First Report of Pseudomonas cichorii Causing Tomato Pith Necrosis in Fujian Province, China

Pith necrosis, an important disease of tomato worldwide, is caused by Pseudomonas corrugata (Buonaurio et al. 1993), P. mediterranea (Trantas et al. 2013), P. viridiflava, and Pectobacterium spp. (Malathrakis and Goumas 1987). More than 233 ha of tomato are harvested annually in Minqing County, Fujian Province, China. During spring 2015 and 2016, pith necrosis symptoms were observed on tomato with an incidence of 30 to 50% in production areas of Minqing County. Symptoms consisted of general plant wilting, yellowing, and brown to black spots or lesions on stem, petiole, and fruit stalk. Internally, pith tissues showed water-soaking, hollowing, and soft rotting, and vascular tissue was yellow to brown in color. To characterize the pathogen, symptomatic tissue was surface sterilized in 0.5% sodium hypochlorite for 1 min, rinsed with sterile distilled water, and then ground in a few drops of sterile distilled water. Subsequently, suspensions were streaked onto nutrient agar and incubated for 48 h at 28°C. Isolated colonies were milky, opaque, and mucoid with smooth margins and fluorescent on King’s medium B under 365-nm ultraviolet light. They were gram negative, arginine dihydrolase and β-galactosidase negative, catalase and oxidase positive, and did not produce levan. The bacteria elicited a hypersensitive response in tobacco leaves and were unable to cause soft rot in potato slices. According to LOPAT tests, they belonged to fluorescent pseudomonads group III. Three representative isolates were selected for further analysis. The polymerase chain reaction-amplified product from four different genes, 16S rRNA (Lane 1991) (GenBank accession nos. MG197708, MH373660, and MH373661 of 1,361 bp), gyrB (accession nos. MG925213, MH423894, and MH423897 of 858 bp), rpoB (accession nos. MG925214, MH423895, and MH423898 of 1,113 bp), and rpoD (Trantas et al. 2013) (accession nos. MG925215, MH423896, and MH423899 of 602 bp), was sequenced. Nucleotide sequences for each gene were compared with nucleotide databases and had 97 to 100% sequence identity with the corresponding sequences of P. cichorii (GenBank accession nos. AB724288, KU174189, JQ437232, JQ267562, and JQ437235). Pathogenicity of three representative isolates was tested in potted tomato plants (Jingaotieqiu) at the five true leaf stage. Bacterial suspensions (approximately 10⁸ CFU/ml) were prepared in sterile distilled water and injected into the stems of three plants per isolate with a hypodermic syringe. Control plants were injected with sterile distilled water. All plants were kept in a mist room with 100% humidity for 4 h and then transferred to a greenhouse at 25°C and 80% relative humidity (12 h light). Symptoms were first observed 7 days after inoculation. After 14 days, all inoculated plants developed brown to black lesions on the stem and pith tissue developed hollowing, whereas control plants remained asymptomatic. The pathogen was reisolated from diseased plants and was identical to the original isolates based on morphology, biochemical characteristics, and molecular characteristics. Thus, the pathogen was identified as P. cichorii. Although the disease has already been reported on tomatoes from Guangdong Province, China (Yu et al. 2017), this is the first report, to our knowledge, of P. cichorii causing tomato pith necrosis in Fujian Province, China. These results suggest that the pathogen is spreading and disease incidence increasing. Thus, tomato pith necrosis poses a serious threat to tomato production in China.