The distribution of mollicutes in periwinkle plants (Catharanthus roseus (L.) G. Don) that had been graft-inoculated with the clover proliferation (CP) agent or the potato witches'-broom (PWB) agent was studied by in situ molecular hybridization with biotinylated single-stranded DNA probes and by DNA staining of hand sections of petioles and stem internode tissues with DAPI (4',6-diamidino-2-phenylindole). In situ hybridization with the biotinylated probes was specific, detecting only the CP

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... PWB agents but not the western aster yellows (AY27) agent, which was detected by DAPI. The mollicutes were detected first in the external primary phloem tissue and then in the recently infected secondary phloem elements. Later, the spread of mollicutes was detected in the internal phloem tissue. In situ hybridization was highly specific and sensitive in detecting, without background signals, mollicutes in young sieve elements. However, the complete absence of hybridization signals and of DNA staining with DAPI coincided with degeneration of affected sieve tubes in the primary phloem elements.