Molecular characterization of Pseudomonas syringae pv. tomato isolates from Tanzania

Bacterial speck caused by Pseudomonas syringae pv. tomato is an emerging disease of

tomato in Tanzania. Following reports of outbreaks of the disease in many locations in

Tanzania, 56 isolates of P. syringae pv. tomato were collected from four tomato- producing

areas and characterized using pathogenicity assays on tomato, carbon source utilization by

the Biolog Microplate system, polymerase chain reaction and restriction fragment length

polymorphism (RFLP) analysis. All the P. syringae pv. tomato isolates produced bacterial

speck symptoms on susceptible tomato (cv. ‘Tanya') seedlings. Metabolic fingerprinting

profiles revealed diversity among the isolates, forming several clusters. Some geographic

differentiation was observed in principal component analysis, with isolates from Arusha

region being more diverse than those from Iringa and Morogoro regions. The Biolog system

was efficient in the identification of the isolates to the species level, as 53 of the 56 (94.6%)

isolates of P. syringae pv. tomato were identified as Pseudomonas syringae. However, only

23 isolates out of the 56 (41.1%) were identified as Pseudomonas syringae pv. tomato. The

results of this work indicate the existence of P. syringae pv. tomato isolates in Tanzania that

differ significantly from those used to create the Biolog database. RFLP analysis showed that

the isolates were highly conserved in their hrpZ gene. The low level of genomic diversity

within the pathogen in Tanzania shows that there is a possibility to use resistant tomato

varieties as part of an effective integrated bacterial speck management plan.

KEYWORDS: Bacterial speck; Biolog system; polymerase chain reaction (PCR); restriction

fragment length polymorphism (RFLP); tomato.