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Peach latent mosaic viroid Detected for the First Time on Almond Trees in Tunisia

Almond (Prunus dulcis Mill) is an important crop in countries of the Mediterranean area. Until now, among viroids, only Hop stunt viroid (HSVd) is known to infect cultivated almond trees (2). In 2004, a survey of almond trees was carried out in orchards in different regions of Tunisia, a major producing and exporting country of almond. Symptoms such as mosaic and necrotic lesions, potentially caused by the Peach latent mosaic viroid (PLMVd), were observed on leaves of cultivated almond trees. Since PLMVd was recently detected in peach and pear trees in Tunisia (4), the presence of this viroid in almond trees was studied. The detection method on the basis of one-tube reverse transcription-polymerase chain reaction (RT-PCR) assays was previously described and validated for the detection of this viroid in fruit trees (4). Amplification products were obtained by using previously reported primer pairs of PLMVd (1). Positive controls included RNA preparations of twigs of PLMVd-infected GF 305 peach seedlings. These materials, provided by B. Pradier (Station de Quarantaine des Ligneux, Lempdes, France), were positive as revealed by chip budding on peach seedling indicator plants grown under greenhouse conditions. RT-PCR analysis of nucleic acid preparations from leaves of almond showed specific amplification products with the expected size of 337 bp for two almond trees among 17 trees tested. Nucleotide sequence analyses of cloned amplification products obtained with the PLMVd primers confirmed a size of 337 bp and revealed a sequence similar to sequences from other PLMVd isolates previously characterized. The sequences shared 94 to 98% identity with the reference isolates of PLMVd from peach (EMBL Accession No. M83545, AF170511, AF170514, and AY685181). The two infected almond trees are proximal to each other and peach trees infected with PLMVd. This suggests that one tree may have served as a source of inoculum for the other through agronomic practices such as pruning or the aphid Myzus percicae (3). Alternatively, PLMVd may have originated in an unknown host and was then transmitted to almond trees. Our investigation shows that almond is a new host for PLMVd.

References: (1) N. Astruc. Eur. J. Plant Pathol. 102:837, 1996. (2) M. C. Cañizares et al. Eur. J. Plant Pathol. 105:553, 1999. (3) J. C. Desvignes et al. Phytoma 444:70, 1992. (4) I. Fekih Hassen et al. Plant Dis. 88:1164, 2004.