Prunus salicina Lindl., also known as Japanese plum, is a temperate-zone fruit tree grown in mountainous areas of Taiwan. The planted area in Taiwan is approximately 3,000 ha. In June 2011, more than 20% of plum fruits harvested in an orchard in Lishan (elevation about 2,000 m) showed black, mostly circular, sunken necrotic lesions. Leaves with a shot-hole appearance and cankered branches were found when investigating the orchard. Bacteria were isolated from symptomatic fruits, leaves, and branches. Isolation on nutrient agar detected colonies that were yellow, mucoid, gram-negative, Xanthomonas-like, and induced hypersensitive responses on tomatoes. Three voucher isolates, BCRC80476, BCRC80478, and BCRC80481, obtained from the fruit, leaf, and branch, respectively, were deposited in the Bioresource Collection and Research Center, Hsinchu, Taiwan. Molecular analyses were conducted for species identification. Sequences of the gyrB gene of the three voucher isolates (GenBank Accession Nos. KC202288, KC202289, and KC202287) were 100% identical to that of Xanthomonas arboricola pv. pruni pathotype strain ICMP51 (2). In addition, DNA fragments of the xopE3 gene (an X. arboricola pv. pruni specific T3E gene, approximately 381 bp) were PCR amplified using the primer pair fw-5′CCGACATTGCCGTCAGCGATCACG3′ and rv-5′AGCGTTCTTGGGTGTGTTGAGCATTTG3′ (1). The bacterial isolates were identified as X. arboricola pv. pruni on the basis of the colony characteristics, sequence homology, and the specific PCR assay. Pathogenicity was confirmed by inoculation of greenhouse-potted P. salicina plants with strains BCRC80476, BCRC80478, and BCRC80481 using bacterial suspensions (6.7 × 108 CFU per ml) in 0.01% Tween 20. Five plants were evenly sprayed with inoculum of each bacterial isolate and covered with plastic bags for 3 days. One week post inoculation, at an average temperature of 19°C, the 15 inoculated plants produced brown-purple spots delimited by a chlorotic margin on the leaves. Three weeks post inoculation, the necrotic leaf spots completely deteriorated, leaving a shot-hole appearance, and the branches showed lesions similar to those observed in the fields. The pathogen was reisolated from the symptomatic tissues, fulfilling Koch's postulates. Control plants sprayed with 0.01% Tween 20 remained symptomless. To our knowledge, this is the first record of X. arboricola pv. pruni causing bacterial spot on P. salicina in Taiwan.
References: (1) A. Hajri et al. Appl. Environ. Microbiol. 78:371, 2012. (2) J. M. Young et al. Syst. Appl. Microbiol. 31:366, 2008.