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In this article we will discuss about Puccinia Graminis. After reading this article you will learn about: 1. Habit and Habitat of Puccinia Graminis 2. Symptoms of Puccinia Graminis 3. Vegetative Structure 4. Life Cycle.
Contents:
- Habit and Habitat of Puccinia Graminis
- Symptoms of Puccinia Graminis
- Vegetative Structure of Puccinia Graminis
- Life Cycle of Puccinia Graminis
1. Habit and Habitat of Puccinia Graminis:
P. graminis is an obligate parasite, polymorphic, macro cyclic and heteroecious rust (Figs. 11, 12). It effects wide range of hosts including wheat, barley, oats and rye. Grass hosts include Agrostis, Dactylis and Agrophyron. P. graminis tritici involves in its life cycle two distinct alternate host plants i.e., wheat (Triticum vulgare fam. Poacae) and Barberry (Berberis vulgaris fam. Berberidaceae).
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The wheat plant is called the primary host and the barberry plant is secondary or alternate host. In Northern India the black rust appears after March, but in South India, the rust may appear as early as in the fourth week of November thus, causing great loss to wheat crops.
2. Symptoms of Puccinia Graminis:
On Wheat:
The symptoms of the disease appear as large, elongated and brown pustules (uredosori) on the stem, leaf, sheath and leaf (Fig. 1 A). Later on these brown pustules change into black coloured large pustules (teleutosori, Fig. 1 B). Grains of the infected plants are shriveled, much lighter in weight and thus reducing the yield.
The pathogen shows a balanced host parasitic relationship. Even in the severe infection, the parasite does not cause much serious damage except that the growth of the plants may be somewhat retarded and the granis may be of reduced size and of poor quality.
On Barberry:
Infection first starts on the dorsal surface of the leaf in the form of minute, dark coloured and flask shaped pycnia which appear as yellow spots (Fig. 2 A). Beneath Pycnia, on the ventral surface, appear cup like projections of aecia (Fig. 2 B) or aecidia.
3. Vegetative Structure of Puccinia Graminis:
The mycelium is dikaryotic (each cell of the mycelium bears two nuclei) on primary host (wheat) and monokaryotic (each cell of the mycelium bears only one nucleus) on the secondary or alternate host (barbery). The monocaryotic mycelium is also called hapiomycelium or primary hyphae and the dikaryotic mycelium is called secondary hyphae.
The mycelium is well developed, branched, septate and does not spread throughout the host, but is localised to isolated patches. It is either intercellular or intracellular, with the former producing bulbous, branched or knotted haustoria into the cells for obtaining nourishment.
The transverse septa are present or long intervals between the cells. Each septum contains a simple central pore. In contrast with many Basidiomycetes the dolipore parenthesome complex is completely absent Cell wall is made up of fungal cellulose. The cytoplasmic membrane surrounds the granular cytoplasm and reserve food material remains in the form of glycogen bodies and oil globules.
4. Life Cycle of Puccinia Graminis:
Puccinia graminis is long cycled rust (macro cyclic). At the time of reproduction it produces five distinct stages in a regular sequence.
These are as follows:
Stage 0: Spermogonia bearing spermatia and receptive hyphae.
Stage I: Aecia bearing aeciospores.
Stage II: Uredia bearing uredospores.
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Stage III: Telia bearing teleutospores.
Stage IV: Promycelia bearing basidiospores.
Out of these five stages, Uredo stage, Teleuto stage are produced on the primary host (wheat) and remaining two stages, (spermogonial and aecial stages) are produced on the secondary host i.e., barberry.
Stages on Primary Host (Wheat):
Uredospore’s or Uredo stage:
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This stage is formed by the infection of the aeciospores brought from the infected barberry plants or by the uredospore’s themselves coming from the neighbouring wheat plants infected earlier. Both the spores are bi-nucleate and on germination, produce a germ tube on wheat leaf.
The germ tube grows over the surface of the epidermis of the host and on reaching a stoma its tip swells up and forms a vesicle like structure called appressorium (Fig. 3). The protoplasm of the germ tube migrates into the appressorium. Now it is cut off from the germ tube by a septum. The appressorium produces a narrow hypha. It enters inside the sub-stomatal chamber through stoma.
Its tip again swells up and forms a sub-stomatal vesicle. The contents of the appressorium migrate to vesicle through narrow hypha. A hypha of the dikaryon (two nuclei) cells develops from this vesicle (Fig. 3). It branches and produces hyphae which spread in between the cell (intercellular) but occasionally produce hausotria.
