In this article we will discuss about the stem rust of wheat caused by basidiorycetes.
Host: Tritlcum Vulgare
Pathogen: Puccinia Graminis Tritici.
Introduction to the Stem Rust of Wheat:
The disease occurs in all wheat growing countries of the world. In India it appears at different times of the year in different parts of the country. It appears in the month of March in Northern India. In Southern and Peninsular India it appears very early in the 4th week of November.
Effects and Importance of Stem Rust Disease:
The stem rust is a serious threat to wheat in India. It causes serious damage in moist areas and moist season. The nature and extent of damage varies from slight to almost complete failure of the crop.
The annual loss from rusts in India was estimated to be about Rs. 40 million (Butler and Hayman, 1906). Mehta (1942) put this loss at about Rs. 60 million annually both in wheat and barley.
Prasad (1960) reported that nearly one million tons wheat worth about Rs. 392 million was lost during 1958-59. The losses include reduction in the quantity of the thrashed grain (low yield), its poor quality and shrivelled lighter grains.
These effects are due to two factors:
1. Robbing off of the food by the pathogen for its own use.
2. Excessive transpiration due to numerous slits in the host tissue (epidermis). These factors result in lessened metabolism, in partial sterility, reduction in the number of grain in the head and shrivelled grain.
Samborski and Shaw (1956) reported increase in the respiratory activity of rusted plants especially at the site of infection. Shaw and Colotelo (1961) found increase in total nitrogen, protein nitrogen, soluble nitrogen and the ratio of soluble to insoluble nitrogen as the rust develops. Prasad (1967), opined that amino acids and proteins are synthesized by the rust fungus itself.
Symptoms on the Grain Host:
Elongated pustules or streaks appear chiefly on the stem and leaf bases, leaf sheaths and even on the glumes. These are the uredia or uredosori. Each uredosorus contains a mass of reddish brown or rusty red, one-celled, binucleate uredospores.
They are exposed by the rupture of the leaf epidermis. The torn epidermis forms a collar-like structure around the oblong sorus. The uredospores are produced by the dikaryotic or secondary mycelium parasitizing the tissue of the host.
This spore stage of the parasite is called the red rust or summer stage. Later in the season the sori turn black. This is due to the appearance of two- celled, black teleutospores or teliospores in the place of uredospores in the old uredia.
When the grains are almost ripe, new and independent oblong to linear teleutosori make their appearance on the stem of the host. They are smooth and dark brown or black in colour. The overlying epidermis is ruptured and the spores are exposed.
The stems at this stage are dry and cracked. The teleutosori contain masses of black two-celled spores called the teleutospores. There is no other marked abnormality in the appearance of the plants. However, the plants look sickly and fail to develop normal ears in the case of a severe attack.
Symptoms on Barberry Host:
The infected barberry leaves show small circular, slightly elevated, yellowish or orange yellow spots on the upper surface. Each spot is dotted with an indistinct pimple, an ostiole of the spermagonium.
A sticky liquid begins to exude through the ostiole. On the lower side of the leaf there appear cup shaped aecidia or- aecial cups. They usually appear opposite the spermogonial or pycnidial spots. The aecidial cups are orange yellow in colour.
Each one of them contains a mass of orange coloured, binucleate spores called the aecidiospores or aeciospores. The infection causes no serious damage to the Barberry host.
Causal Organism:
The causal agent of this disease is Puccinia graminis tritici Erikss. and Henn. (P. graminis Pers.,). It is a heteroecious parasite which completes its disease cycle in two hosts namely wheat and barberry (Berberis) or Mahonia.
Disease Cycle (Fig. 22.17):
Eriksson on the basis of his observations in cereal rusts, especially Puccinia glumarum proposed the “mycoplasm theory”. He held that on the onset of winter, the fungal hyphae degenerated in the host plant. The fungus cytoplasm mingled with that of the host protoplasm in the cell.
As soon as the winter was over, the mycoplasm (fungal cytoplasm) migrated into the intercellular spaces of the host. There the mycoplasm reforms the hyphae and haustoria. The mycoplasm theory was vehemently opposed.
It was not widely accepted because of opposition and thus fell by the wayside. The commonly held view is that normally teleutospores are the overwintering structures (A). But in the plains in India uredospores constitute the primary inoculum.
They are carried from the distant high altitude hills by wind. After the usual resting period the teleutospores germinate in situ (on wheat stem and stubbles in the field). Each cell produces a short promycelium (epibasidium) into which the synkaryon migrates (B).
