P. deliense Meurs.
In India this disease has been reported from almost all states, including Kerala, Rajasthan, Himachal Pradesh, Orissa, Maharashtra, Tamil Nadu, Andhra Pradesh, and Sikkim. Soft rot reduces the potential yield to a great extent in the field, storage, and market and may cause losses of even more than 50 percent (Joshi and Sharma, 1980). Crop loss depends on the growth stage at which infection starts. Total loss results if the infection occurs in the early stage of crop growth. In Kerala, the loss can be as high as 90 percent during heavy infection (Rajan and Agnihotri, 1989). Sinha and Mukho-padhyay (1988) reported losses up to 50 to 90 percent under storage (Table 8.2).
Ginger plants are susceptible to Pythium infection at all stages of growth. Buds, roots, developing rhizome, and collar regions are the main points of infection. When the seed rhizomes are infected, they fail to sprout due to the rotting of buds (Figure 8.1). After sprouting, the infection takes place through roots or through the collar region, finally
reaching the rhizome. Symptoms appear initially as water-soaked patches at the collar region. These patches enlarge and the collar region becomes soft and watery, and then rots. Sprouts turn yellow and collapse.
In mature plants, infection leads to yellowing of leaves. This yellowing starts from the leaf tip and spreads downward, mainly along the margins resulting in death of leaves. The dead leaves droop and hang down the pseudostem until the entire shoot becomes dry (Figures 8.2 and 8.3). The basal portion of the plant exhibits a pale translucent coloration. This area later becomes water soaked and soft to such an extent that the whole shoot either topples or can easily be pulled out.
Rhizomes first turn brown and gradually decompose, forming a watery mass of putrefying tissue enclosed by the tough skin of the rhizome (Figure 8.4). The fibrovascular strands are not affected and remain isolated within the decaying mass. Roots arising from the affected regions of the rhizome become soft and rot (Dohroo, 1982). The rotten parts emit a foul smell. Rotting attracts opportunistic fungi, bacteria, and insects. Severe infestation can lead to total crop loss (Figure 8.5).
Butler (1907), who reported the disease for the first time, identified the causal organism as Pythium gracile Schenk. Several species of Pythium have since been reported to cause the disease in different parts of the world: P. aphanidermatum (Edson) Fitz. (Mitra and Subramanian, 1928), P. butleri Subram. (Thomas, 1938), P. complectens Braun (Park, 1934), P. deliense Meurs (Haware and Joshi, 1974), P. gracile (deBary) Schenk (Butler, 1907), P. graminicolum Subram. (Park, 1935), P. myriotylum Drechsler (Uppal, 1940; Park, 1941; Bertus, 1942), P. vexans deBary (Ramakrishnan, 1949), P. pleroticum T. Ito (Sharma and Dohroo, 1980), P. zingiberum (Ichitani and Shinsu, 1980), and P. ultimum (Dohroo and Sharma, 1985).
Butler and Bisby (1931) considered P. butleri and P. gracile to be identical with P. aphanidermatum. Three species of Pythium—P. aphanidermatum (Haware and Joshi, 1972), P. deliense (Haware and Joshi, 1974) and P. myriotylum are reported to be responsible for soft rot in Madhya Pradesh, India. On the West Coast of Madras in India, Pythium spp.
occurs in association with Sclerotium rolfsii and causes rhizome rot (Anonymous, 1953). Table 8.2 gives the Pythium species causing soft rot in various parts of India. P. aphani-dermatum and P. myriotylum are the dominant species.
In Himachal Pradesh, P. pleroticum is found in association with Fusarium equiseti. P. pleroticum causes wet rot, whereas F. equiseti is responsible for dry rot under field and storage conditions (Sharma and Dohroo, 1980). Bhardwaj et al. (1988b) reported five pathogens of rhizome rot of ginger in Himachal Pradesh: P. pleroticum, P. aphanidermatum, P. ultimum (Dohroo, 1987), F. equiseti, and F. solani.
P. vexans has been observed at an altitude of 1,170 m in the Wynad area in Kerala, India. The temperature requirement of this species is lower than that of other species, with the maximum tolerance limit being 34°C. For germination of P. aphanidermatum and P. myriotylum, the optimum temperature is about 34°C (maximum is 40°C), and for P. pleroticum, it is 25 to 30°C. However, the optimum temperature for the growth and multiplication of F. equiseti is 30°C (Dohroo, 1979). A warm and humid climate predisposes ginger plants to infection at the sprouting stage itself because of their tender and succulent tissues (Dake, 1995). There are two ways in which the disease is carried over and perpetuated: through diseased rhizomes and through oospores surviving in debris in soil. The infected rhizomes and roots remaining in the field form an important source of primary infection. Such plant parts may contain large numbers of perennating oospores. Oospores have also been detected in the scales of stored rhizomes (Thomas, 1938). The spread of the disease is typical of soilborne diseases because of fairly heavy and well-distributed showers during the crop season from June to October (Dake, 1995).
