Rust diseases of wheat are regarded as the most destructive pathogens on the crop.
However, the wheat crop suffers from three types of rusts:
(1) Black or stem rust caused by Puccinia graminis f. sp. tritici,
(2) Yellow or stripe rust caused by Puccinia striiformis, and
(3) Brown or leaf rust caused by Puccinia recondita.
The damage to wheat is mainly due to brown or leaf rust and yellow or stripe rust. Black or stem rust appears quite late in the main wheat growing areas and normally the crop escapes severe damage.
Type # 1. Black or Stem Rust of Wheat (Triticum Vulgare):
Black or stem rust of wheat is distributed throughout the world wherever susceptible wheat varieties are grown. In India, particularly in the northern part, this disease appears quite late during the growing season (late in March or even later). It is usually the time when the wheat is maturing and, therefore, the crop escapes much damage.
Black or stem rust attacks aerial parts like leaves, leaf-sheaths, and stems; stem being often most severely affected. As a result of this disease, the quality and yield of grains is considerably reduced. The infected plants usually produce fewer tillers, set fewer grains comparatively to healthier ones, and seeds are smaller in size.
The causal organism of stem rust, a fungus called Puccinia graminis, was first recognized by Persoon in 1797. But, its true life history was explored in 1865 when de Barry, by inoculation experiments, was successful to demonstrate that P. graminis needs two hosts, wheat and barberry, to complete its life-cycle.
Symptoms:
The symptoms appear in the form of elongated, narrow, elliptical reddish-brown pustules on stem, leaf-sheath, and leaves. The stems become often most severely affected.
These pustules (called uredopustules or uredia as they bear a type of spores called uredospores) usually run parallel with the long axis of the infected parts, vary from one quarter of an inch to more in length, frequently coalesce with each other and burst early exposing powdery mass of reddish or rust-coloured uredospores. One can see, however, the prominent fringes of ruptured epidermis all around the burst part of uredopustules.
Later in the season when the wheat plants are about to mature, a different kind of pustules (called teleutopustules or telia as they bear a type of spores called teleutospores or teliospores) develop either as a result of transformation of uredopustules or independently of uredopustules.
In the first condition, the production of uredospores in uredopustules is gradually replaced by the production of teleutospores; this results in pustules gradually changing from reddish-brown to black in colour as the teleutospores are dark-brown in colour. The teleutopustules burst in the same manner as the uredopustules and expose teleutospores.
Causal Organism:
The fungal pathogen is a macro-cyclic, heteroecious rust completing its life-cycle by means of five different types of spores – uredospores, teleutospores (tetiospores), basidiospores, pycniospores or spermatia, and aeciospores. Uredo- and teleutospores occur on wheat, basidiospores on stubble or straw fallen off in soil as a result of germination of teleutospores, and pycniospores (spermatia) and aeciospores on barberry (Berberis vulgaris) leaves.
Barberry is the alternate host of the pathogen. The pathogen is a biotroph. However, the mycelium of P. graminis tritici is colourless and intercellular sending small, round or branched haustoria inside the host cells. A mass of hyphae collect beneath the epidermis and develops pustules.
Uredospores are dikaryotic (bi-nucleate), oval, single-celled, brick-red or rusty coloured, provided with minute spiny structures on wall, and each measures 25-30 x 17-20 µm. They possess four germ pores along an equatorial band and are the only kind of stem rust spores that can infect the host on which they are produced, i.e., the uredospores are produced on wheat and can then reinfect wheat producing more uredia and uredospores.
Teleutospores consist of two thick-walled smooth, superimposed, and dikaryotic (binucleate) cells. They are spindle-shaped, thick walled, dark- brown in colour, with rounded or pointed apex, and measure 40-46 x 15-20 µm. Each of the teleutospore cells consists of single germ pore. Teleutospores are the final spores produced on wheat and serve as the “resting spores”.
The two nuclei lying in each cells of a mature teleutospore fuses together (karyogamy), when it is still on wheat or during the resting period in soil resulting in a diploid nucleus. Each diploid cell now germinates giving rise to a “promycelium” (generally referred to as a “basidium”) in which the diploid nucleus migrates and undergoes meiosis producing four haploid nuclei. Meiosis is followed by septation giving rise to four cells in a promycelium each containing a single haploid nucleus.
