Occurrence of Peach Latent Mosaic Viroid (PLMVd) on Plum in Italy

Peach latent mosaic disease was originally described in France in 1976 (1). The disease is often latent in peach trees but may induce mosaic symptoms on leaves, irregularly shaped colorless fruit with cracked sutures and enlarged pits, bud necrosis, and delay in foliation, flowering, and fruit maturity. Recently, the disease has been shown to be caused by peach latent mosaic viroid (PLMVd) (2). PLMVd is distributed worldwide on peach. A recent survey in Italian plum orchards has showed the occurrence of PLMVd in two plum cultivars (Black Diamond and Angeleno) at different geographical locations in central Italy. Plum samples showed bark necrosis and bark split on branches and trunk; moreover, 1-or 2-year-old shoots and stem appeared dwarfed because of shortening of internodes and reduced growth. Budwood from plum samples were grafted, in greenhouse, onto peach GF305 indicator plants. Total nucleic acids (TNA) were then extracted from peach GF 305, or plum leaves, after 6 months of postinoculation. The presence or absence of PLMVd was ascertained by bidirectional polyacrylamide gel electrophoretic (dPAGE) analysis and by spot hybridization assay with PLMVd cRNA probe provided by A. Hadidi. A dPAGE analysis of TNA extracted from grafted peach GF305 showed a band with the same electrophoretic mobility as that of D-168 strain of PLMVd positive control. No bands were observed in healthy peach GF305 or from healthy or diseased plum extracts. The lack of detection of a viroid band from diseased plum samples by dPAGE analysis may be due to low titer of the viroid in infected plum. Molecular hybridization analysis of total nucleic acids with PLMVd cRNA probe, however, showed positive hybridization signals with nucleic acids from grafted peach GF305 or infected plum. Thus, PLMVd or a very closely related viroid infects plum trees. Moreover, on the basis of biological and serological assays, apple chlorotic leaf spot trichovirus, in a sample, and prunus necrotic ring spot ilarvirus, in the second one, were found in association with PLMVd. The same symptoms observed in the two PLMVd-infected plums occurred in other plants infected only by the two viruses (alone or in association). These data suggest that, very probably, PLMVd does not play a specific role in the etiology of the field symptomatology described above. In any case, this is the first report for the occurrence of PLMVd on plum in Italy. References: (1) J. C. Desvignes. Acta Hortic. 67: 315, 1976. (2) R. Flores and G. Llacer. Acta Hortic. 235:325, 1988.