Relationships between phytoplasmas associated with elm yellows
Phytoplasma diseases causing elm yellows have been described in North America and in Europe. In USA, significant outbreaks of a lethal yellows occur sporadically on indigenous elms (Ulmus americana, U. rubra, U. alata, U. serotina and U. crassifolia), but elms of Eurasian origin are rarely damaged by this disease. In Europe (notably in Italy and France), several phytoplasma strains have been identified on declining Eurasian elm species and hybrids. In most cases, it has been shown that these phytoplasma strains belonged to the Elm Yellows group sensu lato (group 16SrV). In addition to elms, phytoplasmas of this group occur in many other plant species: e.g. Apocynum, Prunus, Rubus, Vitis, Ziziphus. Molecular studies (RFLP analysis of 16S rDNA, plus 16S-23S spacer region for some samples) have been carried out to obtain a better understanding of the relationships between these phytoplasmas. The study included: 9 phytoplasmal DNA samples obtained from symptomatic Ulmus in USA and Italy (U. americana, U. rubra, U. minor (= U. carpinifolia), U. chenmoui and a hybrid clone Lobel); 1 sample obtained from an Apocynum cannabinum plant growing near an outbreak of elm yellows and infected by a phytoplasma belonging to the 16SrV group; 5 samples obtained from previous studies and collected from other plants (Rubus fruticosus, Vitis vinifera, Ziziphus jujuba, Prunus).
Results showed that although all these strains belong to the 16SrV group there is some variation among them. Strains from Ulmus can be differentiated from strains collected on other hosts. In particular, the strain from Apocynum cannabinum collected near diseased elms showed a unique RFLP pattern, and was more closely related to strains from Vitis vinifera and Rubus fruticosus than from elms. Among phytoplasmas associated with elms, European strains are related to but different from North American ones. It appears that there is more variation among American phytoplasmas than among European ones. A comparison was made between the Italian strains and an already known French strain, and they appeared similar. The authors concluded that their study supports the concept that phytoplasmas associated with elm yellows compose a taxonomically discrete but non-uniform sub-group (16SrV-A). More studies are needed.
Note: So far, the lethal disease associated with phloem necrosis symptoms in USA has been attributed to elm phloem necrosis phytoplasma. This pathogen was listed as an EPPO A1 quarantine pest, assuming that it was different from phytoplasmas associated with elm yellows in Europe. Molecular studies appear to support this hypothesis, but more work is still needed in this rapidly evolving area of phytoplasma taxonomy.
Sources
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Griffiths, H.M.; Sinclair, W.A.; Boudon-Padieu, E.; Daire, X.; Lee, I.M.; Sfalanga, A.; Bertaccini, A. (1999) Phytoplasmas associated with elm yellows: molecular variability and differentiation from related organisms.
Plant Disease, 83(12), 1101-1104.
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