Various control measures has been tried to combat the disease with limited success. Bacterial wilt is a major problem and one of the constraints in the production of ginger and other vegetable crops because of its wide host range, the genetic variability it exhibits, and the complexity of its epidemiology and modes of transmission. The general strategies for management of bacterial wilt are: selection of healthy rhizome material from a disease-free area; selection of field with no previous history of bacterial wilt; preplant treatment of rhizomes by application of heat or chemicals; strict phytosanitation in the field, including restrictions on movement of farm workers and irrigation water across the field; clean cultivation and minimum tillage; crop rotation with nonhost plants such as paddy and maize; insect pest and nematode control in the field; and soil amendments, including biological control agents. Some of these control methods are considered in greater detail in the following.
Selection of Healthy Rhizome Material from Disease-Free Area
The use of rhizomes collected from previously disease-affected areas as planting material invariably results in severe disease when such material is planted in virgin soil or fallowed soil or even soil that has been rotated with nonhost crops. This experience emphasizes the need for pathogen-free seed in order to prevent disease outbreaks. In the absence of effective chemical and biological control methods, the best possible approach would be planting of pathogen-free rhizomes in pathogen-free soil in order to avoid or prevent the occurrence of bacterial wilt epidemics (Pordesimo and Raymundo, 1963; Supriadi, 2000). Techniques have been standardized to detect the pathogen in rhizome using nitrocellulose membrane-enzyme linked immunosorbent assay (NCM-ELISA) (Kumar et al., 2002) (Figure 9.6). In spite of the availability of excellent pathogen-detection technologies to detect the pathogen in rhizomes, soil, and irrigation water, their use in the indexing of planting material is negligible among ginger farmers. Although very sensitive and selective, these techniques for detection of R. solanacearum in ginger rhizomes are not readily adapted to the processing of large volumes of planting material; they are almost impractical under the farming conditions of developing nations in Asia and, therefore, they have not been adopted. The only method that has been used is selection of seed rhizomes from disease-free fields is visual inspection. This unscientific method of planting material selection often results in severe epidemics of bacterial wilt disease in India and other southeast Asian nations. Moreover, pathogen-free rhizomes are not readily available to all farmers owing to scarcity of seed material during peak seasons of planting, especially in crops like ginger, which require 1 ton of seed rhizome per acre of land.
Selection of Field with No Previous History of Bacterial Wilt
Site selection is one of the most important factors that contribute to the successful control of bacterial wilt of ginger. It has been observed that a soil with no history of bacterial wilt often results in healthy crops of ginger if the rhizomes are free from the pathogen. Traditionally, ginger is cultivated in a previously fallowed soil, virgin forest soil, or rubber plantations after 20 to 25 years of rubber crop in the Kerala state of India. This long crop rotation often results in a healthy crop of ginger. Another alternative is to plant underneath perennial trees or in social amenity forests with
NCM-ELI5A of Ginger isolates of Ralstonia sotanacearum
regulated shade (Supriadi, 2000). French (1994a,b) has pointed out that pathogen-free soil and the use of certified seed tubers contribute most to the avoidance of brown rot of potato, and it is likely that the same is true for the avoidance of bacterial wilt of ginger. Soil can also be indexed for the presence of the pathogen by sensitive methods like the polymerase chain reaction (PCR). Techniques have been standardized to detect the pathogen in soil using double antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) and PCR (Kumar et al., 2002; Priou, 2001; Priou et al., 1999, 2002). The PCR-based method for detection of the bacterium in soil has been based on universal primers specific for R. solanacearum (Opina et al., 1997, Kumar, unpublished data) (Figure 9.7).
Solarization of soil prior to planting has been widely used to control soilborne pathogens and pests in various crops including potato, ginger, onion, carrot, and peanut, without consequential damage to the environment as occurs when methyl bromide is used for
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