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Title: First report of apple (Malus sylvestris) as a host of ‘Candidatus Phytoplasma pruni’in the United States

item NIKOLAEVA, E - Pennsylvania Department Of Agriculture
item WELLIVER, R - Pennsylvania Department Of Agriculture
item ROSA, C - Pennsylvania State University
item JONES, T - Pennsylvania State University
item PETER, K - Pennsylvania State University
item Costanzo, Stefano
item Davis, Robert

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/7/2016
Publication Date: 2/1/2017
Citation: Nikolaeva, E.V., Welliver, R., Rosa, C., Jones, T., Peter, K., Costanzo, S., Davis, R.E. 2017. First report of apple (Malus sylvestris) as a host of ‘Candidatus Phytoplasma pruni’in the United States. Plant Disease. 101:378

Interpretive Summary: Phytoplasmaas are tiny bacteria that infect plant life, are spread by plant-feeding insects, and cause serious diseases of crop plants and plants in natural habitats worldwide. One such phytoplasma is called X-disease phytoplasma, which causes disease in stone fruits including peach trees in the United States. Prior to the study reported here, the X-disease phytoplasma had not been found in apple trees, but this study discovered that some apple trees in the state of Pennsylvania were infected by this phytoplasma, also known as ‘Candidatus Phytoplasma pruni’. The available data seem to be consistent with the concept that the observed infection of apple trees in Pennsylvania by ‘Candidatus Phytoplasma pruni’ represents an unusual event. Nevertheless, the work reveals a previously unknown feature of this phytoplasma’s biology, and it emphasizes the need for further research to increase understanding of the phytoplasma’s biology and to devise improved measures for controlling spread of the phytoplasma. Such progress will aid the cause of food security, trade, and sustainable agricultural production. This report will be of interest to diagnostics laboratories, research scientists, farmers, and quarantine agencies internationally.

Technical Abstract: During a survey of Pennsylvania fruit tree orchards in 2013-2015, some apple trees were found to exhibit abnormally small fruits, clumps of small leaves, blind wood, or premature reddening and curling of leaves. DNA was extracted from symptomatic leaves collected from three apple trees, and the DNA was used as template in tests to screen for presence of phytoplasmal DNA by using a Real-time PCR protocol. The results indicated phytoplasmal infections in the three apple trees. Amplified 16S RNA, SecY, and ribosomal protein encoding genomic regions were amplified in polymerase chain reactions and subjected to automated DNA sequencing. Analysis of the 16S rRNA gene regions indicated that the detected phytoplasma strains were identical in the three apple trees and can be classified in subgroup 16SrIII-A (X-disease phytoplasma subgroup) and are strains of ‘Candidatus Phytoplasma pruni’. Analyses of secY and ribosomal protein gene sequences supported this conclusion. To our knowledge, this work provides the first evidence to indicate that M. sylvestris can serve as a host of ‘Ca. Phytoplasma pruni’. Although based on the results we cannot absolutely attribute the disease in apple to ‘Candidatus Phytoplasma pruni’, we propose a new name for the apple disease, “Apple X-Disease,” to distinguish it from X-Disease on Prunus species.