Occurrence of atypical Pseudomonas viridiflava strains on different host plants in southern Italy

Since 1997, severe bacterial disease symptoms were observed during surveys carried out in farms with field-grown Molfettese Catalogna chicory (Cichorium intybus var. sylvestre, Catalogna group), sugarloaf chicory (C. intybus var. porphyreum), curly type lettuce (Lactuca sativa var. crispa) cvs Langero and Aleppo, and safflower (Carthamus tinctorius) located in the Apulia and Basilicata regions of southern Italy. Convex colonies with yellowish mucoid material on hypersucrose medium (5% sucrose nutrient agar), producing fluorescent pigments on King's medium B, were isolated from all symptomatic plants. Based on conventional biochemical and nutritional tests, Biolog assays, and analysis of partial 16S rDNA and rpoB sequences, these atypical fluorescent Pseudomonas isolates were identified as Pseudomonas viridiflava. The pathogenicity of the isolates was ascertained by artificial inoculation on their respective original hosts. The analysis of rpoB gene sequences and RAPD-PCR profiles showed a high genetic variability among the P. viridiflava isolates. To the best of our knowledge, this is the first report of bacterial diseases on horticultural crops caused by atypical strains of P. viridiflava in Italy and the first report of the bacterium on safflower. In addition, P. viridiflava is reported for the first time as a causal agent of leaf spot on potato (Solanum tuberosum). The economic importance of chicory and curly type lettuce in southern Italy and recent spread of atypical and highly virulent P. viridiflava strains require more extensive and careful phytosanitary surveillance, as well as an adjustment of sustainable crop protection management to cope with this new challenge.