Elm and alder yellows: etiology, symptomatology and epidemiology

Elm yellows (EY) and alder yellows (ALY) are decline diseases associated with the presence of phytoplasmas that affects several Ulmus (elm) and Alnus (alder) species, respectively. EY is widespread in North America and Europe whereas ALY is restricted to Europe. These diseases are mainly associated with the presence of closely related phytoplasmas, the EY agent ‘Candidatus Phytoplasma ulmi’ and the ALY agent, which are members of the EY or 16SrV group, subgroups 16SrV-A and -C, respectively. EY symptoms vary among the elm species. Foliar yellowing, extensive phloem necrosis and death are predominant in North American species. In contrast, European and Asian species are primarily characterized by witches’ brooms, do not show phloem necrosis and are less prone to severe decline. In all ALY-affected species the symptoms are similar and include yellowing, sparse foliage, premature autumn coloration, reduced terminal growth, dieback and decline. However, latent infections are also common in both plant species. The elm leafhopper Scaphoideus luteolus is the insect vector of EY phytoplasma in North America, whereas Macropsis mendax has been reported as vector in Northern Italy. The ALY phytoplasma is transmitted by Oncopsis alni. This leafhopper, which feeds occasionally on grapevine, can transmit ALY strains from alder to grapevine