GENETIC RELATEDNESS OF MYCOPLASMALIKE ORGANISMS DETECTED IN ULMUS SPP IN THE UNITED-STATES AND ITALY BY MEANS OF DNA PROBES AND POLYMERASE CHAIN-REACTIONS

DNA fragments of an elm yellows (EY) mycoplasmalike organism (MLO) from diseased periwinkle (Catharanthus roseus) were cloned in plasmid vector pSP6 and Escherichia coli strain JM83. DNA probes were prepared by nick translation of EY-specific recombinant plasmids with biotinylated nucleotides. None of the EY probes hybridized with DNA from four representative strains of the aster yellows MLO strain cluster. None of the probes tested at 50 C hybridized with DNA of the MLOs of ash yellows, potato witches-broom, Canadian peach X, clover proliferation, and beet leafhopper-transmitted virescence diseases, but two tested at 42 C hybridized with DNA of one or another of these MLOs. All probes tested hybridized with MLOs detected in Ulmus americana and U. parvifolia in the United States and in U. carpinifolia in Italy, revealing a close relatedness among these MLOs. These data support the recognition of a unique strain cluster, the elm yellows MLO strain cluster, and identification of strains from the United States and Italy as members of this cluster. Polymerase chain reactions using oligonucleotide primer pairs, derived on the basis of the nucleotide sequence of probe pEY11, provided means for sensitive detection of EY MLOs in infected elm tissue and for differentiation among EY MLO variants. Preliminary results indicated the existence of various strains of EY MLOs in North America that were distinct from a strain of EY MLO present in Italy.