These four cells produce four “basidiospores” on sterigmata. The mature basidiospores are forcibly discharged, are taken away by wind, fall on barberry leaves, and germinate thereupon. They develop flask-shaped structures called “pycnia’ or “spermogonia” on the upper surface of barberry leaves. Pycnia are ostiolate and consist of pycniospores or spermatia of ‘+’ or ‘male’ type and receptive hyphae of ‘-‘ or ‘female’ type and vice versa depending upon the ‘+’ or ‘-‘ nature of pycnia (spermogonia). Sexual fusion occurs between spermatia and receptive hyphae of opposite strains.
The dikaryotic mycelium thus developed then forms another type of structures, namely, “aecia” on the lower surface of barberry leaves. Aecia are cup-shaped, yellow structures enclosed by peridium (wall), which ruptures later on imparting ‘bell-shaped’ appearance to aecium (now called aecidial cup).
Each aecidial cup contains ‘aeciospores’ produced in chains. Aeciospores are yellow, echinulate, each with six germ pores. On germination, aeciospores produce germ tubes, which disinfect only wheat and never the barberry. The wheat plant thus infected with aeciospores produces uredopustules containing uredospores.
Very interesting aspect of this rust causing pathogen, which has been studied in recent past in India and elsewhere, is that the life-cycle has least concern with the disease-cycle and annual recurrence. The disease- cycle has concern usually only with uredospores particularly in our country.
Disease Cycle:
(i) Perennation:
Though the fungus perennates in cooler regions as teleutospores, which remain dormant on stubble or straw generally fallen off onto the soil for several months (at least 18 months) to complete their life cycle, the high temperatures prevailing in Indian plains after harvest of wheat crop kill the uredospores as well as teleutospores.
This shows that there is no local source of primary inoculum in plains of India. Mehta (1940, 1952) considered the survival of source of primary inoculum during summer in the form of uredospores produced in the cooler conditions of hills such as Himalayas in the north, particularly central Nepal, and Nilgiri and Pulney hills in the south.
But, if the Himalayas were the active foci of source of primary inoculum, stem rust should have appeared normally in the foothills and adjoining plains as early as the first week of February, but it appears much later at the end of March or in April. On the other hand this rust appears as early as December-January at many places in South India.
Joshi (1986) reported, however, that the main source of primary inoculum of stem rust of wheat in plains lies in the southern hills and the hills in north India contribute very little because the environmental conditions in the plains at the time when primary inoculum comes from northern hills are not favourable for the rust. The cyclonic disturbances originating in the Bay of Bengal help disseminating uredospores (the source of primary infection) through air currents and rains.
(ii) Primary Infection:
The uredospores carried to the plains of India work as the source of primary infection. They germinate on healthy hosts giving rise to dikaryotic mycelia. The mycelia so produced enter inside the host, frequently branch, spread in the intercellular spaces, develop uredopustules, and result in the production of new crop of uredospores only within 5-6 days if conditions are favourable.
(iii) Secondary Infection:
The uredospores produced as a result of primary infection are the main source of secondary infections during the growing season. Since the uredospores are capable of germinating and producing new crop of uredospores within a short span of time (5-6 days only) if conditions are favourable, they result in many secondary infection cycles during one growing season thus adding to the severity of the disease. This is the reason why uredospores are also called “repeating spores”.
Predisposing Factors:
Temperature plays a crucial role in the incidence of stem rust. The minimum, optimum, and maximum temperatures for germination of uredospores are 20°C, 24°C, and 30°C, respectively. When the temperatures are below than that stated, the uredospores take more time to cause infection, i.e., the incubation period is increased. This results in fewer infection cycles during the growing season and very slow development of disease.
Humidity also is a factor which determines stem rust infection. Availability of free water and dew deposit on the surface of the leaves and other susceptible parts of the host atleast for two hours is essential for quick germination of uredospores. However, if one takes temperature and moisture together, the disease becomes severe in a season of abundant moisture when temperatures for major part of the day are around 15-20°C.
Management:
(i) Though the fungus grows well on its alternate host to complete its life- cycle, this survival of pathogen least contributes to disease development in India. In our country, however, the problem is of perennating the pathogen on self-sown wheat plants and other collateral hosts growing on hills. If this source could be destroyed, possibilities of rust incidence in plains could be minimized or completely eliminated.
