The following points highlight the four destructive diseases of wheat. The diseases are: 1. Loose Smut 2. Black Rust or Stem Rust 3. Yellow Rust or Stripe Rust 4. Brown Rust or Orange Rust.
Disease # 1. Loose Smut of Wheat:
This serious disease of wheat is responsible for heavy losses in wheat production in the world. It is caused by Ustilago nuda var. tritici (Sub-division-Basidio-mycotina, Class-Teliomycetes, Order-Ustilaginales, and Family-Ustilaginaceae). The disease is worldwide in occurrence and occurs in India in almost all wheat- producing states, and mainly in U.P. and Punjab.
1. The pathogen is seed borne and the disease appears only when the plants are in the ear-forming stage. Prior to the healthy ears, the diseased ears come out of the boot.
2. Soon the flowers in the ear are replaced by black powdery mass of spores.
3. The infected inflorescence consists of deformed spikelets filled with dry, black powdery mass of spores. Glumes and all other floral parts are entirely replaced by spores.
4. Only the inflorescence axis, ends of glumes and awns remain in the entire inflorescence. All other parts are replaced by spores.
5. The general appearance and growth of plants is not much affected.
1. Caused by Ustilago nuda var. tritici the spores of this loose smut disease are pale olive, spherical to oval, have minute echinulate walls and measure 5-9 µ in diameter. They are produced in very large number and reach the ovaries of the healthy plants in dry season.
2. The spores germinate by producing germ tube. After dikaryotization this enters the young tissues of the ovary. The hyphae remain dormant in such infected grains until next season when such grains are again sown.
3. Mature spores are diploid and called smut spores or teliospores or chlamydospores.
1. Use of resistant varieties is the best control measure.
2. The disease can also be controlled, to some extent, by hot-water treatment to seeds.
3. Use of systemic fungicides, such as Vitavax, Benlate, Thiram or Maneb, also helps in checking the disease.
Disease # 2. Black Rust or Stem Rust of Wheat:
This is still the most serious disease of wheat (Triticum aestivum) throughout the world. In some years, up to 90 per cent reduction in grain yield has been reported from some countries due to this disease In India, the present loss in grain yield from this disease is estimated to be at least 15%, which amounts a loss of over Rs. 1000 million to our farmers.
In India black rust appears slightly late in the season, and in Northern India it is not seen until March. In Southern Indian states, however, it is seen during November-December.
1. The early symptoms appear in the form of elongated, reddish-brown pustules, primarily on stem, followed by leaf sheaths, lamina and floral parts. Since the stem is most severely attacked, the disease is called ‘stem rust’.
2. These pustules of uredia contain several uredospores. The uredial pustules enlarge, merge with each other, rupture the host epidermis and finally expose several uredospores.
3. Slightly late in the season, these uredopustules change into teleutopustule. The teleutopustules or telia are darker and change from brown to black in colour. Because of the black colour, the disease is also called ‘black rust’. Telia contain several teleutospores. Black coloured teleutopustules first develop on stem and then also on leaves and other plant parts.
4. The diseased plants are stunted, and produce small spikes and shrivelled grains or no grain at all.
1. The disease is caused by Puccinia graminis tritici (Subdivision-Basidiomycotina, Class- Teliomycetes, Orafer-Uredinales, Family-Puccinia-ceae), a heteroecious and macrocyclic rust.
2. The mycelium is intercellular with small, round haustoria. Uredospores develop in uredia or uredosori. Each uredospore is stalked, oval and brown coloured. It contains two nuclei and remains surrounded by a thick warty or spiny exine and inner smooth intine. It contains four germ pores. On dissemination the uredospores germinate and propagate the disease very quickly on healthy plants.
3. The same mycelium, which was producing uredia early in the season, starts producing telia containing several teleutospores late in the season. Telia develop more frequently on stem and then on leaf sheaths, lamina and other plant parts. Telia are oblong to linear and dark brown to black structures. Each teliospore or teleutospore is a stalked and bicelled structure with a smooth thick wall.
