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First Report of Bacterial Speck of Tomato Caused by Pseudomonas syringae pv. tomato Race 1 in Portugal

Protected and open field tomato crops are economically important for Portuguese agriculture. In 1983, Pseudomonas syringae pv. tomato (Okabe, 1933) Young, Dye & Wilkie, 1978 was first reported affecting protected crops (3) and then later under open field conditions (1). In the 2009 spring/summer season, several outbreaks of bacterial speck of tomato showing an unusual degree of severity were observed in open fields from the Tagus Valley Region. Typical symptoms included necrotic specks surrounded by a yellow halo on younger and older leaves with losses higher than 60% due to the heavy floral bud abortion. Abnormal lesions on the stems, as well as on the petioles and fruits, together with reduced growth of the entire plant, which is normally uncommon, were frequently observed in affected plants from distinct tomato cultivars (H-9665, H-9776, and CDX 255), two of them carrying the Pto resistance gene. Samples collected from different fields and cultivars were observed and used for isolation of the causal agent on King's medium B. The isolates were characterized (2) and Koch's postulates were fulfilled by carrying out pathogenicity tests. Ten plants from three commercial cultivars carrying the Pto resistance gene (CXD 255, Defender F1, and H-9775) were inoculated by spraying bacterial water suspensions (10⁸ CFU ml^--1) and kept under environmental conditions favorable for disease development. Positive and negative controls were also performed using P. syringae pv. tomato type strain (CFBP 2212T; race 0) and sterile distilled water, respectively. Cultural and biochemical characterization of the isolates showed their ability to produce levan, use sucrose, and induce a hypersensitivity reaction on tobacco leaves. Moreover, the isolates were oxidase negative, did not hydrolyze arginine nor produce soft rot on potato slices, and did not use erythritol as well as DL-lactate, identifying them as P. syringae pv. tomato. Typical and severe bacterial speck symptoms were produced in the Pto resistant tomato plants 4 days after inoculation and the isolates could be recovered after reisolation. Negative control plants showed no disease symptoms and CFBP 2212 was unable to produce typical lesions, except for a few in the older leaves. Altogether these results pointed to P. syringae pv. tomato race 1 as the disease causative agent. Further confirmation was achieved by partial sequencing of the rpoD gene using primers PsrpoD FNP1 and PsrpoDnprpcr1 (4). rpoD sequences, obtained from two isolates (CPBF 1288, GenBank HM368535; CPBF 1290, GenBank HM368537), were compared by nucleotide BLAST at NCBI displaying a 100% level of DNA similarity with strain Pto T1 belonging to P. syringae pv. tomato race 1. To our knowledge, this is the first report of the occurrence of Pseudomonas syringae pv. tomato race 1 in Portugal.

References: (1) L. Cruz et al. ATTI Giornate Fitopatol, 2:399, 1992. (2) R. Lelliott and D. Stead. Methods for the Diagnosis of Bacterial Diseases of Plants. Blackwell Scientific Publications, Oxford, 1987. (3) H. Oliveira and J. Santa-Marta. Pseudomonas syringae pv. tomato (Okabe, 1933) Young, Dye & Wilkie, 1978. Uma nova bacteriose do tomateiro em Portugal. Publicação do Laboratório de Patologia Vegetal “Veríssimo de Almeida”, 1983. (4). S. Sarkar et al. Genetics 174:1041, 2006.