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Pigeon pea plants are susceptible to infection by the wilt pathogen at all stages of development. The main symptom of the disease is wilting characterized by gradual, sometimes sudden, yellowing, withering and drying of leaves followed by drying of entire plant or some of its branches (Singh, 1973). Such plants exhibit loss of leaf turgidity, interveinal clearing and chlorosis before death. Isolated wilted plants may appear about a month after sowing and patches of dead plants in the field, usually at the flowering and podding stage, are the first indication of wilt (Reddy et al., 1990). The characteristic symptom of the disease in the adult plants is a purple band extending upwards from the base of the main stem. The band can clearly be seen in pigeon pea, when the green stems of healthy plants are compared to the diseased stems with coloured lines. Partial wilting is quite common and distinguishes the disease from termite damage, drought and Phytophthora blight, which also kill the plants. Partial wilting is due to infection of lateral roots, while total wilt is a result of tap root infection (Reddy et al., 1990). If an infected plant is split open below the purple band, browning of the stem and brown to black discoloration of xylem vessels is visible. Sometimes lower branches show die-back with the purple band extending from the tip downward and intensive xylem blackening (Reddy et al., 1993b).
Resistant Crop Cultivars
There has been considerable research on the control of this disease. The major emphasis has been on the identification of resistance and the production of wilt-resistant cultivars (Deshpande et al., 1963). Pot and field screening techniques have been developed for the identification of resistance (Nene et al., 1981a; Haware and Nene, 1994). For a pot-screening technique, autoclaved pigeon pea stem pieces are mixed with non-autoclaved alfisol in pots (Reddy et al., 1998). The soil is inoculated with a F. udum culture multiplied on sand-pigeon pea flour (9:1) medium. A susceptible cultivar is sown and the wilted plant material is re-incorporated in the pots for three cycles.
Wilt-diseased plots have been used to screen crops against several vascular wilts. At ICRISAT Centre (Patancheru, India), it was found that the wilt disease develops more quickly in alfisols than in vertisols and shows up earlier in alfisols (Nene et al., 1980). The best way to develop the disease is to incorporate stubble from diseased plants into the soil and to grow wilt-susceptible cultivars in intermittent rows all over the field (Reddy et al., 1998). The search for sources of resistance to wilt in pigeon pea began as early as 1905 at Pune, India (Butler 1908, 1910). Many sources of resistance were identified at locations throughout India (Alam, 1931; Reddy et al., 1998). Despite the availability of large number of sources of resistance, only a few cultivars were popular with the farmers.
Multilocation tests conducted in India and eastern Africa helped to identify several lines resistant to wilt at several locations. Moderately resistant lines are available in all maturity groups. Some of these lines also show resistance across locations and seasons (Nene et al., 1981b, 1989; Amin et al., 1993). These lines includes ICP 8863 (Maruti), ICP 9145, ICP 9174, ICP 12745, ICPL 333, ICPL 8363, ICPL 88047, BWR 37. DPPA 85-2, DPPA 85-3, DPPA 85-8, DPPA 85-13, DPPA 85-14, Bandpalera, ICP 47769, ICP 9168, ICP 10958, ICP 11299, C11 (ICP 7118), BDN1 (ICP 7182). ICP 8864 and ICP 9145 have been released for commercial cultivation in Malawi and have become very popular (Reddy et al., 1995). The lines that showed resistance in Kenya were ICP 8869, ICP 9145 and ICP 10960. The lines that were resistant in Malawi are ICP 7855, ICP 9145, ICP 9154, ICP 9174, ICP 9177, ICP 10958, ICP 11297, ICP 11299 and ICP 12738. Maruti (ICP 8863), which has been released for commercial cultivation in India (Konda et al., 1986), has become very popular in peninsular India. Lines which combine wilt resistance and resistance to other diseases have been identified. For example, ICPL 87 and C11 possess resistance to wilt, sterility mosaic and Phytophthora blight; ICP 7667, ICP 8861, ICP 8662 (Hy3C) to wilt, sterility mosaic and powdery mildew; ICPL 81 to wilt and halo blight; BDN1 to wilt, Phytophthora blight and halo blight; ICP 8661, ICP 8662, ICP 8867, ICP 8869, ICP 10962 to wilt and Alternaria blight.
Inheritance of resistance
Limited information is available on the inheritance of resistance to the disease. In a cross between one resistant (ICP 8863) and two susceptible (ICP 2376 and LRG 3C) lines, resistance was found to be controlled by a single recessive gene. The gene was designated pwr1 (Jain and Reddy, 1995).
Nature of resistance
The nature of resistance was analysed in cultivars that were resistant and susceptible to wilt. The extract from a resistant cultivar (C-11-6) inhibited spore germination and the growth of the germtube. The inhibitory compounds in the resistant cultivar were identified as chlorogenic acid and caffeic acid (Murthy and Bagyaraj, 1983).
Pigeon pea is generally grown in inter- and mixed-cropping systems in rotation with other crops. However, since the fungus survives on deep-seated roots of the host, below the depth of ordinary cultivation, the success of rotation will depend upon the field sanitation (removal of affected plants with their roots), hot weather cultivation etc. A 4-5-year rotation has been found to free the field completely of the wilt pathogen (Singh, 1973). Field studies conducted at ICRISAT Centre (Patancheru, India) have shown that crops such as sorghum, castor, maize and groundnut inhibit the soil population of F. udum (Himani Bhatnagar, 1995). One-year breaks with either sorghum or fallow reduced wilt in the following pigeon pea crop from 60-90% to 16 and 31%, respectively (Natarajan et al., 1985). Pigeon pea rotation with tobacco has been recommended as a possible means of control because of the adverse effect of tobacco root exudates on the pathogen (Bose, 1938). Wilt incidence was increased with increasing pigeon pea biomass (Reddy et al., 1994).
Root exudates from a range of crops, which are frequently intercropped with pigeon pea, and green manuring reduce wilt incidence either by reducing population of F. udum or by increasing the antagonistic activity of the microbial population in the soil. A higher incidence of wilt in sterilized soil than in unsterilized soil was attributed to the action of antagonistic organisms such as Aspergillus niger, Rhizopus nigricans and Bacillus subtilis (Vasudeva and Roy, 1950; Vasudeva and Govindaswami, 1953). In later studies, Singh (1973) reported the production of 'bulbiformin' by B. subtilis, which inhibits the growth of F. udum. The antibiotic remains active in soil for 35 days. It was also found that the rhizosphere of resistant cultivars contains a greater population of Streptomyces spp., which are antagonistic to F. udum, than the rhizosphere of susceptible cultivars. The soil composition and certain cultural practices can affect wilt incidence (Shukla, 1975; Upadhyay and Rai, 1981). These findings indicate that the disease can be kept under control if conditions can be created in the soil that are suitable for the development of antagonistic organisms through organic amendments.
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
In an ICRISAT survey conducted from 1975 to 1980, annual losses in grain yield due to wilt were reported to be up to US$ 36 million (Kannaiyan et al., 1984). In eastern Africa, losses were estimated at US$ 5 million (Kannaiyan et al., 1984). Losses caused by the disease are dependent on the stage of wilt occurrence. If wilt occurs prior to podding, loss is total; however, only partial loss may result if wilt occurs at pod filling stage or later (Kannaiyan and Nene, 1981). Long-duration varieties may compensate for loss of early wilted plants. Yield loss in cultivars that are infected but do not show wilt symptoms have not yet been quantified. If wilt occurs during pod filling, the seed may become infected. Such seed may become a source of primary inoculum if not properly treated with fungicides.