Each cell of teleutospore has a single germ pore. Two nuclei present in each cell of teleutospore fuse and form a diploid nucleus. Differing from uredospores, the teleutospores cannot again infect the wheat plant. Each cell of teleutospore germinates to form a four-celled promycelium. From each cell of promycelium develops a haploid basidiospore.
4. These basidiospores cannot infect the wheat plants. They are blown by wind to very far distances, and infect the bushes of Berberis vulgaris, the alternate host. These bushes are present on hills.
5. On the upper surface of the leaves of Berberis vulgaris, the basidiospores germinate and produce pyenidia or pyenia. Each pyenidium is a flask shaped body containing an ostiole, receptive hyphae and spermatia or pycniospores. The receptive hyphae and spermatia undergo fusion and form the dikaryotic mycelium. From this dikaryotic mycelium develop aecidia or aecia or aecial cups on the lower surface of leaf of Berberis vulgaris.
6. The aecia are yellow and cup shaped structures containing aeciospores arranged in chains. The aeciospores are polyhedral, binucleate structures arranged in basipetalous manner. The aeciospores have six germ pores and infect the leaves of wheat plant.
1. All measures of proper sanitation of crop and complete eradication of diseased plants should be adopted.
2. Rust resistant varieties of wheat (e.g., N.R 770, N.P. 809, N.P. 829, N.P. 846, etc.) should be used as seed as far as possible.
3. A programme should be launched to eradicate the alternate host.
4. Mixed cropping of wheat and barley should be practised.
5. Spray of several fungicides and other chemicals on leaves, and treatment of seeds with several chemicals, have also been quite helpful in controlling the disease. These fungicides and other chemicals include Zineb, sulphur dust, Nabam, ZnSO4, Dithane M-45, Vitavax, Plantavax and Several antibiotics.
Disease # 3. Yellow Rust or Stripe Rust of Wheat:
This highly destructive disease of wheat occurs mainly in eastern and northern India. Found also in Nilgiri and Pulney hills of south India, this rust is not found in peninsular India. On the hills it is first seen in November-December while in plants it appears during January-February.
1. In early stages, long streaks appear first on leaves and then on leaf sheaths, stalk and glumes. These streaks contain rows of uredosori.
2. In each row, the pustules are oval in shape and 4. Telia or teleutosori develop only rarely. If present, lemon-yellow in colour. They are arranged in rows they are small, oval to linear and black coloured, or stripes, hence called ‘stripe rust’.
3. Even the very minute sori are quite distinct from one another and remain arranged in linear fashion.
4. Only in very late stages, teleutosori or telia appear as black patches or spots. The teleutospores usually do not burst through the epidermis.
1. It is caused by Puccinia striiformis (= Puccinia glumarum).
2. The uredospores are not oval but globose bodies. Their exine contains several fine spines, and the germ pores are 6-11 in number.
3. The telia remain covered for a long time by the epidermis, and teleutospores remain surrounded by brown paraphyses.
4. The apex of teleutospores is not as thick and pointed as of black rust. The teleutospores areoblong with somewhat flat apex and they are slightly constricted at the septum.
Same as of black rust.
Disease # 4. Brown Rust or Orange Rust of Wheat:
Since this rust of wheat attacks only the leaves and only very rarely the leaf sheaths and stem, this is also called ‘leaf rust’. It is also more common in northern India than in south India. In U.P., Punjab and Bihar this is the earliest rust to occur on wheat and occurs even earlier than yellow rust sometime in November.
1. Round or oblong, orange coloured pustules are irregularly distributed on leaves. They are very rarely found on leaf sheaths and stem and never occur in rows or stripes.
2. The pustules are bigger than that of yellow rust.
3. The uredosori burst usually on upper surface of leaves.
4. Telia or teleutosori developed only rarely. If present, they are small, oval to linear and black coloured.
1. The disease is caused by Puccinia recondita, a heteroecious rust. Its uredial and telial stages develop on wheat while pycnial and aecial stages on Thalictrum.
2. The uredospores are brown or orange coloured, spherical with minutely echinulate wall and 7-10 germ pores. They measure 16-28µ in diameter.
3. The teleutospores are bicelled, oblong to cuneiform and slightly constricted at the septum. The apex of teleutospores is rounded and prominently thickened.
Same as of black rust.