Rhizome rot disease is prevalent in countries of Indian subcontinent, Japan, and Fiji Islands. In India, the disease occurs wherever the ginger crop is grown but is quite severe in southern part. The most badly affected state is Kerala, which produces almost more than two third of total ginger in our country.
In Tamil Nadu, thread blight disease of ginger caused by Pellicularia spp. (Rhizoctonia solani), which is a similar rot disease, simultaneously attacks the crop and both in combination prove to be very destructive. Rhizome rot is not confined only to the crop in the field but also causes 80-90% destruction of conns in storage. However, the disease causes normally 8-15% loss in the field but it may result in total loss in low lying infested fields if all the conditions become favourable.
Symptoms of Rhizome Rot Disease:
In the beginning, the green colour of the leaves starts fading (chlorosis) from tip downward through margins leaving the central portion of the leaf normally green. Infected leaves later wither and die and the symptoms then appear on stems. While withering leaves are still attached to the stem, translucent brown discolouration appears on the base of the stem and its portion just above the soil line becomes watery and soft. The disease progresses from base region gradually to the rhizomes in the soil. The latter decompose and turn into a decaying mass of tissues enclosed by the comparatively tough rind.
If the infected rhizomes are sown, the young shoots arising from them damp off. Pseudostems wither and die resulting in poor crop production if the pathogen infects the plants after the shoots have arisen. If the infection takes place in early stages, mostly the rhizomes are not formed.
Causal Organism of Rhizome Rot Disease:
Rhizome rot of ginger is caused by 8 different species of Pythium in different parts of the world. Pythium aphanidermetum, P. myriotylum, P. butleri, P. graminicola, P. vexans (= P. complectans), P. pleroticum, and P. deiense are the species that have been found causing the disease in India.
The hyphae of P. myriotylum are straight or sinuous, up to 8.5 µm in diameter, sparsely branched, and forming numerous clavate or knob-like appressoria in clusters on host surface. The hyphae produce terminal or intercalary sporangia sometimes consisting of portions of hyphae on which swollen, lobulated or digitate outgrowths are attached laterally. The sporangia produce zoospores within them or in the vesicle developed apically on them.
The zoospores are reniform (kidney-shaped) with two flagella (one smooth and other tinsel) attached in the middle groove. Sexual reproduction is oogamous. Oogonia are terminal or intercalary in position on hyphae and are abundantly produced. They are nearly round, smooth, and thin-walled.
Antheridia are diclinuous, clavate or crook-naked, and are formed at various distances below the oogonium. As many as 10 antheridia (usually 3-6) have been found attached to a single oogonium. Oospores are sub-spherical, light yellow, and contain a single reserve globule and a sub-spherical or flattened refrainment body. The average diameter of oospores is about 29 µm and their wall is 1-2 µm thick.
Rhizome Rot Disease Cycle in Ginger:
(i) Perennation:
The disease is both soil-borne and seed-borne. Oospores present in diseased plant debris fallen off the infected plant survive in soil and act as the main source of soil-borne primary inoculum in favourable conditions. In addition, the pathogen also survives mainly in the form of oospores in diseased rhizomes and the latter used for seed introduce the pathogen in the field.
(ii) Primary and Secondary Infections:
Although the method of germination of oospore is not clear, it is assumed that it corresponds to Pythium causing damping- off disease. The perennating oospores in soil germinate and result in zoospores/germ tube which, on coming in contact, causes primary infection. Secondary infections are caused by zoospores produced as a result of primary infection during the same growing season. During the late stage of growth of the host, the pathogen undergoes sexual reproduction resulting in the formation of oospores that perennate and serve as the source of primary infection during the growing season.
Predisposing Factors:
40°C temperature is most favourable as zoospores are produced abundantly at this temperature. Low lying fields and water-logging conditions are conducive to disease development. However, the incidence of the disease is very high in virgin soil containing abundant decomposing matter.
Management of Rhizome Rot Disease:
1. Since rhizomes may carry pathogen inoculum, use of healthy rhizomes as seed is the first requisite to manage the disease.
2. Water-logging in the field must not be allowed.
3. Since the pathogen is often deep-seated in infected rhizome, a half hour dip of seed rhizomes in 0.25% Antracol (propineb; zinc propylene bisdithiocarbamate), 0.3% Fycop (copper oxychloride), or 0.3% Blitox-50 is recommended to control the disease.
4. Spraying of the soil with Bordeaux mixture once before sowing (6:6:50) and then after germination (5:5:50) at 2-3 weeks interval partially controls the disease.
5. The disease can be best controlled by treating both seed rhizome and soil with metalaxyl (Ridomil 5G or Apron 35 WS).
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