The Pathogen

Bacterial wilt of ginger is caused by a prokaryote, Ralstonia solanacearum Yabuuchi (Smith), (synonym Pseudomonas solanacearum E.F. Smith), that has a wide host range including both dicots and monocots. R. solanacearum (Yabuuchi et al., 1992, 1995) is gram negative, rod shaped, and motile with one or more polar flagella or nonmotile without flagella. Erwin Frink Smith originally described the pathogen as Bacillus solanacearum in 1896. R. solanacearum belongs to the rRNA homology group II pseudomonads based on rRNA:DNA homology (Palleroni et al., 1973) and to the beta subclass of Proteobacteria. The bacterium is endemic in most subtropical and tropical regions of the world. Colonies of R. solanacearum on tetrazolium medium are distinguished from other bacterial colonies by their fluid smooth white appearance with red central whirling pattern (Figure 9.3) (Kelman, 1954). Conventionally, strains of R. solanacearum are grouped into races based on host range and biovars based on the ability to oxidise or utilize selected sugars (Buddenhagen et al., 1962; Hayward, 1960, 1964). The two groupings have been used in epidemiological studies, although they are imperfectly correlated, except that biovar 2 is equivalent to race 3. Almost all isolates from naturally infected ginger have proven to be either biovar 3 or biovar 4; they show variable degrees of pathogenicity to ginger. Isolates from ginger in Queensland showed a differential disease reaction correlated with biovar (Hayward et al., 1967). Biovar 4 produced a wilt on ginger in 14 or 21 days after stem and root inoculation, respectively, whereas biovar 3 isolates produced a wilt 6 weeks postinoculation. Biovar 4 was accordingly described as producing a rapid wilt and biovar 3 a slow wilt. The relationship between virulence on ginger and biovar is not consistent. In India, for example, where biovar 3 has been

Figure 9.3 Typical colony of R. solanacearum causing bacterial wilt of ginger.

identified on ginger (Sarma et al., 1978), a rapid wilt of ginger was produced on inoculation.

Studies have shown that biovar 3 isolates from ginger cause slow browning of the leaves over a period of 6 weeks, whereas biotype 4 isolates from ginger caused typical wilting in Lycopersicon esculentum L, Solanum tuberosum L., Zinnia elegans Jacq, Capsicum frutescens L., Physalisperuviana L., and Solanum melonga L. in 7 to 14 days. R. solanacearum from other hosts such as tomato, Chromolaena, chili, and potato was nonpathogenic on ginger. It is interesting to note that the R. solanacearum isolates from Chromolaena, a common weed of ginger fields, were not pathogenic on ginger even though they belong to biovar 3. Similarly, isolates from potato, tomato, and capsicum were not pathogenic to ginger (Kumar and Sarma, 2004). However, in India biovar 3 causes wilt in ginger in 5 to 7 days after artificial stem inoculation and in 7 to 10 days following soil inoculation of the pathogen (Kumar and Sarma, 2004). Biovar 4 is encountered infrequently in India. Zheng and Dong (1995) reported the predominance of biovar 3 in China among 129 isolates of R. solanacearum causing bacterial wilt of plants including ginger.

R. solanacearum is a heterogeneous species showing significant genotypic and pheno-typic diversity (Cook et al., 1989; Hayward, 1991, 1994). In Hawaii, strains affecting Strelitzia (bird of paradise) and ginger, both relatives of banana, were affected by two different strains, neither of which could wilt triploid banana (Quinon et al., 1964). Two clusters within strains of R. solanacearum have been reported based on restriction fragment length polymorphism (RFLP) (Cook et al., 1989); Division 1 includes biovars 3, 4, and 5 and Division 2 biovars 1, 2, and N2. Division 1 was referred to as an Asian subdivision and Division 2 as an American subdivision because of the geographic origin of the isolates represented. A subcluster of R. solanacearum isolates belonging to Division 2 (Cook et al., 1991) has recently been recognized by Taghavi et al. (1996) based on 16S rRNA gene sequence information; this subcluster contains isolates from Indonesia and also Pseudomonas syzygii and the blood disease bacterium. This close relationship of R. solanacearum, the blood disease bacterium, and P. syzygii led Taghavi et al. (1996) to coin the term "R. solanacearum species complex" to describe the complexity of the species as revealed by RFLP analysis and sequencing of conserved genes. The use of 16S rRNA gene sequences for the classification and identification of R. solanacearum has shown up to 99 percent similarity between isolates. R. solanacearum isolates from ginger similarly showed a very high degree of homology (Kumar et al., 2004). Other measures of genetic diversity are needed before it can be concluded that there is limited genetic diversity among ginger isolates. Wilt of the ornamental gingers—Hedychiumflavum, H. coronarium, and H. gardenarianum—is caused by R. solanacearum and the strains causing wilt are similar (Aragaki and Quinon, 1965). Zingiber zurumbet, a close relative of edible ginger, is susceptible to bacterial wilt caused by ginger strains of R. solanacearum (Figure 9.4). Isolates of R. solanacearum from ginger on tetrazolium medium are similar in appearance. This and other evidence suggests that strains of R. solanacearum from ginger may have evolved in a particular location and then spread to other parts of the world through planting material. All of the ginger strains belong to the Asian division of Cook et al. (1989).

Bacterial soft rot and rhizome decay is caused by either Pectobacterium (Erwinia) caro-tovorum subsp. carotovorum or Pectobacterium (Erwinia) chrysanthemi (M. Stirling, personal

Figure 9.4 Susceptibility of Z. zurumbet to R. solanacearum.

communication) and bacterial rhizome rot is caused by Pseudomonas marginalis in Korea (Choi and Han, 1990) and Nigeria (Nnodu and Emehute, 1988). However, their importance under field conditions is not known. Bacterial rot caused by Pseudomonas zingiberi has been reported from China (Li et al. 1994).

R. solanacearum is considered to be one of the most important plant pathogenic bacteria as it causes great economic losses worldwide (Hayward, 1991). The bacterium has an unusually wide host range; plant species susceptible to the pathogen have been observed to occur in over 50 plant families (Hayward, 2000). The host range includes solanaceous plants (tomato, potato, tobacco, eggplant), leguminous plants (such as groundnut, French bean), monocotyledons (mainly banana, ginger), and several tree and shrub hosts (such as mulberry, olive, cassava, eucalyptus). Recently it was shown that certain ecotypes of the model plant Arabidopsis thaliana are also susceptible to the pathogen (Deslandes et al., 1998).

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