Occurrence of tomato pith necrosis caused by Pseudomonas marginalis in Italy

In 2006, a serious outbreak of tomato pith necrosis (TPN) with approximately 90% disease incidence was observed in two greenhouses in Sicily. Adult fruiting plants showed chlorosis and slight wilting of shoot apices. Internal stem browning along the entire length of the plant was observed. Pith tissues were soft but not rotted or hollowed.Only fluorescent colonies developed on King’s B medium (KB) after isolation from infected tissues. Ten pure isolates were all levan, oxidase, potato soft-rotting and arginine dihydrolase positive and were negative for tobacco hypersensitivity; they produced 2-ketogluconate, nitrate reductase, acid from sucrose and lecithinase from egg yolk, and were thus LOPAT group IVa of the fluorescent pseudomonads, which corresponds to Pseudomonas marginalis.

Tomato cv. Marmande plantlets (ten plantlets per isolate) were prick inoculated at the axil of the first true leaf with a pin coated with 24 hr-old cells grown on KB. After inoculation, plants were covered with plastic bags for 48 h, then uncovered and kept in a growth chamber at 27°C with a photoperiod of 16/8h. Two weeks after inoculation, all showed extensive stem pith browning extending 2-4 cm from the inoculation point. Two representative bacterial isolates were identified as P. marginalis by using the Biolog Identification System (version 4.2; Biolog, Inc., Hayward, CA, USA) with a similarity of 0.93.Their partial 16S rDNA sequences were identical and they shared 99%-100% identity with other P. marginalis strains available in GenBank. These results confirm their identity as P.marginalis.

Pseudomonas corrugata and P. mediterranea are considered the main causal agents of TPN (Catara, 2007) althougha number of fluorescent pseudomonads have also been identified in different countries as causal agents. They include P. cichorii, P. viridiflava, P. fluorescens (Lo Cantore & Iacobellis, 2002; Alippi et al., 2003; Sutra et al., 1997) and the fluorescent Pseudomonas strains assigned to three unnamed genomospecies by Sutra et al. (1997). This is, to our knowledge, the first report of P. marginalis as a causal agent of TPN.