But, this appears practically not possible because such hosts may be present in very inaccessible places where the search for them may be almost impossible. However, it has been suggested that if immune or resistant varieties for the hilly regions could be raised and the area saturated with them, the amount of inoculum could be drastically minimized.
(ii) Mixed cropping of wheat and barley with suitable crop renders good crop insurance even if the main crop fails.
(iii) Reduction in proportion of nitrogen in the N.RK ratio in a fertilizer can help reduce rust incidence in a susceptible variety.
(iv) A variety of chemicals have been tested against rust incidence. These include – Sulphur dust; Nabam and ZnSo4 at 14 day interval; Zineb, Maneb, and Zinc as four foliar spray at 10-14 day interval; Dithane M-45; antibiotics; and systemic fungicides like Vivatx, Plantvax as foliar sprays and seed treatment. Propiconazole (Tilt) at 0.1% has been found effective in controlling stem rust (also brown and yellow rusts) because it persists for 12 days and completely inhibits uredospore germination.
(v) Since the pathogen evolves new physiological races, the resistant varieties of one time become susceptible on the other. This is the reason why this aspect requires continuous investigation followed by time to time recommendation of resistant varieties. All India coordinated Wheat Improvement Programme recommends varieties resistant to rusts time to time. At present, varieties like HD 2278. HW 741, WL 614, Sonara 63, 64, etc. are considered resistant to stem rust of wheat.
Type # 2. Yellow or Stripe Rust of Wheat:
Yellow or stripe rust is mainly restricted to the north and northwestern region of India and also occurs in Nilgiri and Pulney hills of south India. However, it is totally absent from peninsular India.
This rust has been found on the new crop of wheat sown on hills to appear as early as November-December, and in the foot hills of north India in December or in the first fortnight of January. It is usually seen first in the northern foot hills and adjacent areas such as Gurudaspur and Roper in Punjab and Pantnagar in the tarai region of Uttaranchal.
Yellow rust causes extensive losses in the field due to foliage destruction followed, sometimes, by sterility of spikelets, or in the production of badly shrivelled grains. The extensive cultivation of Kalyansona variety of wheat over a large area of northwestern India has aggravated the yellow rust problem during 1968-69 and its breakdown during 1970-71 was as a result of a new virulent race of the pathogen.
Symptoms:
The symptoms of this disease normally become evident earlier than the black or stem rust. When the host is mildly attacked, the symptoms appear mainly on leaves, but, if the infection is severe then the other aerial parts such as leaf-sheath, stalks, and glumes are also attacked and they show bright-yellow coloured pin-head like uredopustules arranged in long streaks. Severe infection results in drying of leaves. The teleutosori (telia) appear mostly on the lower surface of leaves in the form of black coloured streaks.
Causal Organism:
The pathogen is a biotroph and its primary host is wheat. It produces uredospores in uredosori and teliospores in teliosori. The uredospores are wind-dispersed and germinate on the leaf surface and penetrate indirectly through stomatal openings. Inside the host leaf, fungus grows in the intercellular spaces in the form of much branched dikaryotic mycelium. Intracellular haustoria are formed, which penetrate the host cells and absorb nourishment. Soon it starts producing uredo sori below the epidermis.
The uredospores are nearly round, echinulate with a hyaline epispore. Each uredospore possesses 6-10 scattered germ pores. The teleutospores are oblong to cuneiform with almost flattened apex. Both the pustules are covered by host epidermis. The teleutospores are two-celled and are interspersed with brown coloured unicellular paraphyses.
No intervening hosts for pycnial and aecial stages of the fungus have been discovered.
Disease Cycle:
The fungus is heteroecious but its alternate host is not yet known. The fungus survives in the hills during summer months at an altitude of about 1990 m and above in volunteer wheat or on some weeds. About 90 weeds (e.g., Agrophyron semicostatum, Bromus catharticus, B. japonicus, Hordeum murinum, etc.) are known to act as collateral hosts of the fungus.
A grass, Muhlenbergia hygelli, growing wild in the Shimla hills, has been found to harbour yellow rust, but its role in the annual recurrence is yet to be ascertained. Source of primary infection are uredospores, which come from hills and germinate on wheat plants. The teleutospores are capable of germination immediately without a resulting period.
Management:
(i) Use of resistant varieties is the best way of controlling the rust.