Each synkaryon undergoes meiosis in the promycelium or the epibasidium. Segregation of the sexual strains takes place. Walls are laid between the haploid nuclei so that each promycelium or epibasidium becomes septate and four-celled (B).
Each cell of the basidium produces a single, uninucleate haploid basidiospores at the end of a sterigma. Of the four basidiospores thus produced two are of plus strain and two minus strain.
The basidiospores are disseminated by air currents. While floating in the air they may chance to fall on young barberry leaves. If temperature and moisture conditions are favourable the basidiospores germinate.
Each basidiospore develops a germ tube or a primary hypha (C). It penetrates the cuticle directly and brings about infection of the new host. Within the host tissue the primary hypha branches freely to form a monokaryotic or haplomycelium.
The hyphae constituting it ramify in the intercellular spaces between the mesophyll cells. The cells are uninucleate. The nuclei in the mycelium are either of plus strain or minus strain depending upon the nature of the germinating spore. The mycelium feeds and grows vigorously.
Eventually it enters the reproductive phase and forms thick mats of hyphae here and there beneath the upper and lower epidermis. The hyphal mats beneath the upper eipdermis function as primordia of spermogonia.
Those beneath the lower epidermis function as primordia of aecidia or aecia. In about a week’s time the primordia beneath the upper epidermis produce small flask-shaped fruiting bodies called the spermagonia.
They are embedded in the tissue in orange yellow spots on the upper surface of the leaves of barberry bush (E). Each spermagonium opens to the outside through a small aperture called an ostiole which projects above the surface of the leaf.
The spermagonium contains three types of hyphal threads:
1. Periphyses:
These are slender sterile, hyphae guarding the ostiole and projecting through it.
2. Spermatiophores:
These are numerous, fine, elongated hyphae arising from the interior of the swollen portion of the spermagonium. They abstrict small, hyaline spermatia at their tips in succession. The abstricted spermatia lie free in the cavity of the spermagonium.
3. Receptive or Flexuous Hyphae:
These are the fine, hair-like hyphal threads seen interspersed between the periphyses. They extend out through the ostiole and project much beyond the periphyses.
The contents of the spermagonium are entirely plus or minus according as the spermagonium has developed from a plus or a minus mycelium. The spermatia emerge in a viscous sugary liquid through an ostiole to the leaf surface along with the flexuous hyphae.
Sexual union or spermatisation, as it is called, takes place between spermatia of one strain and flexuous hyphae of the other strain. The intervening walls between the spermatium and the flexuous hypha dissolve at the point of contact.
The spermatium nucleus passes into the receptive or flexuous hypha through the pore. The spermatium nucleus now passes down the receptive hypha through the septal pores and reaches the basal cell which becomes binucleate or dikaryotic.
The dikaryotic cell develops into a secondary or a dikaryotic mycelium. The transference of spermatia from leaf to leaf is the work of insects. They are attracted by the nectar and visit one spermagonium after another.
Meanwhile the hyphal mats beneath the lower epidermis develop into spherical masses of cells. These are known as the protoaecidia. By this time the secondary mycelium formed from the dikaryotic cells at the base of the receptive hypha reaches the young aecidium.
Its cells mingle with the haploid tissue of young aecidium. As a result a palisade-like layer of binucleate cells is formed at the base of aecidium. These binucleate basal cells produce binucleate aecidiospores in terminal chains. The wall of the aecidium splits open.
The aecidium now assumes a cup-shaped form (E). The lower epidermis also ruptures and the aecidiospores are now exposed. They are unable to reinfect barberry.
The aecidiospores (F) are binucleate and are carried by air currents to the wheat host. Here the aecidiospores germinate each by putting out a germ tube or an infection hypha which enters the host tissue through a stoma (G).
The tip of the infection hypha swells to form an appressorium which covers the mouth of a stoma. The contents of the aecidiospore migrate into the appressorium. A narrow, peg like infection hypha emerges from the appressorium, passes through the stomatal opening and enters the substomatal cavity to form a vesicle.
From the substomatal vesicle arise hyphae which proceed intercellularly into the parenchymatous tissue of the host leaf to form the intercellular mycelium (H). The hyphae send small round or branched haustoria into the host cells.
About ten days after the infection of the grain host (wheat) rusty red and powdery masses of uredospores appear on the stem and the leaves in oblong to circular sori (I). They appear in the months of February to March.
