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Title: First Report of a New Phytoplasma Subgroup, 16SrIII-S, Associated with Decline Disease Affecting Sweet and Sour Cherry Trees

Author
item VALIUNAS, D - VILNIUS, LITHUANIA
item JOMANTIENE, R - VILNIUS, LITHUANIA
item IVANAUSKAS, A - VILNIUS, LITHUANIA
item ABRAITIS, R - VILINUS, LITHUANIA
item STANIENE, G - LITHUANIA
item Zhao, Yan
item Davis, Robert

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/10/2009
Publication Date: 5/1/2009
Citation: Valiunas, D., Jomantiene, R., Ivanauskas, A., Abraitis, R., Staniene, G., Zhao, Y., Davis, R.E. 2009. First Report of a New Phytoplasma Subgroup, 16SrIII-S, Associated with Decline Disease Affecting Sweet and Sour Cherry Trees. Plant Disease. 93:550.

Interpretive Summary: Phytoplasmas are small bacteria that lack a rigid cell wall and are carried from plant to plant by insects. They cause diseases in many plant species, including fruit trees such as cherry. We found severely diseased cherry trees in Lithuania, and through the technology of DNA fingerprinting, we discovered that the trees were infected by two different species of phytoplasma. One was a previously unknown phytoplasma; it was present in trees suffering from a disease that we have named cherry decline. The other was very similar to a known phytoplasma, but this was the first finding of the phytoplasma in diseased cherry trees. We classified the previously unknown phytoplasma in a distinct new subgroup (III-T) within the X-disease phytoplasma group (group 16SrIII), which contain diverse phytoplasmas, some of which are known to be capable of infecting stone fruits. The results of this study expand knowledge of diseases in fruit trees, and they provide base information to aid efforts to reduce the spread of fruit tree diseases. The findings will be of interest to researchers, students of horticulture, plant quarantine agencies, and enterprises involved in the production of disease-free planting stock and growing of fruit trees.

Technical Abstract: During July 2007, we observed sweet cherry (Prunus avium) and sour cherry (Prunus cerasus) trees exhibiting disease symptoms suggestive of possible phytoplasma infection in a large orchard in the Kaunas region of Lithuania. Samples of leaf tissue were collected from sweet cherry trees that were affected by a decline disease (designated cherry decline, ChD) characterized by symptoms that included leaf reddening and premature leaf drop, and from sour cherry trees exhibiting symptoms that included proliferation of branches and non-seasonal flowering. To assess the diseased trees for phytoplasma infection, DNA was extracted from the samples and used as template in nested polymerase chain reactions (PCRs) primed by phytoplasma universal primer pairs P1/P7 and R16F2n/R16R2 for amplification of 16S ribosomal (r) DNA sequences. Phytoplasma-characteristic 1.2 kbp DNA sequences were amplified from all samples, indicating phytoplasma infection in all 15 trees. On the basis of collective restriction fragment length polymorphism (RFLP) patterns, the amplified DNA sequences were derived from two different phytoplasmas. One, designated ChD phytoplasma and found in sweet cherry trees and sour cherry trees, was tentatively classified as a member of new subgroup T in 16S rDNA RFLP group 16SrIII (X-disease phytoplasma group). The amplified ChD phytoplasma 16S rDNA was cloned in Escherichia coli, sequenced, and the sequence deposited in the GenBank database (GenBank accession no. FJ231728). Nucleotide sequence alignments revealed that the ChD phytoplasma 16S rDNA shares 99.8 % sequence identity with 16S rDNA from one operon (rrnB, GenBank acc. no. AF370120) from a phytoplasma previously found to be associated with dandelion virescence (DanVir) disease in Lithuania. The high similarity of the ChD 16S rRNA gene sequence to that of DanVir rrnB suggests the possibility that ChD and DanVir may belong to a single phytoplasma species and that dandelion is possibly an alternate host of ChD phytoplasma. The other phytoplasma, found in two sweet cherry trees, was classified in subgroup 16SrI-B of group 16SrI (‘Candidatus Phytoplasma asteris‘ and related strains) and was designated cherry proliferation (ChP) phytoplasma (GenBank acc. no. FJ231729). Thus, in Europe, cherry may be affected by diseases associated with phytoplasmas belonging groups 16SrI, 16SrIII, 16SrX and 16SrXII. Interestingly, the 16S rDNA from ChD phytoplasma shared only 97.0% sequence identity with 16S rDNA from Canada X-disease (CX) (GenBank acc.no L33733) phytoplasma, indicating that ChD phytoplasma may represent a species that is distinct from that represented by strain CX. Since phytoplasmas belonging to two groups (16SrI, subgroups I-B and I-Q and 16SrIII, subgroup T), representing at least two distinct species, have been found in cultivated cherries in Lithuania, further study of Prunus and other plant hosts growing in natural habitats seems warranted. Infections by diverse phytoplasma strains and species underscore the need for production of phytoplasma-free planting stock and for intensified research to reduce ecological and economic impacts of these phytoplasmas.