Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/2/2014
Publication Date: N/A
Citation: Interpretive Summary: Liberibacter is a new and economically important bacterium that severely damages several crops including potato in the Americas and New Zealand and carrots in Europe. This plant pathogen is transmitted to these crops by psyllids, serious insect pests in the United States. Researchers at USDA-ARS Wapato in Washington, in collaboration with scientists at the Institute for Agricultural and Fisheries Research in Belgium and the National Institute for Agricultural Research in Morocco, discovered time that this bacterium was infecting carrot crops in Morocco, posing a serious threat to the vegetable industry in Africa. Information from this research will help affected carrot producers in Africa and elsewhere reduce damage caused by this important plant pathogen by effectively monitoring and controlling its psyllid insect vectors to prevent spread of the bacterium.
Technical Abstract: In March of 2014, carrot plants (Daucus carota L. var. Mascot) exhibiting symptoms of yellowing, purpling, and curling of leaves, proliferation of shoots, formation of hairy secondary roots, general stunting and plant decline were observed in commercial fields in the Gharb region of Morocco. The symptoms resembled those caused by phytoplasmas, Spiroplasma citri, or ‘Candidatus Liberibacter solanacearum’ infection. About 30% of the plants in each field were symptomatic and plants were infested with unidentified psyllid nymphs; psyllids are known vectors of ‘Ca. L. solanacearum’. A total of 10 symptomatic and 2 asymptomatic plants were collected from three fields. Total DNA was extracted from petiole and root tissues of each of the carrots, using the CTAB buffer extraction method. The DNA samples were tested for phytoplasmas and spiroplasmas by PCR but none of these pathogens were detected in any of the samples. The DNA extracts were then tested for ‘Ca. L. solanacearum’ by PCR using specific primer pairs OA2/OI2c, Lso adkF/R and CL514F/R, to amplify a partial fragment of the 16S rDNA, the adenylate kinase gene and rpIJ/rpIL 50S rDNA ribosomal protein genes, respectively, of ‘Ca. L. solanacearum’. DNA samples from all 10 symptomatic carrots yielded a specific band of 1168 bp for the 16S rDNA fragment, 770 bp for the adenylate kinase fragment and 669 bp for rpIJ/rpIL, indicating the presence of ‘Ca. L. solanacearum’. No ‘Ca. L. solanacearum’ was detected in the 2 asymptomatic plants. DNA amplicons of three plant samples (one plant/field) with each primer pair were directly sequenced (Macrogen Inc., Amsterdam, The Netherlands). BLAST analysis of the 16S rDNA amplicons resulted in a single consensus sequence (deposited in GenBank as Accession No. KJ740159) that showed 99% nucleotide identity to the 16S rDNA sequence of ‘Ca. L. solanacearum’ from carrots in Finland (Accession No. GU477255). A single consensus sequence was also obtained from the adk amplicons and was deposited in GenBank as Accession No. KJ740162. The sequence was 98% identical to the analogous adk sequence of ‘Ca. L. solanacearum’ from Texas (Accession No. CP002371). Similarly, the rpIJ/rpIL amplicons resulted in a single consensus sequence (deposited in GenBank as Accession No. KJ754506) that was 100% identical to the sequences of the analogous rpIJ/rpIL ribosomal protein gene of ‘Ca. L. solanacearum’ from mainland Spain (Accession No. HQ454310) and the Canary Islands (Accession No. HQ454321). To our knowledge, this is the first report of the occurrence of ‘Ca. L. solanacearum’ in Morocco and Africa, suggesting a wider distribution of the bacterium in carrot crops in the Mediterranean region, including North Africa. ‘Ca. L. solanacearum’ has caused economic damages to carrot and celery crops in the Canary Islands and mainland Spain, France, Sweden, Norway, and Finland. This bacterium has also caused millions of dollars in losses to potato and several other solanaceous crops in the United States, Mexico, Central America, and New Zealand. Given the economic impact of ‘Ca. L. solanacearum’ on numerous important crops worldwide, it is imperative that preventive measures be taken to limit its spread.