Pythium has well-developed mycelia often with appressoria. Zoosporangia are filamentous, and not differentiated from vegetative hyphae. The hyphae are colorless, although occasionally appear yellowish. Pythium has specific cultural requirements. On corn meal agar and potato carrot agar, most species do not produce aerial mycelia, whereas in oatmeal agar most species produce profuse aerial mycelia. Various colony patterns on agar media have been recognized that depend on the medium and incubation temperature (Van der Plaats Niterink, 1981). Van der Plaates Niterink (1981) gave the morphological characters of various species in his monograph on the genus.
Histopathological investigations of rhizome rot are lacking. However, it is known that Pythium spp. are intercellular as well as intracellular. Tissue degeneration occurs in advance of colonization of the host tissue. Dissolution of the middle lamella leads to soft rot symptoms. In this process of rotting amylase, invertase, macerating, and oxidative enzymes are found to play their roles (Dohroo, 1982; Dohroo et al., 1984b; Sharma and Dohroo, 1985; Dohroo, 1989b).
The primary source of soft rot is the oospore present in the diseased rhizome or the soil. The oospore germinates directly or indirectly. In the first case, the oospore produces a germ tube that elongates and either produces a sporangium or penetrates the host directly. In the second case, the oospore germinates, producing a sporangium and zoospores. It has been reported that host root exudate causes the accumulation of zoospores around the root zone and accelerates their encystment, germination, and infection (Tripathi and Grover, 1978). In addition, infection can also be by the appresoria formed by the hyphal elements.
Spread of disease is by waterborne zoospores or hyphal fragments. Such waterborne zoospores are attached to the ginger root, where they encyst and produce germ tubes that infect roots. The lesions appear at the point of entry within 72 hours under ideal weather conditions. Sporangia are produced on the surface of host lesions. Oospore formation takes place in the host tissue or in the soil. The oospores are dormant structures and help in perennation.
Several factors influence the Pythium infection and disease development. The important ones are weather and soil factors. High soil water, high relative humidity, and relatively low temperature favor the disease development and spread. Ginger planting often coincides with monsoon rains, and during this time, the soil water and ambient temperature (25 to 30°C) become conducive to the onset of disease (Sarma, 1994). Once disease starts, it spreads to the adjacent clumps mostly through soil water by means of zoospores and hyphal fragments. The disease is generally less in well-drained soils while water stagnation aggravates the disease incidence (Sarma, 1994).
Healthy Rhizome Selection: Infected rhizomes are the primary source of perennation and spread of soft rot. The best method to manage the disease is by the use of disease-free rhizomes for planting (Park, 1941; Bertus, 1942; Shahare and Asthana, 1962; Dohroo, 1993).
Narrow Ridge Cultivation: Kim et al. (1998) reported that narrow ridge cultivation reduced the disease (P. myriotylum) effectively compared to the unridged control plots in all the fields tested.
Mulching: Das (1999) showed that the plots mulched with maha neem (Melia aza-dirachta) leaves (2.5 kg/m2) were completely free from rhizome rot (P. aphanidermatum).
Soil Solarization: Soil solarization is a soil disinfection practice achieved by covering moist soil with transparent polythene film during the period of high temperature and intense solar radiation. Such a situation leads to the eradication or substantial reduction of soilborne inocula, and consequently a substantial reduction in disease incidence (Katan, 1981) Compared to other soil disinfection methods, solarization is nonhazardous, more economical, and leaves no residue. Soil solarization as a process of preplant soil disinfection was first advocated by Katan et al. (1976), and is presently used widely all over the world.
Soil solarization involves the following steps: preparing the field to make it ready for planting, spreading a transparent polythene sheet evenly on the soil surface, taking care to avoid formation of air pockets, and sealing the side of the sheet. The polythene sheet should be ideally about 300 ^ thick. Adequate soil moisture is necessary during solarization to increase the thermal sensitivity of the target organism, to improve heat conduction in the soil and to enable biological activity (Katan and De Vary, 1991). Soil is generally irrigated once before covering with polythene. Solarization should be done during the months of March to May under Indian conditions when the solar radiation is most intense. The soil should be kept covered for 45 to 60 days.
Soil solarization helps in pathogen and disease control, and as a result leads to significant yield increase (Davis, 1991). Solarization also helps in weed control. Pythium control through solarization has been reported by many workers (Chen and Katan, 1980;
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