Development of Uredospore’s:
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Within 5-6 days, the mycelium absorbs sufficient food from the host. It begins to aggregate near the surface of the infected organs and forms a compact mass. These are called uredia. From these uredia arise vertically a layer of bi-nucleate parallel cells known as basal cells. The basal cells elongate vertically and divide transversely into a lower cell (foot cell) and an upper cell (uredospore mother cell).
The upper cell divides again and its upper daughter cell swells to form a single, bi-nucleate, oval, uredospore or uredinospore, while the lower daughter cell matures into a stalk (Fig. 4 A). Thus, the uredospore’s are formed in a group and each such group is called as uredospore’s or uredinium (Fig. 4 A). The developing uredospore’s exert pressure on the over-lying epidermis. By this pressure the epidermis bulges out and later breaks up and the uredospore’s get exposed.
The uredospore’s are golden brown and oblong, ovate or ellipsoidal in shape. They are double walled, echinulate, binucleate (the two nuclei belonging to opposite strains) and possess four equatorially arranged germ pores (Fig. 4 B).
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A uredospore can infect only a wheat plant. After falling on a suitable host it germinates within a few hours and produces a dikaryotic mycelium. The mycelium is capable of producing uredospore’s again within 10-12 days after germination. Thus, these spores cause several successive infections during the season, and spread the fungus and the disease from field to field provided the environmental conditions are favourable (sufficient moisture). On the basis of the aforesaid behaviour these uredospore’s are also known as repeating spores.
Teleutospores or Telial Stage:
Towards the end of the growing season of wheat crop, the environmental conditions become unfavorable (hot and dry) for the growth of the uredospore’s. Now uredosori produce another kind of spores called teleutospores. First, they develop among the uredospore’s within the same sorus, but later they develop in separate sori known as teleutosori or teleutopustule (Fig. 5 A).
As the crop matures, the number of uredospore’s is reduced and the sori contain only teleutospores. This stage is known as the black stage and hence the name black rust is given to the disease (Fig. 5 A, B). The teleutospores are dark brown or black in colour. They are bi-celled and spindle shaped structures (Fig. 5 C) with a pointed apex and thick smooth wall.
Each cell of a teleutospore has a single germ pore and two nuclei (one of plus strain and the other of minus strain, Fig. 5 C). As the teleutospores reach towards maturity, karyogamy takes place and the two nuclei fuse to form a diploid nucleus (Fig. 5 C). The development of teleutospores is entirely similar to the uredospore’s.
At this stage the teleutospores undergo a period of rest. During resting period they lie on the ground or still attached to the host. These are the dormant cells and are capable of tiding over unfavorable period.
Basidial Stage:
After the renting period, the teleutospores germinate during the early part of spring. They germinate in situ and either one or both of its cells give rise to a germ tube. known as promycelium. The promycelium together with the teleutospore cell is called basidium. However, many authors prefer to call the teleutospore cell as the hypo-basidium and the promycelium as the epibasidium (Fig 6 A-C).
The diploid nucleus of the teleutospore migrates into the promycelium and divides meiotically into four haploid nuclei (Fig. 6 C). The septa appear between the nuclei and divide the promycelium into four haploid cells. Each haploid cell of the promycelium produces a slender, short, lateral, tube-like structure known as sterigma (Fig. 6 D). The sterigma swells up at the end to form a spore like cell.
The haploid nucleus from each promycelium cell migrates into this developing spore cell through its respective sterigma. Thus, at the tip of each sterigma, a minute spore is formed.
This spore is called basidiospore (Fig. 6 D). Each cell of promycelium produces a single basidiospore. Thus, from a single cell of teleutospore four haploid, unicellular, uninucleate basidiospores are formed. Two, out of the four telutospore basidiospores are of ‘+’ strain and the other two of strain.
Soon after the basidiospore formation they are forcibly ejected by the ‘water droplet method’. (In this method a liquid begins to collect in the form of a droplet at the base of the basidiospore.
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This droplet gradually attains a bigger size and suddenly pushes off the basidiospore forcibly into the air to a short distance.) The basidiospores are carried away by wind. They are capable of germinating only on Barberry plants (Berberis vulgaris) available on hills. They perish soon if the alternate host is not available.
