GENETIC RELATEDNESS OF MYCOPLASMALIKE ORGANISMS AFFECTING ELM, ALDER, AND ASH IN EUROPE AND NORTH-AMERICA

DNA from strain ULW of the European elm witches'-broom mycoplasmalike organism (MLO) maintained in periwinkle was enriched by repeated bisbenzimide-CsCl density gradient centrifugation and cloned in Escherichia coli. Three cloned DNA fragments of the ULW MLO were used as probes to hybridize Southern blots of HindIII-digested DNA prepared from MLO-infected elm, alder, and ash trees and from periwinkle plants infected by various MLOs. Hybridization profiles similar to that of the ULW MLO were obtained with DNA from elm witches'-broom-affected Ulmus minor (U. carpinifolia) and U. glabra collected at various locations in Europe, from American strain EY1 of the elm yellows MLO, from elm yellows affected U. americana and U. parvifolia collected in New York State and Michigan, and from alder yellows affected Alnus glutinosa and A. incana collected in various countries of Europe. Thus, European elm witches'-broom and North American elm yellows, as well as European alder yellows, may be caused by closely related MLOs. The MLOs affecting A. rubra (A. oregona) in Washington State differ from the ULW MLO, because only one of the three ULW probes hybridized with all DNA samples from this species. Two of the probes hybridized to DNA from field-collected samples of ash yellows-diseased Fraxinus americana and from a periwinkle-maintained strain of the ash yellows MLO. However, the sizes of the ash yellows MLO fragments hybridizing to the probes differed from those of the probes. Restriction fragment length polymorphisms were observed among the ash yellows samples. More distantly related to the MLOs occurring in elm and in alder in Europe than the ash yellows MLOs were eight periwinkle-maintained MLO strains from various woody and herbaceous hosts.