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Viroids are the smallest known pathogens of plants. They are single-stranded, circular, rod-like RNAs with no protein capsid no any detectable messenger activity. Citrus is one of the most important commercial fruit crops grown in Morocco. Seven viroids reported to infect Citrus spp. belong to four genera of the family Pospiviroidae. Hop stunt viroid (HSVd) belongs to genus Hostuviroid and consists of a 295-303 nucleotides circular single-stranded RNA. It has been found in a wide range of hosts including several woody and herbaceous crops. Cachexia, an economically important disease of citrus, has been associated with HSVd infection. HSVd isolates are divided into five phylogenetic groups according to the sequence variants: plum-type, hop-type, citrus-type, plum-citrus-type and plum-hop-citrus-type. In this work we present the molecular characterization of six new sequence variants of HSVd obtained from the main Moroccan citrus growing areas: Gharb, Haouz, Loukkos, Moulouya, Souss and Tadla, respectively. The genetic diversity among the Moroccan variants was investigated. Phylogenetic analysis showed that Moroccan citrus HSVd variants were clustered into one group within the citrus-type and the sequence variability seems neither related to the host nor to symptomatology. Despite an accumulation of nucleotide changes, the secondary structure of the viroid mol­ecule was conserved. Since citrus viroids are readily mechanically transmissible via working tools, they affect both old and young plantings in all the citrus groves surveyed.

Keywords: Hop stund viroid, Survey, Molecular characterization, Morocco