(ii) The chemical control measures used for black or stem rust can be used to control the disease.
Type # 3. Brown or Leaf Rust of Wheat:
Brown or leaf rust is restricted to wheat and certain grasses and attacks leaves almost exclusively, rarely leaf sheaths, and very rarely stems. This rust occurs in all parts of our country but is more common in northern and eastern parts than in peninsular India. It is the earliest rust that appears on wheat in parts of Bihar, Uttar Pradesh and Uttaranchal, and Punjab and, together with the yellow rust, causes severe damage resulting in great loss to the crop.
Brown or leaf rust occurred in epidemic form in northwestern region of the country during 1971-72 and 1972-73 resulting in the loss of 0.8 to 1.0 million tons and 1.5 million tons of wheat, respectively. It has been reported that the epidemic of brown rust on Sonalika variety of wheat in Uttar Pradesh and parts of Bihar caused a loss of one million tons of wheat.
Symptoms:
The symptoms of disease appear on leaves in the form of bright orange coloured pustules. These pustules, on maturity, turn brown in colour. The pustules, which turn orange to brown, are the uredosori.
The latter are never in rows but may be gathered in small clusters or may be irregularly scattered all over the leaf surface. They are bigger in size than the uredia of yellow rust disease. However, the teleutosori (teliosori) either fail to develop or develop at the fag end of the disease. These are small, over to linear, black in colour, and remain covered by the epidermis.
Heavy rusting of the foliage results in poorly developed root system, poor quality and quantity of grains, and reduced yield of straw.
Causal Organism:
The pathogen is a biotroph and heteroecious. Its primary host is wheat and the secondary host is Thalictrum. In wheat, the secondary dikaryotic mycelium grows in the intercellular spaces. It penetrates special branches intracelullarly in the cells of host, which get modified to haustoria. The haustoria are the main organs of nutrition and absorb nourishment from the host.
Usually the fungus produces uredosori in host leaves. The uredospores are brown, round or oblong, echinutate, stalked and have 7-10 germ pores. These spores are dispersed in the air and cause infection to other plants. The infection by germ tubes from uredospores takes place through stomata on either side of the leaf.
The teliospores are bi-celled with flattened top of the upper cell, brown and smooth. They are produced in teliosori, which are small, over to linear, black in colour, and covered by the epidermis. Paraphyses, which are abundantly present, divide the teliosorus into numerous chambers. The number of chambers in each teliosorus is more than in the yellow rust.
The teliospores germinate to produce basidia and basidiospores, which normally cause infection in alternate host. The functional alternate host, which has not been reported from India, is Isopyrum fumariodes. In India, the non-functional host is Thalictrum, which commonly occurs in hills. The spermogonial and aecial stages of the fungus have not been reported.
Disease Cycle:
Brown or leaf rust survives during summer at altitudes of 1450-2050 m on Nilgiri and Pulney hills in south and Himalayas in north India because it can withstand warm weather better than the yellow or stripe rust. It has two foci of infection in India.
If we take the whole country into account, the uredospores are air-dispersed from site of survival and the rust gets established in early January and sometimes even earlier in the plains of Tamil Nadu and Karnataka in the south, and in the foothills of the Himalayas in the north.
The first build-up of inoculum takes place in the plains of Karnataka in south and uredospores are wind-blown northwards to Maharashtra and Madhya Pradesh. At the same time the inoculum (uredospores) of the foothills of Himalayas moves to the northern plains.
Therefore, the brown rust appears slightly later in the western foothills of north India. The rust populations of the North and South, moves in opposite directions, finally merging into each other. This may result in epidemics in any part of the country if climatic condition becomes favourable.
Once the infection is established on wheat, uredospores are quickly and repeatedly produced in uredosori, are dispersed on to healthy leaves causing secondary infections. The latter may take place several times during a particular growing season.
Management:
(i) Like black rust, use of resistant varieties in considered to be the best measure for disease management.
(ii) Management using chemical measures is almost the same as in case of black rust. However, one or two foliar sprays of fungicides such as dithiocarbamates and RH-124 in field conditions have given good control of the disease.
(iii) Since nitrogen tends to increase susceptibility while potash renders opposite effect, reduction in the proportion of nitrogen in NPK ratio can help reducing rust incidence in susceptible varieties.
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