Candidatus Phytoplasma pyri
Significance: Pear decline (PD) is an important insect-transmitted phytoplasma disease that occurs mainly in Europe and North America. Infected trees of susceptible pear varieties may die within a few years after infection, or alternatively may live for many years in a slow state of decline. Fruits, if produced, are smaller and fewer in number compared to those produced on healthy trees. Phytoplasma detection and characterization is now primarily based on PCR amplification of the ribosomal RNA gene (rDNA). According to rDNA molecular evidence, phytoplasmas are currently divided into about 20 major phylogenetic groups, with the typical pear decline phytoplasma of Europe and North America being tentatively placed as a distinct definable taxa (i.e., ‘Candidatus phytoplasma pyri’) within the Apple proliferation subgroup (e.g., see Seemüller and Schneider, 2004). In 1994, Asia pear trees (Pyrus pyrifolia) exhibiting symptoms somewhat typical of PD disease were observed in orchards of central Taiwan (Dungshr and Heping areas). Initial symptoms developed on infected trees in the fall and included premature reddening of foliage followed by early leaf drop. In the spring, leaves on affected trees remained small and pale and shoot development was minimal. During hot dry weather conditions that followed, quick decline and death of trees occurred within just a few weeks. Molecular characterization of the Pear decline Taiwan phytoplasma (PDTW) agent showed it to be closely related to the phytoplasmas of the apple proliferation group, but also supported the view that the PDTW phytoplasma causing pear decline in Taiwan may represent a new, distinct subgroup of the apple proliferation group. Two species of pear psyllas, Cacopsylla qianli and C. chinensis, were found to carry the PDTW phytoplasma.
Issues of Concern: While PD is known to occur within North America, it is controlled within the NAPPO countries through domestic certification programs aimed at ensuring host planting materials, such as cuttings, scions, and rootstocks, are free from the pathogen. This latest research from Taiwan indicates that, based on putative restriction site and phylogenetic analysis of rDNA sequences, the pear decline phytoplasma affected Asian pears in Taiwan is likely a new phytoplasma subgroup, distinct from North American Pear decline phytoplasma. While the risk posed by this new phytoplasma is not yet fully understood, agencies involved in fruit tree certification and testing for PD should in be made aware of this newly identified phytoplasma and take necessary measures to ensure it can be reliably detected and if necessary eliminated from any potential host plant propagative materials (e.g., imported germplasm).
Hsiu-Lin Liu, Ching-Chung Chen, and Chan-Pin Lin. 2007. Detection and identification of the phytoplasma associated with pear decline in Taiwan. Eur. J. Plant Pathol. Vol. 117:281–291. Seemüller E, Schneider B, 2004. ’Candidatus Phytoplasma mali’, ’Candidatus Phytoplasma pyri’ and ’Candidatus Phytoplasma prunorum’, the causal agents of apple proliferation, pear decline and European stone fruit yellows, respectively. International Journal of Systematic and Evolutionary Microbiology, 54(4):1217-1266.