R. solanacearum can survive in the soil for long periods in the absence of host plants. There are conflicting reports on the longevity of R. solanacearum strains in soil, especially in the absence of protected sites (Graham and Lloyd, 1979) and on its resistance to desiccation. Soil types have been differentiated as being either conductive or suppressive to bacterial wilt (Hayward, 1991); their indirect influence on soil moisture determines the population size of antagonistic microorganisms, which affect, in turn, the persistence of R. solanacearum. Many workers have stressed the complexity of the epidemiology of bacterial wilt and involvement of many interacting factors (Kelman, 1953; Buddenhagen and Kelman, 1964; Hayward, 1991). Dissemination on infected vegetative planting material is of major importance in the case of banana, ginger, and potato and the potential for spread may be local or international. Some early evidence indicated that true seed might serve as a means of dispersal of the pathogen. In the case of groundnut, the pathogen is potentially seedborne, but early attempts to demonstrate seed transmission proved negative (He, 1990). It now appears that the pathogen does not survive in the dry seed normally used for planting. However, at high seed moisture content as in succulent plants like ginger rhizomes, survival and transmission are possible. When moist seeds from wilted groundnut plants are harvested and planted immediately in sterile soil, wilting of seedlings occurs. If the moisture content of harvested seed is above 10 percent, then survival is possible in storage; below this level there is no prolonged survival (Zhang et al., 1993a). Irrigation and the "wetting and drying" cycles that occur in the soil may tend to move cells up and down the soil profile and at times concentrate them in the rhizosphere. Also, the application of fertilizers during the growing season may influence root growth and the rhizosphere dynamics of the bacterium. Brown rot of potato caused by R. solanacearum race 3 (biovar 2) occurs in some cool temperate environments in which latent infections commonly occur in progeny tubers (Ciampi and Sequeira, 1980; Hayward, 1991). There is no comparable evidence of latent infection of either banana or ginger being affected by bacterial wilt under relatively cool conditions. Rhizomes used as planting material may show no obvious symptoms and serve as a means of dissemination of the disease.
Vaughan (1944) proved that apparently healthy tomato seedlings taken from infected seedbeds were responsible for dispersal of the pathogen when replanted in areas hundreds of miles away. Insect dissemination of R. solanacearum to banana (Moko disease) is uniquely important in that disease. In banana insects carry bacteria mechanically from the ooze issuing from diseased banana inflorescences to healthy inflorescences (Buddenhagen and Elsasser, 1962). There is little evidence of insects being of the same importance in dissemination of R. solanacearum on other hosts. However, it should be noted that a related pathogen, P. syzygii, the cause of Sumatra disease of cloves in Java and Sumatra, is transmitted by tube-dwelling cercopids of the genus Hindola (Homoptera: Cerco-poidea: Machaerotidae). Few insect pests affect ginger, but their role in disease spread in the field cannot be ruled out. Leaf infection has been reported in a few instances in nature and can occur by inoculation under conditions of high humidity and temperature (Hayward and Moffett, 1978; Moffett et al., 1981). Aerial transmission through rain splash dispersal of epiphytic populations on tobacco leaves has been described in Japan (Hara and Ono, 1985). The worldwide distribution and damaging nature of bacterial wilt on many crops suggests that R. solanacearum is an ecologically competent pathogen able to survive in the absence of its host, and in some circumstances overwinter in temperate zones and survive the dry season in tropical areas (Persley, 1986).
The modes of invasion of R. solanacearum differ from those of most other bacterial plant pathogens. Since ginger is planted as broken seed pieces termed "seed rhizomes," the soilborne bacterial inoculum has ample opportunity to invade the cut ends of the rhizomes during plant emergence. Infection occurs through wounds in roots or rhizomes or at sites of secondary root emergence. The bacterium colonizes the intercellular spaces of the root cortex and vascular parenchyma and produces extracellular enzymes that break down the pectic compound in the host plant cell wall and middle lamella facilitating spread through the vascular system (Vasse et al., 1995). In xylem vessels bacterial populations rapidly reach a very high level (Figure 9.5) (1010 cells/cm of stem in tomato) concomitant with wilting and plant death. The bacterium is then released to the soil, living as a saprophyte until able to infect new host plants. Infection from rhizomeborne inoculum is assumed as a result of simultaneous multiplication of bacterium and plant cells and eventual blockage of the vascular elements by the bacterial cells.
