Avocado sunblotch viroid (ASBVd) is a circular and single-stranded RNA viroid (genus Avsunviroid) with a genome of about 250 nucleotides in length (Roberts et al., 2023). It causes sunblotch disease in infected avocado. The viroid is transmitted through infected propagation material, pollen and seed. ASBVd has been reported in most avocado-producing areas and causes significant reductions in yield and fruit quality (Carabez et al., 2019).

  In December 2022, plants of avocado (Persea americana) cv Hass exhibiting cracked bark, multiple sunken yellowish crevices on the fruits and leaves with variegated areas were observed in an orchard in Murang'a County, Kenya. The disease incidence was about 5% in the 3.1 hectare field. Ten diseased fruits were sampled to be tested for ASBVd. Four fruits from asymptomatic trees were also sampled.

  Total RNA was extracted from both diseased and asymptomatic samples using CTAB. A two-step RT-PCR assay was done using the extracted RNA and ASBVd-specific primers which amplify 88 to 104 nucleotides (Schnell et al., 1997). The amplified products were visualised using gel electrophoresis and amplicons of the expected size were obtained from all diseased fruits. No amplification was obtained from the asymptomatic fruits. The amplicons from three samples were Sanger sequenced bidirectionally and found to be identical (GenBank Accession No. OQ871564). A BLASTn search of the sequence revealed that it shared 99.37% identity with an isolate. from Uruapan, Mexico (KF562705). A phylogenetic tree was constructed using the Maximum Likelihood method (MEGA X program) with ASBVd sequences from different geographical locations.During pest and disease surveys in summer 2017, severe leaf drops were observed in four citrus orchards, two sweet orange (cvs Thompson Navel and Washington Navel) and two mandarin (cvs Nova and Oro Grande) orchards, located in Chlef, one of the most important citrus region in Algeria. Visual inspection of green dropped leaves and leaves on tree canopy revealed unusual symptoms consisting of greasy black protuberant spots on the abaxial side and yellowing on the adaxial surface. A quick survey was conducted to determine the incidence of the disease. The proportions of trees affected by the disease were 50 and 35% for the sweet orange and mandarin orchards, respectively. The incidence of the diseased leaves ranged from 30 to 60% for sweet orange, while 50–70% of leaves were symptomatic in the mandarin orchards. Morphological identification and a multi-locus sequencing approach associated this disease to the fungus Neodidymelliopsis ranunculi. Koch's postulates also confirmed N. ranunculi to be the causal agent. This is the first report of black spot symptoms caused by N. ranunculi on citrus in Algeria. The rapid spread of the disease since its first detection, poses a serious threat to the sustainability of citrus production in Algeria and calls for the implementation of urgent management actions.

  Source of information:https://www.sciencedirect.com/science/article/abs/pii/S0261219423002454