engleski

Elm yellows and Dutch elm disease in Croatia

Elm yellows (EY) is a disease caused by phytoplasmas, non-helical, wall-less, unculturable bacteria of the class Mollicutes and provisional genus ‘Candidatus (Ca.) Phytoplasmaʼ. ‘Ca. Phytoplasma ulmi’ (16SrV-A subgroup) is predominantly infecting elms in Europe, although other phytoplasmas were occasionally detected as well. Dutch elm disease (DED) is caused by ascomycete of the genus Ophiostoma. First DED pandemic was caused by less aggressive species, O. ulmi, and the second, ongoing pandemic is caused by more aggressive species, O. novo-ulmi. In Croatia, DED has spread in the early 20th century and decline of elms observed since then is mainly associated with this disease, while phytoplasmas causing EY have not been studied. However, DED was mainly detected based on external and internal disease symptoms without detailed investigation of the DED pathogen. In this study, samples of Ulmus laevis, U. minor and U. glabra from across Croatia were analyzed. Phytoplasmas were detected by 16S rRNA gene amplification and sequencing in 64 out of 139 samples. Three phytoplasmas were identified: ‘Ca. Phytoplasma ulmi’ (16SrV-A subgroup), ‘Ca. Phytoplasma asteris’ (16SrI group) and ‘Ca. Phytoplasma solani’ (16SrXII-A subgroup), but the infection was most frequently caused by ‘Ca. Phytoplasma ulmi’. Dutch elm disease was confirmed by isolation of the fungus from 55 out of 139 trees and results of fungal growth rate and morphology test indicated that infections were exclusively caused by O. novo-ulmi. Furthermore, for the first time it was proven that phytoplasmas and O. novo-ulmi can cause mixed infection in natural elm populations.

elms; phytoplasma; phytopathogenic fungi; pathogen detection; mixed infection

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