Negative control value: 0.333
Negative control value: 0.333
aMean of two readings. Data in parentheses are standard deviation values.
A405 values greater then three times that of negative control are positive samples.
Absorbance was read at 405 nm 1 hr after adding the substrate solution.
washings of the unexposed rhizomes show a negative result for the presence of R. solanacearum, it has been concluded that R. solanacearum survives in the vascular tissues of inoculated or infected rhizomes.
The effect of microwave treatment on microorganisms appears to be related to heat induction (Vela and Wu, 1979). However, some metabolic effect not related to heating may occur (Barker and Fuller, 1969). The microwave oven has been used as a research tool in several different investigations (Diprose et al., 1978). Susceptibility of microorganisms on seed to microwave heating is determined by altering the power level, amount of sample, and water content, as well as exposure time (Thomas et al., 1979; Puri and Barraclough, 1993). If the test sample (rhizome or seed) is homogeneous, the entire microwave energy penetrates all parts simultaneously and heat is generated evenly throughout the material; since the surface of ginger can lose energy by convection, conduction, or radiation, dielectric heating can result in the interior of the sample becoming hotter than the outside. Treatment of 1 kg soil for 150 seconds is sufficient to eliminate populations of Pythium, Fusarium, and most nematodes in soil. Fusarium species tolerated high-aerated steam temperatures than Rhizoctonia species, but Fusarium was less tolerant to microwave treatment (Bollen, 1969). The effect of 2,450 MHz heating on plant pathogens and soil microorganisms has been reported (Ferris, 1984). When infected rhizomes were subjected to microwaves for 30 seconds, the resulting plantlets were free from bacterial wilt under greenhouse conditions (Figure 9.9) (Kumar
et al., 2003). However, at longer exposure, germination of the rhizomes was adversely affected (Kumar et al., 2003).
An effective method to reduce the occurrence of bacterial wilt of potato in infested potato fields in Peru is crop rotation with maize (Elphinstone and Aley, 1993). In China 3 years of rice cultivation reduced the bacterial wilt incidence of groundnut from 8.3 percent to 1.5 percent (Wang et al., 1983). In India rotation with finger millet or maize reduces wilt of eggplants and tomato (Sohi et al., 1981). In pot culture experiments soil amendments were effective in controlling bacterial wilt (Chang and Hsu, 1988), and similar results were obtained in field trials (Hartman and Yang, 1990; Elphinstone and Aley, 1993). Pegg and Moffett (1971) suggested that the grower should attempt to eradicate weeds known to harbor biovar 3, which has a wide host range including ginger. Experience in India has shown that crop rotation with nonhosts such as cereals and millets results in a reduction in the wilt incidence in the ensuing crop of ginger, but more work is required on cultural methods of disease management comparable to that done with other hosts of R. solanacearum.
Identification of Resistance Sources for Control of Bacterial Wilt of Ginger
In vitro and in vivo techniques are available for screening the germplasm for bacterial wilt tolerance in ginger. Almost all cultivated edible ginger is susceptible to bacterial wilt. Over 600 accessions screened for bacterial wilt tolerance using a soil inoculation method were found to be susceptible to the disease. Incorporating the toxic metabolites of Ralstonia in the culture medium was used for in vitro selection; however, surviving plantlets were found to be susceptible to bacterial wilt in the field (A. Kumar, unpublished data). An efficient in vitro screening technique for tolerance to bacterial wilt has been developed at the Indian Institute of Spices Research, Calicut. Live bacterial cells are added to tissue culture bottles containing ginger plantlets; this method enables screening of large numbers of plantlets in tests of 2 weeks' duration. Susceptible plants became chlorotic (Figure 9.10). A differential reaction of ginger accessions to bacterial wilt was reported by Indrasenan et al. (1982).
Successful biological control agents have the ability to compete with other members of the soil microflora and also to produce antibiotics or induce a response in the host that favors growth of the biological control agent while inhibiting the growth of R. solan-acearum. Bacterial antagonists and avirulent strains of R. solanacearum are effective in the control of wilt in groundnut (He, 1990). The bacterial antagonists include Pseudomonas fluorescens (Kempe and Sequeira, 1983; Ciampi-Panno et al., 1989; Gallardo et al., 1989), Pseudomonas glumae (Wakimoto, 1987; Furaya et al., 1991), Pseudomonas cepacia (Aoki et al., 1991), Bacillus species (Fucikovsky et al., 1989), and Erwinia species (Fucikovsky et al., 1989). Avirulent mutants of R. solanacearum (Chen and Echandi, 1984; Kempe and Sequeira, 1983) show promise for bacterial wilt control (Trigalet and Trigalet-Demery, 1990). Other biological agents have not been very effective in natural environments due to poor colonization and because the level of protection is not sufficient for commercial use (Chen and Echandi, 1984). Endophytic antagonists derived from wild-type strains
are potential control agents (Frey et al., 1993). Some genetically engineered avirulent mutants of R.solanacearum with lesions in the hrp gene cluster have the ability to colonize the host plant multiplying in the rhizosphere and rhizoplane and inside the collar and lower part of the stem. These mutants induce a host defense response, and also have the ability to produce bacteriocin with a wide spectrum of activity, which makes them promising agents for the biological control of bacterial wilt under field conditions.
Dr. A. Kumar is thankful to the International Foundation for Science, Sweden, and the Department of Biotechnology, Government of India, for their financial support for programs on bacterial wilt of ginger at the Indian Institute of Spices Research, Calicut.
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