB does not affect plum (P. domestica) or cherry (P. avium) trees (Abou-Jawdah et al., 2003). Nevertheless, its rapid spread on almond, peach (P. persica) and nectarine (P. persica var. nucipersica) orchards indicate a risk for epidemics in Lebanon and in the other countries of the Mediterranean area.
In Iran, ‘ Ca. P. phoenicium’ has not been identified in peach and nectarine but rather in other plant hosts, such as GF-677 (P. amygdalus × P. persica) and wild almond (Prunus scoparia) (Salehi et al., 2015), and more recently in apricots (P. armeniaca), in which it has been found to cause apricot yellows (Salehi et al., 2018).
Smilax aspera L. and Anthemis sp. are plant hosts preferred by the cixiid insect vectors Tachycixius viperinus (T. viperina) and T. cf. cypricus (T. cypricus) (Tedeschi et al., 2015). The leafhopper vector Asymmetrasca decedens was found feeding on Prunus scoparia (Salehi et al., 2015).

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