First Report of Tomato Pith Necrosis Caused by Pseudomonas cichorii in Tanzania

Tomato (Solanum lycopersicum) is affected by many bacterial diseases, and pith necrosis is increasingly reported in this important vegetable crop. Tomato pith necrosis is caused by various species of Pseudomonas, including P. corrugata, P. viridiflava, P. cichorii, and P. mediterranea (Trantas et al. 2015). During a survey of tomato diseases in the Morogoro Region of Tanzania during May 2014, pith necrosis-like symptoms were observed in low incidence (<1% of plants) in one field in each of two villages. Symptoms included extensive dark brown external stem discoloration and streaking near branch axils and along petioles, adventitious root formation near discolored areas, stem collapse, internal browning of the pith, and laddering and/or absence of pith. Stem samples were washed with tap water and the epidermis removed, then were surface disinfested in 70% ethanol for 30 s and rinsed with sterile water. Tissue was macerated in sterile distilled water, allowed to stand for 5 min, and the suspension was streaked on Pseudomonas F agar medium (PF). Plates were incubated for 48 h at 30°C, and the dominant colony type was purified on PF. Pathogenicity trials were performed at Sokoine University of Agriculture. Seven isolates were cultured on PF agar for 24 h at 30°C. For each isolate, five 4-week old tomato ‘Bonnie Best’ seedlings were inoculated by stabbing the axils of the second and third true leaves with a sterilized toothpick dipped in an individual colony. The negative control was stabbed with a toothpick dipped in sterile water. Plants were bagged for 24 h to increase humidity and evaluated 4 weeks after inoculation. Pith browning was observed for six of the seven isolates and laddering of the pith was observed for two of seven isolates. Twisting of the upper stem and leaves was observed for one isolate. No symptoms developed in the noninoculated control plants. Bacteria were reisolated from the inoculated plants as described above and five isolates were selected for further study. All isolates, including all field isolates and those reisolated from pathogenicity tests on tomato, were confirmed to be P. cichorii through biochemical tests (Saygili et al. 2008) and DNA sequencing conducted at OARDC. All isolates were fluorescent on PF, gram negative, levan negative, oxidase positive, nonpectinolytic on potato and crystal violet pectate medium, arginine dehydrogenase negative, and caused a hypersensitive response on tobacco. DNA sequences were obtained for all isolates following amplification using 16S primer pair 8F/1492R (Turner et al. 1999). Sequences exhibited at least 99% sequence similarity to GenBank accessions of P. cichorii (AB724285 and AB724294), and one representative sequence for each village was deposited in GenBank (KP295476 and KP295477). To our knowledge, this is the first report of pith necrosis of tomato caused by P. cichorii in Tanzania. While incidence was low in the villages in which it was found, losses of even a few plants potentially reduce tomato yields and the livelihoods of smallholder farmers in Tanzania.