Detection of Mycoplasmalike Organisms in Peach and Chokecherry with X-Disease by Fluorescence Microscopy

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Abstract
Colonization of peach (Prunus persica) and chokecherry (P. virginiana) by the mycoplasmalike organism associated with X-disease (XMLO) was examined throughout two growing seasons in relation to external symptom development. Leaf midveins and petioles were sampled biweekly in 1983 and weekly in 1984. Peach trees were visually rated for symptom severity using a rating scale 0-4, where 0, 10, 50, 90, and 100% of the canopy exhibited X-disease symptoms. Samples were fixed in glutaraldehyde and sectioned with a cryostat. Tissue sections were stained with the DNA-specific fluorochrome 4''-6-diamidino-2-phenylindole (DAPI) and examined in blind tests. Presence or absence and relative abundance of fluorescent particles (DAPI-stained DNA of XMLO) varied with sampling date, date of initial symptom expression, overall symptom severity, and host species. Observations made by fluorescence microscopy were verified by electron microscopy. XMLO were found in all peach and chokecherry samples from trees that became symptomatic, but none were detected in samples from trees that remained apparently healthy. XMLO-invasion of sieve tubes was more extensive in severely than in mildly symptomatic peach and was often evident up to 6 wk before external symptoms were expressed. In comparison, XMLO-invasion of symptomatic chokecherry was always earlier and more extensive than that of peach. XMLO were consistently detected before abnormalities developed in the phloem.