Though the wilt disease of arhar (pigeon pea) occurs commonly throughout our country, it causes severe damage in parts of Maharashtra, M.P., U.P., Bihar and, Tamil Nadu. This is perhaps the most destructive fungal disease of this valuable pulse crop in our country.
Continuous cropping of arhar in the same field results in as much as 50% plant mortality due to the disease. Almost about 5-10% damage to standing arhar crop is a common feature every year in Uttar Pradesh and Bihar. However, most of the attacked plants succumb completely to wilt disease and their recovery is rare.
Symptoms of Wilt Disease:
Though the arhar plants are susceptible to the attack of wilt disease throughout their development, the main symptoms of the disease is the wilting of seedlings and adult plants as if they have suffered from water shortage even though there may be plenty of moisture in the field. The wilting is characterized by gradual, sometimes sudden, yellowing, withering, and drying of leaves followed by drying of entire plant or some of its branches.
When one examines the main roots and the base of the stem, he usually finds that the tissues are blackened, either uniformally or especially in the early stages, in streaks. This blackening is best seen on removing the bark. Streaks can be traced down to the roots and are found to arise from the main or lateral roots, which have become rotten. The rotting is dry and no exudates are given out.
Causal Organism of Wilt Disease:
The fungus is confined to vascular tissues and is both inter- as well as intracellular. The septate hyphae run across the cells, growing rapidly along the inside of the walls of larger vessels. This results in the plugging of such vessels.
The mycelium is hyaline and produces spores of three types inside the host. These spores are – micro-conidia, macro-conidia, and chlamydospores. The micro-conidia are small, elliptical, curved, and unicellular or with one or two septa, measure 5-15 x 2-4 µm, and are produced on hyphal branches. The macro-conidia are produced in small cushions of somatic mycelium on the surface of the host near ground level.
They are long, curved (fusaroid), pointed at the tip and knotched at the base, septate (usually 3-4 septa), and measure 15-50 x 3-5 µm. The chlamydospores are produced on the host generally when the latter becomes older. They are oval or spherical, single or in chains, terminal or intercalary, and have ability to persist in soil for long period of time.
Wilt Disease Cycle:
(i) Perennation:
The pathogen is soil-borne surviving in the soil as a saprophyte in the absence of the living host. The roots of the host are generally left to remain in the soil for long durations after harvest and so the pathogen perennates for years on such debris. Apart from the saprophytic mycelium, the conidia and chlamydospores persist for even considerably longer and more longer duration respectively in the soil and prove to be the source of primary inoculum.
(ii) Primary Infection:
The primary infection occur mainly through the fine rootlets, which are penetrated by the hyphae perennating in the soil or the hyphae formed as a result of the germination of conidia and chlamydospores present in soil. From infected fine rootlets the pathogen mycelia ultimately pass into the larger roots, which are not directly penetrated by the hyphae unless they are injured. It has been observed that the above-ground plant parts are never attacked by the pathogen.
(iii) Secondary Infection:
Secondary infection in case of this disease is rare as the conidia produced on aerial parts of the host seldom succeed in causing infection.
Predisposing Factors:
Though the wilt of arhar develops at a wide range of temperatures, the temperatures between 17-29°C are considered conducive to disease development. Manuring with superphosphate and cattle dung also help increase in disease incidence. However, Shukta (1975) have reported that the sandy soils are also more conducive to severe occurrence of wilt disease of arhar.
Management of Wilt Disease:
(i) Since the disease is soil-borne and no treatment of soil on a field scales in possible, the only method practicable to control wilt disease of arhar is employing cultural practices.
Though several cultural practices have been recommended, crop rotation is considered the best way of reducing soil-borne infection. A 4-5 year crop rotation programme has been found making the field completely free from the pathogen. Tobacco, sorghum, pearl millet, cotton, and resistant pigeonpea cultivars are recommended as rotation crops. However, the success of crop rotation depends on field sanitation (removal of affected plant parts), deep ploughing during summer, and similar practices.
(ii) Mixed cropping with Sorghum (Jowar) has been found to be most effective. Two cycles of sorghum followed by pigeonpea has been found to reduce the disease incidence by 16%. It is considered that the root exudates of sorghum have a suppressive effect on the pathogen in soil thereby suppressing infection of pigeonpea.
(iii) Employing green manures in the field has been found reducing wilt incidence. This has been as a result of increase in antagonistic microbial population in soil. The main antagonistic microorganisms to lower the incidence of wilt disease are – Aspergillus niger, Rhizopus nigricans, and Bacillus subtilis.
(iv) The use of resistant varieties is of foremost importance. Varieties C11, C28, C36, F18, T17, NP15, NP38, and NP41 are resistant to wilt of pigeonpea lines ICP4769, 0063, 9168, 10958, 11299 and cultivars C11 (ICP7118) and BDN 1 (ICP 7182) have been found showing medium to long duration resistance in all the 5 years of multi-locational trials (15 different locations) in Bihar, U.P., Maharashtra, Bengal, Delhi, and Karnataka.
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