Chard (Beta vulgaris var. cicla L.) is a biennial herbaceous plant in the Chenopodiaceae family. It is rich in vitamins and minerals and is one of the most popular traditional vegetables in Taiwan. Chard accessions VI048530 and VI050121 growing in fields at Shanhua, Tainan, showed wilting symptoms in March and April 2013. The initial symptoms of wilt were observed on young green leaves. These symptoms progressed over time to chlorosis, interveinal necrosis, and finally blight. Finally, the plants collapsed and died. Vascular and pith tissues were discolored, especially at the stem base. A whitish mass oozed from the cut end of diseased stems. A total of eight bacterial strains were isolated from stems and roots of wilted chard plants. On tetrazolium chloride (TZC) medium (4), colonies were round to oval, fluidal, and white with a pink or red center after incubation at 30°C for 48 h. A typical hypersensitive reaction was induced within 24 h when the strains were infiltrated into tobacco leaves. Koch's postulates on chard plants were confirmed using the eight strains within a greenhouse, under natural light, with temperature and humidity ranges from 25 to 34°C and 56 to 98%, respectively. Fifteen chard (VI048530) plants at the four- to six-leaf stage were inoculated by soil drenching with 30 ml of a ~108 CFU/ml bacterial suspension. Sterile water was used as negative control. After 4 to 6 days, the first symptoms of wilt were observed on the young chard leaves. The progression of symptom development was identical to that observed in the field. The colony morphology on TZC medium of isolates from the inoculated plants was identical to that previously described from field samples. Pathogenicity of the strains was also tested on tomato (VI005790), eggplant (VI046095), and pepper (PBC1367) plants using the previously described inoculation procedure. The mean disease incidences on tomato, eggplant, and pepper plants were 100% (120/120), 100% (120/120), and 79.2% (95/120) respectively. Latent infection was found in asymptomatic pepper plants (16/120) by a printing method. Polymerase chain reaction (PCR) amplification of total DNA from each strain using the Ralstonia solanacearum-specific primer pair AU759f and AU760r (5) produced the expected 282-bp amplicon. All the isolated strains were identified as biovar 3 based on their capacity to utilize three hexose alcohols (mannitol, sorbitol, and dulcitol) and three disaccharides (lactose, maltose, and cellobiose) (2) to produce acid. Based on the phylotype-specific multiplex PCR assay and the partial egl gene sequence (GenBank accession numbers KM100442 to KM100449) (1), all chard isolates were identified as R. solanacearum phylotype I, sequevar 34. Bacterial wilt symptoms have also been observed on beet (Beta vulgaris L.), a close relative of chard, but beet has not been confirmed as a host plant (3). To our knowledge, this is the first report of chard as a host of R. solanacearum worldwide.
References: (1) M. Fegan and P. Prior. Page 449 in: Bacterial Wilt Disease and the Ralstonia solanacearum Species Complex. C. Allen et al., eds. APS Press, St. Paul, MN, 2005. (2) A. C. Hayward. J. Appl. Bacteriol. 27:265, 1964. (3) A. Kelman. The Bacterial Wilt caused by Pseudomonas solanacearum. Tech. Bull. No. 99. N.C. Agric. Exp. Stn., 1953. (4) A. Kelman. Phytopathology 44:693, 1954. (5) N. Opina et al. Asia Pac. J. Mol. Biol. Biotechnol. 5:19, 1997.