Genetic relatedness of phytoplasmas of elm, alder and ash in Europe and North America
Diseases of many forest and amenity trees are induced by phytoplasmas. In eastern and central North America, several species of elm (Ulmus americana, U. rubra, U. alata, U. serotina and U. crassifolia) are affected by elm phloem necrosis (EPPO A1 quarantine pest), also called elm yellows. In Europe, a disease associated with elm witches' broom phytoplasmas has been observed in U. minor (U. carpinifolia) in former Czechoslovakia, Italy and France. Alder yellows have been observed in Alnus glutinosa and A. incana in Europe and on A. rubra in Washington State (USA). In United States and in south-eastern Canada, several ash species (Fraxinus sp.) are affected by ash yellows. The aim of this study was to compare DNA samples (by Southern blot analysis) from phytoplasma-infected elm, alder and ash trees located in Europe and North America and also to compare them with strains of several phytoplasmas maintained in periwinkle (Catharanthus roseus).
Three cloned DNA fragments of a strain of the European elm witches' broom phytoplasma maintained on periwinkle were used as probes to hybridize with DNA prepared from phytoplasma infected elm, alder and ash trees and also from periwinkle plants infected by various phytoplasmas. The results showed that European elm witches' broom and North American elm yellows (elm phloem necrosis phytoplasma), as well as European alder yellows, are genetically related and may be caused by closely related phytoplasmas. In addition, they are distinct from all other phytoplasmas tested in this study. Ash yellows appeared to be slightly more distantly related to elm and alder yellows, it is nevertheless more closely related to elm and alder phytoplasmas than to all other phytoplasmas included in this comparison. The authors concluded that not only elm yellows (elm phloem necrosis) and elm witches' broom are caused by closely related phytoplasmas (see also EPPO RS 94/065), but very similar organisms also affect alder in Europe. They felt that the most appropriate name for the pathogen affecting elms would be elm yellows. But, although similarities have been found, differences among the tested material, such as host specificity, have not been detected with the characterization methods used. Further studies are therefore necessary for a better understanding of the identity, host specificity, variability and epidemiology of such phytoplasmas, as this may have implications on plant quarantine.
Mäurer, R.; Seemüller, E.; Sinclair, W.A. (1993) Genetic relatedness of Mycoplasmalike Organisms affecting elm, alder and ash in Europe and North America.
Phytopathology, 83 (9), 971-976.