First Report in the Seychelles of Xanthomonas axonopodis Genetic Cluster 9.2 Causing Bacterial Leaf Spot of Avocado

Small, black, angular leaf lesions, which sometimes coalesced, were collected from avocado (Persea americana Miller) leaves in a government nursery located at Grand Anse, Mahé, Seychelles archipelago in 2003. Patterns of diseased plants were highly clustered, suggesting local dispersal in the nursery. Yellow-pigmented Xanthomonas-like bacterial colonies were isolated on KC semiselective medium (3). Amplified fragment length polymorphism (AFLP) analysis was performed on two avocado strains together with reference strains of the genetic clusters of Xanthomonas axonopodis (4) and the type strain of all other valid Xanthomonas species using SacI/MspI and four primer pairs (unlabeled MspI + 1 [A, C, T, or G] primers and 5′-labeled – SacI + C primer for the selective amplification step) (1). The two avocado strains showed identical fingerprints and were closely related to X. axonopodis genetic cluster 9.2 (4). One strain, JZ103-1, was further analyzed by MultiLocus Sequence Analysis (MLSA) using portions of three housekeeping genes (atpD, dnaK, and gyrB) as described previously (1). MLSA data confirmed that the xanthomonad associated with avocado was most closely related to X. axonopodis genetic cluster 9.2. No other strain in this genetic cluster shared an identical sequence type. Avocado cv. Grand collet leaves from the youngest growth flush were infiltrated with a needleless syringe (10 inoculation sites per leaf and three replicates) with bacterial suspensions. Typical, water-soaked lesions that developed into black necrotic spots appeared 6 to 8 days after infiltration on all inoculated leaves when suspensions containing ~1 × 106 CFU ml–1 were used (i.e., ~7 × 102 CFU per inoculation site), while no lesions developed on leaves inoculated with Tris buffer or with suspensions containing ~1 × 104 CFU ml–1. One month after inoculation, mean Xanthomonas population sizes determined on KC semiselective medium (3) from ~1 cm2 leaf fragments showing black lesions ranged from 2 × 106 to 4 × 106 CFU per lesion, typical of a compatible interaction. A few colonies that recovered from lesions obtained after inoculation were typed by AFLP and were identical to the inoculated strain. An extensive branch and trunk canker of avocado caused by a Xanthomonas sp. has been reported in California (2). This bacterium did not cause lesions of avocado leaves or fruit after inoculation (2). This appears to be the sole previous report of a xanthomonad being pathogenic to avocado and the symptoms observed in the Seychelles appear therefore very different from the ones reported in California. No major outbreak of bacterial leaf spot of avocado has been reported in the Seychelles archipelago since 2003. References: (1) N. Ah-You et al. Int. J. Syst. Evol. Microbiol. 59:306, 2009. (2) D. A. Cooksey et al. Plant Dis. 77:95, 1993. (3) O. Pruvost et al. J. Appl. Microbiol. 99:803, 2005. (4) J. Rademaker et al. Phytopathology 95:1098, 2005.