Rust of bajra or pearl millet commonly appears in the warmer regions and has been reported from many parts of the world including North America, Sudan, Kenya, Nigeria, Uganda, South Africa, Rhodesia, Tanzania, Pakistan, and India. In India, it occurs in every belt particularly Tamil Nadu, Maharashtra, Madhya Pradesh, U.P., and Bihar. The disease causes extensive damage to both dry and irrigated crops. When the plants are infected early in the season the disease results in heavy reduction in grain yield.
Symptoms of Rust Disease:
The symptoms appear in the form of small, brownish-yellow pustules in groups, arranged often in longitudinal series, on the leaves. Usually the uredia begin to develop at the time the crop is about to flower though in some cases they have been observed much earlier. The sori first appear in the upper half of the leaf near the tip. Soon, however, the intensity of infection increases and they are found throughout the leaves covering the entire surface in severe cases.
Uredia are deeply yellowish brown and occur on both sides of the leaf, being more common on the upper surface. Telia develop much late than the uredia. They are smaller, isolated or in groups and long remain covered by the epidermis.
Very often the tissues of the leaf around the sori quickly dry up, with the result that the telia are seen in the middle of straw-coloured lesions. The whole-leaf may wither completely. When the incidence of rust is heavy the field presents a scorched appearance.
Causal Organism of Rust Disease:
The pathogen is biotrophic and heteroecious. Its uredial and telial stages develop on bajra, while the pycnial and aecial stages commonly occur on brinjal (Solatium melangena); i.e., the latter is an alternate host of the pathogen. Several minute, orange-yellow, dot-like pycnia develop on upper surface, whereas orange coloured aecia develop on the lower surface of the leaves of brinjal. Pycnia and aecia may develop on the petioles also.
The uredia and telia, develop on bajra leaves. The uredia are sub-epidermal. At maturity, they rupture the epidermis, which forms flakes round the sori and the uredospores are then exposed. The uredospores are borne on hyaline pedicels, which are longer than the spores.
They are oval, pyriform or elliptic with four equatorial genu pores, measuring 33-38 by 23-30 µm in diameter. The uredospores are yellowish, brown with a coloured wall and sparsely echinulate and the echinulations being abundant near the apex. Paraphyses are present mainly along the margins, which are longer than the pedicels of the spores. Uredospores germinate producing one or more genu tubes.
The telia are also sub-epidermal like uredia and remain covered by the epidermis for a long time without bursting. They are dark brown or black in colour. The teleutospores are bright brown, cylindrical to long oval, rounded or flattened at the tip, which has a much thickened wall, slightly narrowed at the septum and smooth.
They measure 40-60 by 16-20 µm and have a short colourless stalk. The apical germ-pore is indistinct, but they germinate at the apex in the usual way, with the formation of a 4-celled promycelium (basidium), which bears elliptical sporidia (basidiospores), 11 x 6 µm in diameter.
Rust Disease Cycle:
The disease is air-borne. The uredial cycle is repeated on bajra resulting in subsequent new infections, while the basidiospores (sporidia) germinate on brinjal, the alternate host, or other solanaceous species such as S. torvum, S. xanthocarpum, etc. It is advocated that the rust may be able to survive on brinjal crop, which grows all the year round. However, the role of alternate and collateral hosts in the perennation of the pathogen and the causation of primary infection on bajra during next growing season needs further investigations.
Management of Rust Disease:
1. The disease can be effectively managed using resistant varieties. The latter should be evolved and distributed to growers.
2. Preventive sprays with 100 ppm of Cupramar and Dithane S-31 have been found to control the disease to some extent.
3. Effective control of rust of bajra has been reported using bio-control agents (e.g., Chaetomium globossum, Trichoderma koningii, Fusarium oxysporum, etc.) by Sinha and Kapooria in 1966.
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