Location: Molecular Plant Pathology
Title: First report of sweet cherry virescence disease in China and its association with infection by a ‘Candidatus Phytoplasma ziziphi’-related strain Authors
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 5, 2013
Publication Date: N/A
Interpretive Summary: Sweet cherry (Prunus avium L.) is a deciduous tree highly valued for its timber and fruit. Having originated in the Black Sea and Caspian Sea region, sweet cherry has been cultivated and naturalized on all continents including the Americas. Last spring, a new disease affecting sweet cherry trees occurred in northern China. Diseased trees developed abnormal flowers that failed to set fruit; some affected trees died prematurely. Using DNA fingerprinting technology, we determined that the sweet cherry disease was associated with infection of trees by a bacterium called phytoplasma. We further found that the phytoplasma was closely related to previously identified phytoplasmas responsible for serious diseases in jujube, plum, apricot, and elm. Our results indicated that the plant host range of this group of pathogen has expanded. By revealing a new plant disease and determining its apparent cause, the work will impact plant pathological research and aid in developing strategies to control the disease. The findings from our work also signals the increased need for improved diagnostic technologies and new quarantine regulations to curb the spread of pathogens and phytoplasmal diseases into the United States. This report will be of interest to diagnostics laboratories, research scientists and extension personnel who are concerned with phytoplasma disease management. The information is also important to the U.S. and international quarantine agencies for formulation and implementation of new quarantine regulations.
Technical Abstract: Sweet cherry (Prunus avium L.) is a deciduous tree originating in the Black Sea/Caspian Sea region where Asia and Europe converge. Being highly valued for its timber and fruit, sweet cherry has been cultivated and naturalized on all continents. Over the past decade, the area of sweet cherry cultivation increased rapidly in China and has reached 140,000 hectares. In April 2013, sweet cherry trees (cultivar Summit) exhibiting floral virescence symptoms were observed in two orchards located at suburban Taian, Shandong Province, China. The diseased trees developed flowers having white petals with green veins or abnormal floral structures having cupped, green petals. The affected flowers failed to set fruit. A month following the first appearance of the virescence symptoms, the diseased trees became wilted and eventually died. Leaf and stem samples were collected from nine symptomatic and two nearby symptomless trees. Total DNA was extracted from each sample using the Plant Quick DNA Extract Kit (TianGen, Beijing, China). Nested-PCR was carried out using phytoplasma-universal primer pairs P1/P7 and R16F2n/R16R2(1). All PCR assays with DNA templates from symptomatic samples yielded an amplicon of 1.25 kb, corresponding to the full-length F2nR2 region of phytoplasmal 16S rDNA. No amplicon was generated in PCRs containing DNA templates from symptomless plants. The amplicons were cloned into plasmid vector pMD18-T (TaKaRa, Dalian, China) and sequenced. The obtained sequences were nearly identical, and a representative sequence was deposited into the GenBank (Accession No. KF268424). An analysis of the sequence through the iPhyClassifier(2) (http://plantpathology.ba.ars.usda.gov/cgi-bin/resource/iphyclassifier.cgi) revealed that the sweet cherry virescence (SCV) disease was associated with an infection by a phytoplasma closely related to the reference strain of ‘Candidatus Phytoplasma ziziphi’. The 16S rDNA F2nR2 region of the SCV phytoplasma shared 99.8% nucleotide sequence identity with that of ‘Candidatus Phytoplasma ziziphi’ reference strain (GenBank Accession No. AB052876). A computer-simulated restriction fragment length polymorphism (RFLP) analysis of the SCV phytoplasma 16S rDNA F2nR2 sequence with a set 17 restriction enzymes(3) resulted in a collective RFLP profile identical to the reference pattern of the elm yellows phytoplasma group, subgroup B (16SrV-B). To our knowledge, this is the first report of a phytoplasmal disease in sweet cherry in China, and the SCV phytoplasma is a new member of the subgroup 16SrV-B. Presence of 16SrV-B phytoplasmas and their etiological association with various plant diseases in China have been reported previously; affected host plants included jujube, hemp fiber, paper mulberry, Chinese cherry, plum, apricot, red barberry, clover, dianthus, elm, and sunshine tree(4). Our identification of the SCV phytoplasma signals further expansion of the plant host range of the 16SrV-B phytoplasma lineage. The impact of the SCV phytoplasma in the regional ecosystem and in sweet cherry production is being assessed.