Latently infected planting material is the major means of dispersal of R. solanacearum between locations, states, countries, and continents (Hayward, 1991). Traditionally, ginger is cultivated in previously fallowed soil or virgin soil. The occurrence of bacterial wilt in such fields is indirect evidence of the rhizomeborne nature of R. solanacearum in ginger (Pegg et al., 1974; Indrasenan et al., 1981; Kumar et al., 2002). Bacterial wilt of banana has long been known to be passively disseminated, primarily on seed pieces, by root wounding during transplanting or by tools and by root-to-root spread (Rorer, 1911; Sequeira, 1958). Serological evidence for the rhizomeborne nature of R. solanacearum in ginger has been reported (Prasheena 2003, Supriadi et al., 1995). In vegetatively propa-
gated crops such as potato, banana, and ginger, infected planting material is the major means of dispersal of R. solanacearum from place to place and also from season to season (Buddenhagen, 1961). Spread of the potato race in tubers is important in parts of Africa, Australia, and South America, and this race has been introduced in England and Sweden in tubers from Egypt and other Mediterranean countries (Anonymous, 1960). Since the pathogen is mainly transmitted through tuber seed, the use of healthy planting material is the most effective means to control the disease (Hayward, 1991).
Root-to-root spread of the bacterium has been recorded (Kelman and Sequeira, 1965) but there is little evidence of long-distance spread from field-to-field except in circumstances where floodwater is responsible for the movement of infested soil and infected plant debris (Kelman, 1953). A synergistic interaction between the root knot nematode, Meloidogyne incognita, and bacterial wilt has been reported. It has been shown that the intensity of bacterial wilt on tomato increased with an increased number of nematodes in the field (Hutagalung and Widjaya, 1976). However, nematicide application did not result in bacterial wilt control.
Survival of R. solanacearum in Soil, Planting Material, and Weeds
In Mauritius bacterial wilt is known to be endemic all over the island, including forest soils and sugarcane fields (Ricaud and Felix, 1971). Soil contaminated with bacterial wilt even to a high level may not give a high wilt incidence when susceptible plants are grown without adequate moisture, which confirms the role of high soil moisture in bacterial wilt incidence (Felix and Ricaud, 1978). Long-term survival of R. solanacearum in soil has long been attributed to the weed population in the field prior to cultivation. There is evidence of the saprophytic and parasitic survival of the bacterium in the rhizosphere of certain weeds in Queensland (Pegg and Moffett, 1971; Moffett and Hayward, 1980). However, there are few reports on the role of weedborne populations of the bacterium in actual bacterial wilt epidemics.
Being a vascular pathogen, it is presumed that R. solanacearum survives in ginger rhizomes at a very low inoculum level without affecting the normal state of the ginger. In general, the bacterial wilt pathogen will not multiply in intact rhizomes during storage unless the dormancy is broken and sprouting initiated. R. solanacearum can survive better under conditions of high soil moisture than in desert areas even under irrigation (Buddenhagen and Kelman, 1964). R. solanacearum survives in soil as well as the seed piece, and this forms a potential source of primary inoculum for the ensuing crop (Indrasenan et al., 1981). The potato race (race 3) is a low-temperature—adapted pathovar and it survives at cool temperatures in plant debris and latently infects potato tubers. R. solanacearum race 1, also a pathogen of potato, differs from race 3 in geographical distribution and the ability to survive under different environmental conditions (Graham et al., 1979). Alternative weed hosts and nonhost plants play an important role in the survival of R. solanacearum in the absence of susceptible crops (Granada and Sequeira, 1983). R. solanacearum has the ability to invade the roots of resistant cultivars of host species, such as tomato and persimmon, and nonhosts, such as bean and corn, without any symptoms. Disease control is made difficult by the ability of the pathogen to survive in the absence of a susceptible host (Granada and Sequeira, 1981). Lum (1973) found that both biovars 3 and 4 survived in soil for 20 months during a severe drought in Malaysia.
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