Situation of elm yellows associated with ‘Candidatus Phytoplasma ulmi’ in Europe
Elm yellows is a widespread and serious phytoplasma disease of elm trees, in particular in the eastern half of the USA. It was reported in the 1930s and initially called ‘elm phloem necrosis’. The disease causes a rapid, general decline of American elms, such as Ulmus americana and U. rubra, which lead to the death of infected trees usually within one growing season after the onset of symptoms. Symptoms include leaf yellowing and wilting, death of branches. The inner phloem of the lower trunk and roots develops a butterscotch colour and finally turns necrotic. Because such symptoms had never been observed in Europe, elm phloem necrosis was added to the EPPO A1 List in the 1970s. Until the 1950s, elm yellows was thought to be restricted to North America but few reports of a disease called elm witches’ broom were made from the former Czechoslovakia, Italy and France on Eurasian elm species (e.g. U. glabra, U. minor, U. pumila). In 2004, it was considered that elm yellows was associated with a new tentative phytoplasma species belonging to the elm yellows group and called ‘Candidatus Phytoplasma ulmi’. Finally, recent studies have confirmed that ‘Ca. Phytoplasma ulmi’ occurs in the USA and in several European countries but the diseases which are associated with it on the two continents are nevertheless very different.
In Europe, the disease was often referred to as ‘elm witches’ broom’, as symptoms usually involved mild leaf yellowing, reduced growth, and witches’ broom. In European countries, symptoms are not conspicuous, disease reports are very occasional, and the disease does not seem to spread rapidly. In grafting experiments, Eurasian elms appear to be tolerant to the disease as they can develop witches’ brooms but not phloem necrosis. Another difference concerns the insect vectors of the disease. In the USA, the main vector is Scaphoideus luteolus (Homoptera: Cicadellidae – EU Annexes), a species which has not been recorded in Europe. Studies carried out in Italy concluded that the main vector in Europe was probably Macropsis mendax (Cicadellidae) and that other species such as Philaenus spumarius (Cercopidae) and Allygidius atomarius (Cicadellidae) could play a minor role in disease transmission. In addition, during preliminary studies conducted in France, ‘Ca. Phytoplasma ulmi’ was detected in Iassus scutellaris (Cicadellidae), Cixius sp. (Cixiidae), Allygidius furcatus (Cicadellidae) but transmission still remains to be demonstrated. The geographical distribution of ‘Ca. Phytoplasma ulmi’ in Europe needs to be further studied, but for the moment the phytoplasma itself or disease symptoms have been recorded from the following countries:
- Czech Republic: ‘Ca. Phytoplasma ulmi’ was detected during surveys carried out from 1997 to 2007 in South Moravia. Affected elm trees showed witches’ broom, smaller leaves, partial leaf yellowing, and irregular twig growth but the disease was rare.
- France: 1 strain was isolated on U. minor near Avignon in the 1990s. Surveys conducted in 1998-2000 showed that symptoms of elm yellows were quite frequent in the elm conservatories of Nogent-sur-Vernisson (Loiret, region Centre) and Guémené-Penfao (Loire-Atlantique, region Pays de la Loire), where approximately 30% of the trees observed were mildly symptomatic. In addition, ‘Ca. Phytoplasma ulmi’ was detected in a small number of samples collected from symptomatic elm trees in Loire-Atlantique, Franche-Comté (U. glabra), and Basse Normandie (U.minor).
- Germany: at least 1 phytoplasma strain was isolated near Stuttgart on U. glabra in the 1990s.
- Italy: elm yellows has been recorded from several regions (e.g. Emilia-Romagna, Friuli-Venezia Giulia, Lombardia, Toscana) on Ulmus glabra, U. minor, and U. pumila. In 2006, ‘Ca. Phytoplasma ulmi’ was detected for the first time on bonsai plants of Ulmus parvifolia and Zelkova serrata in a nursery near Ancona (Marche region). Affected plants showed total or partial leaf chlorosis, foliar reddening, stunting, and witches’ brooms. This was the first record of elm yellows in bonsais, and in Zelkova.
- Serbia: in September 2007, mild leaf yellowing symptoms were observed on 18 elm trees near the villages of Srednjevo, Ljubičevo and Šuvajić (north-east Serbia). Molecular tests confirmed the presence of ‘Ca. Phytoplasma ulmi’ in samples of U. minor (from Srednjevo and Ljubičevo) and in samples of U. laevis (from Šuvajić).
Comparison studies between elm inhabiting phytoplasmas from USA and Europe have showed that they were very closely related, although some minor differences could be observed (when other gene sequences were compared in addition to 16SrRNA). For the moment, it seems that elm yellows observed in USA and Europe might be associated with different strains of the same species ‘Ca. Phytoplasma ulmi’ (rather than with different species of phytoplasmas). The differences observed in the field between the two continents might be explained by different epidemiological situations (e.g. susceptibility of host trees, insect vectors) but more studies are needed to verify these assumptions. Finally, the implications these new findings may have on the phytosanitary status of ‘elm phloem necrosis’ probably need to be discussed within EPPO.
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