LAUSR

Strawberry (Fragaria × ananassa) is an economically important crop in Zhejiang, China. In the autumn of 2021, crown necrobiosis and angular leaf spot was observed in commercial strawberry fields (cultivar 'fenyu') in Cixi, Ningbo, Zhejiang, China (N30°9'55″, E121°21'13″). The disease incidence ranged from 5 to 8 % in the field, but could reach 50 to 60 % in some heavily affected plastic tunnels. In the affected field, this disease could reduce strawberry production by 50%. Early symptoms were water-soaked lesions around the vein of the abaxial leaves; subsequently, reddish-brown irregular spots and coalesced lesions developed. In humid conditions, a sticky bacterial ooze exuding from lesions was observed. Finally, the crown of the diseased plant was necrotized, and several pockets were observed inside the crown after dissection. To isolate the causal agent, the infected leaves and crown tissues from six different plants were surface-sterilized with 75% ethanol for 1 min, rinsed twice with sterile distilled water, cut into small pieces, and soaked in 5 ml of sterile distilled water for 20 min. The supernatant from the cut-up pieces was serially diluted and spread on nutrient agar medium. After 2 to 3 days at 28℃, several yellow colonies were grown on the medium. The colonies from five infected plants were gram-negative, anaerobic rods, yellow, viscous, and gloss, which are typical characteristics of Erwinia anana (Wells et al. 1986). To confirm the identity of the causal bacteria, PCR was conducted for six randomly selected colonies to amplify 16S rRNA (Monciardini et al. 2002), fusA, and gyrB (Stice et al. 2002). The amplicons were sequenced and blasted, and the results showed that the six colonies were identical. The 16S rRNA, fusA, gyrB sequences of the isolate CM3 were deposited in GenBank with accession number ON754076.1, OP587277, and OP587278; BLAST search showed 99.93% (1445 bp out of 1446 bp), 100% (746 bp out of 746 bp), 99.64% (1371 bp out of 1376 bp) similarity with strains of Pantoea ananatis (KT741001.1, MH015093.1 and CP066803.1 accessions, respectively). The resulting concatenated data set of 16S rRNA-fusA-gyrB was used to build a multilocus phylogenetic analysis (MLSA) by maximum likelihood criteria. The MLSA tree indicated that the isolate CM3 belonged to Pantoea ananatis. The isolate's identity was further confirmed by P. ananatis-specific primers pagyrB-F/R (Xiao et al. 2022). Thus, this isolate was designated as P. ananatis CM3. To fulfill Koch's postulates, two old leaves were broken off each of the ten 2-month-old strawberry (cultivar 'fenyu') plants to create wounds, each plants was sprayed with a cell suspension of P. ananatis (107CFU/ml, 0.5 ml) on the stem base. Ten plants were sprayed with water to serve as a control. All plants were kept at 28/25°C (day/night) under a 12-h/12-h photoperiod. All plants were covered with transparent plastic bags to maintain humidity. After 48 h, the bags were removed. After 2 weeks, water-soaked lesions on some leaves were observed similar to those in the field . Three to five weeks after inoculation, the crown of the inoculated plants was necrotized, which was similar to the symptoms in the field. No symptoms were observed in the control plants. The experiment was repeated three times. The bacteria were successfully reisolated from the inoculated crown tissues and leaves and confirmed as CM3 according to the same methodologies used for the initial identification. Bacterial leaf blight in strawberry caused by Pantoea ananatis has been reported in Nova Scotia, Canada, and Egypt (Bajpai et al. 2019; Abdel-Gaied et al. 2022). To our knowledge, this is the first report of Pantoea ananatis causing crown necrobiosis on strawberry in China. This report provides a basis for further research on this disease and its management and control.