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First Report of Pear Blossom Blast Caused by Pseudomonas syringae pv. syringae in China

In the spring of 2006, a new bacterial disease was noted in pear orchards near Hangzhou, Zhejiang Province, China. The disease caused severe blossom blast on pears (Pyrus pyrifolia; cv. Cuiguan). Early symptoms of the disease included blackening of the calyx end of developing fruit, blackening of blossom clusters while leaves of affected blossom clusters appeared normal, or death of clusters consisting of both blossoms and leaves. Later, tips of twigs turned dark brown and died. No bacterial ooze was observed. Twelve bacterial isolates were recovered from ten samples of buds and blossoms. Six isolates were selected for identification. They were similar to those of the reference strains of Pseudomonas syringae pv. syringae LMG5570 and LMG 2230 from Belgium in phenotypic tests on the basis of the Biolog Microbial Identification System (version 4.2; Biolog Inc., Hayward, CA), pathogenicity tests, gas chromatographic analysis of fatty acid methyl esters (FAMEs) using the Microbial Identification System (MIDI Inc., Newark, DE) with aerobic bacterial library (TABA50), and electron microscopy (TEM, KYKY-1000B, Japan). All isolates tested were gram-negative, aerobic rods measuring 1.5 to 2.4 × 0.5 to 0.6 μm with 2 to 4 polar flagella. Fluorescent green diffusible pigment was produced on King's Medium B. Colonies were gray-white and slightly raised with smooth margins on nutrient agar. They produced levan on sucrose nutrient agar. A hypersensitive reaction was observed on tobacco cv. Benshi 24 h after inoculation. All isolates were identified as P. syringae pv. syringae with Biolog similarity index of 0.57 to 0.86 and FAME similarity index of 0.58 to 0.81. Identification as P. syringae pv. syringae was confirmed using 16S rDNA universal primers (2,3): 5′-AGA GTT TGA TCA TGG CTC AG-3′ forward primer, 5′-ACG GTT ACC TTG TTA CGA CTT-3′ reverse primer. The PCR fragments of the three isolates were sequenced and compared with sequences in GenBank. They had 99% similiarity with P. syringae pv. syringae 16S rRNA gene strain NCPPB 3869. Koch's postulates were conducted on buds of the original pear cultivar growing in pots and detached pear blossoms in flasks by spray inoculation with cell suspensions containing 10⁸ CFU/ml of the six isolates at 18 to 22°C with two replications. The bacteria induced symptoms on buds and blossoms similar to those observed in the field. The bacterium was reisolated from symptomatic pear buds and internal ovary tissues. P. syringae pv. syringae was first reported in England as the cause of pear blossom blast in 1914 (1). After searching all the Chinese agricultural databases and major journals (National Knowledge Infrastructure database, Vip Chinese periodical database, Chinese wanfang database, China InfoBank, Scientia Agricultura Sinica, Acta Phytopathologica Sinica, Acta Phytophylacica Sinica, and Journal of Fruit Science), to our knowledge, this is the first report of pear blossom blast caused by P. syringae pv. syringae in China. The disease cycle on pear trees and the control strategies in the regions are being further studied.

References: (1) B. P. Barker et al. Ann. Appl. Biol. 1:85, 1914. (2) U. Edward et al. Nucleic Acids Res. 17:7843,1989. (3) B. Li et al. J. Phytopathol. 34:141, 2006.