The uredospores are shortly stalked, oblong, echinulate structures with four equatorially arranged germ pores on the outer wall (J). These spores are binucleate. Being exposed they are easily carried by the wind to other wheat plants.
Here, within few hours each uredospore germinates in the surface moisture provided by rain or dew. It develops a germ tube (K) which enters the host tissue through a stoma. Within a week’s time provided the weather conditions are favourable, the new dikaryotic mycelium produces a new crop of uredospores.
They are disseminated in the same way. In this way the disease is spread rapidly and widely during the growing period. The uredospores are the only spores in the life cycle which can reinfect the host plant on which they are produced.
When the grain is almost ripe, black teleutospores begin to appear in the uredosori. Soon after the teleutospores develop in new and independent dark brown or black teleutosori or teliosori (A). The teleutospores are two-celled, thick-walled structures. They are the resting spores.
The part of the life cycle which is passed on the grain host or the wheat plant represents the dikaryophase (H-L). During this phase each cell of the mycelium, each uredospore and each cell of teleutospore has a pair of nuclei called the dikaryon.
One of these nuclei is of plus strain and the other of minus strain. Towards maturity plus and minus strain nuclei in each cell of the teleutospore fuse to form a synkaryon or the fusion nucleus. The mature teleutospores thus represent the reduced diplophase (A).
Puccinia graminis tritici is a polymorphic species as it produces a succession of different types of spores. However, investigations carried out by late Dr. K.C. Mehta (1923-40) in India revealed that there is no local source of infection in the plains.
The uredospores perish in the high summer temperature of the plains. There are no barberry bushes in the plains and thus the teleutospores which are unable to survive the high summer temperatures following the harvest remain ineffective.
Hence, the sexual stage comprising the spermagonium and aecidia is cut out from the life cycle. The actual disease cycle is completed with uredospores alone. The teleutospores are produced but they are non-functional.
This eliminates the chances of perennation of the rust on the alternate barberry host in the hills. This has been confirmed by the fact that the races of P. graminis found on the barberry bushes on the hills are different from those occurring on the cereals in the plains.
In spite of the above-mentioned facts, there is annual recurrence of the rusts in the plains in India. It is due to the fact that the uredospores remain viable on the hills. The summer temperature at an altitude of 5,000 ft. and above is quite congenial for their survival.
There at different altitudes, the uredospores over summer on the out of season wheat plants, stubbles, and grass hosts in the uredial stage. These serve as primary inoculum for the wheat crops when sown.
The infection thus starts from the hills. The uredospores are wafted down from the higher altitudes to the foot hills. From there the infection is carried to the crops in the plains.
Control Measures of Stem Rust Disease:
1. Cultivation of Rust Resistant Varieties:
The cultivation of varieties immune to the rust disease is an important means of combating the disease. Some rust resistant varieties of wheat are available in India. Np 710, Np 718 and Np 770 find favour with the farmers.
The newly bred hybrids Np 822, Np 823 and Np 825 have given good results. They possess high degree of rust resistance. In addition they are high yielders. The recently introduced dwarf Mexican wheat varieties such as Sonora 64 and Lerma Rojo are almost completely resistant to black rust.
2. Eradication of Barberry:
Formerly it was believed that the eradication of the less important alternate host barberry is a possible means of eliminating the disease. We now definitely know that the control of stem rust by this method is not possible in India.
The uredospores which are able to survive on stray and self-sown wheat plants on the hills serve as an inoculum.
3. Use of Fungicides Including Antibiotics:
Despite ceaseless efforts to control wheat rusts through resistant varieties, much success has not been achieved. Practically no variety is resistant for a long period due to emergence of new physiological races.
Hence the use of fungicides to control rust diseases has received much more attention in the recent years than in the past. Grewal and Dharam Vir (1959), Mathur et al (1961) and Tandon et al (1968) recommended Zineb and Maneb to control wheat rusts effectively.
Srivastava, Rai and Aggarwal (1972) recommended the use of Dithane Z-78 and Dithane M-45. The latter controls the disease even at lower conc. (1.5 lbs/acre) when applied with sticker. The yield increased by 33.3% over the control.
A number of other chemicals like RH-124 and Plantavax have as well given quite encouraging results. Heagle and Key (1973) reported that Ozone (O3) inhibits infection, hyphal growth and uredospore formation of wheat stem rust fungus.
Actidione has been recommended to be a useful antibiotic as a fungicide for the control of rusts plus zinc sulphate at fifteen days interval from the first week of February are quite effective.