First Report of Bacterial Soft Rot on Tagetes patula Caused by Dickeya dieffenbachiae in China
Authors: Zhang J. X. , Lin B. R. , Shen H. F. , Pu X. M. , Wang Z. W. , Zeng D. Q. , Huang N. 
Published: January 2013

French marigold (Tagetes patula L.), originally from Mexico, is an annual herb widely planted in China because of its beautiful color, long flowering, and strong adaptability, and has been used widely for ornamentation and decorating. French marigold is also rich in patuletin, quercetagetin, and patulitrin, and is therefore applied medicinally for treating colds and coughs. In early summer 2012, soft rot symptoms on French marigold were found at three flower nurseries in Guangzhou, Guangdong Province, P. R. China, and approximately 25% of the plants had the symptoms. The symptoms included tissue collapse of the stems at the soil line followed by wilting of the whole plants. Within 1 week, the infected stems showed vascular discoloration, turned brown and then inky black, and eventually the whole plant collapsed after the basal stem was infected. Bacteria were successfully isolated from eight symptomatic plants on nutrient agar media incubated at 30°C for 48 h. Ten isolates were selected randomly for further characterization. They were gram negative, degraded pectate, negative for oxidase and positive for indole production, and utilized malonate, glucose, and sucrose but not glucopyranoside, trehalose, or palatinose. Polymerase chain reactions (PCR) were performed using the 16S primers 27f and 1495r (4) for molecular identification. Subsequent DNA sequencing showed that the representative tested strain TP1 (GenBank Accession No. JX575747) was 99% identical to that of Dickeya dieffenbachiae (JF419463) using BLASTn. Further genetic analysis of strain TP1 was performed targeting several housekeeping genes, i.e., dnaX (GenBank Accession No. JX575748) with primers dnaxf and dnaxr (3), gyrB (JX575749) with primers of gyrbf1 and gyrbr1 (1), and gapA (JX575750) with primers of gapa326f and gapa845r (2). They were most homologous to the sequences of D. dieffenbachiae, since they had 97%, 96%, and 97% identity with GenBank accessions GQ904794, JF311653, and GQ891968, respectively. Pathogenicity was confirmed by injecting all 10 original bacterial isolates into each of 10 French marigold seedlings, with approximately 100 μl of a bacterial suspension at 1 × 108 CFU/ml. Ten plants inoculated with 100 μl of sterile water served as controls. Plants were placed in a greenhouse at 30 to 32°C and 90% relative humidity. Within 48 h, soft rot symptoms appeared on all inoculated seedlings, while the control plants appeared normal. D. dieffenbachiae was reisolated from the diseased tissues, and confirmed to be the same as the inoculated pathogen by conducting a 16S rDNA sequence comparison. Previously, black spot, botrytis blight, oedema, powdery mildew, southern bacterial wilt, and damping off have been found on T. patula. To our knowledge, it is the first report of a soft rot caused by D. dieffenbachiae on French marigold. Because of the popularity and high economic value of French marigold, identification of this progressing bacterial disease is important to maintain safe production and beautiful scenery.

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