Stages on Barberry Plant:
The basidiospores, which fall over the upper surface of barberry leaf start germinating soon. They germinate by giving out a germ tube which penetrates through the epidermis.
The germ tube elongates and divides inside and develops into hyphae. These hyphae grow between all the cells lying in between the lower and upper epidermis. The hyphae are composed of uninucleate haploid cells (primary mycelium) and are of ‘+’ or strain.
Several basidiospores of different strains may infect the same Berberis leaf. Thus, haplomycelium of two different strains (‘+’ or ‘-‘) is formed. It remains haploid for sometime and the fusion between these two strains (‘+’ or ‘-‘) may occur at a later stage.
Spermogonial or Pycnidial Stage:
This stage is also known as Pycnial or spermatial stage. After about four days of the infection, the haplomycelium collects and forms dense mats both beneath the upper and lower epidermis. The mycelial mats beneath the upper epidermis are known as primordium of spermogonium while the mats beneath the lower epidermis are known as primordium of aecidium or protoaecidium.
In 7 to 10 days after infection, each primordium of spermogonia matures into a small flask shaped structure called spermogonium or pvcnidium. The pycnidia appear as minute yellowish specks on the upper surface of the leaf (Fig. 2 A). A vertical section through these specks reveals that each spermogonium opens on the upper surface of the host leaf through a pore like structure known as ostiole (Fig. 7).
Its wall consists of three kinds of hyphae:
(i) Periphysis:
The ostiole is surrounded at the fringe by the long, delicate, sterile hyphae known as periphysis. They develop near the ostiole from the spermogonial wall and project from the ostiole.
(ii) Flexuous or Receptive Hyphae:
They also arise from the lateral wall of the spermogonium. They are slender, delicate, cylindrical, septate, simple, branched or un-branched with blunt ends. They are present amidst periphysis and sometimes it is difficult to distinguish between the two.
(iii) Spermatiophores or Pycnidiophores:
These are slender, short, vertical, uninucleate hyphae which arise form the base of the spermogcnium (Fig. 7). Each spermatiophore (or sporophore) produces several small uninucleate spermatia or pycnidiospores at its tip by abstraction method (Fig. 8 A).
The spermatia are unicellular, small, oval to spherical, hyaline and smooth walled structures. The spermatia fill the spermogonial cavity and are exuded from the ostiole in a droplet of nectar, which is a thick, sticky, sweet liquid.
According to Craigie (1927) and Butler (1927) the spermatia function as male cell while receptive or flexuous hyphe represent the female sex organs (Fig. 8 B). The spermatia may be ‘+’ or in their sexual nature depend r.g upon the mycelium, produced by the basidiospores ‘+’ or ‘-‘. The insects are attracted by this necter.
The spermatia are dispersed from one spermogonium to another spermogonium on the same leaf – this necter leaf to another leaf. As a result, the ‘-‘ spermatia are transferred to ‘+’ receptive hyphae and are transferred to receptive hyphae. Now the spermatization takes place.
The spermatia of strain come in contact with the tip of the receptive hyphae of opposite strain. The intervening wall at the one stain of contact between these two dissolves and the spermatium nucleus passes downwards through septal pores and form a bi-nucleate cell (Fig. 9 A-C). This pair of nuclei of opposite strains is called a dikaryon and this process is called dikaryotization.
Aecial or Aecidial Stage:
The haplomycelium forms the primordium of aecidium or protoaecidium beneath the lower epidermis. The further development of protoaecidium into aecidium takes place only after the dikaryotization.
The spermatial nucleus (male nucleus) by mitotic division forms a second male nucleus, which moves to the next cell, through septal perforation. In this way the male nuclei produced by successive mitotic divisions pass down and all the cells of primary mycelium are dikaryotized.
The dikaryotic basal cells of the protoaecidium arrange themselves vertically beneath the lower epidermis and are called as sporophores. Each bi-nucleate basal cell then cuts off a chain of bi-nucleate cells in basipetal succession on the side towards the lower epidermis of the host.
These cells are the aecidiospore mother cells (Fig. 7). These cells further divide transversely to form a large cell and a small cell. The large cell develops into aecidiospore while the small cell remains sterile and is known as disjunctor or intercalary cell. The latter dissolves and sets free the aeciospores.
With the development of the aeciospores some of the bassal cells lying at the periphery of protoaecidium mature into a one-celled thick protective layer called peridium.
This entire structure is cup shaped and is known as aecium. The developing aeciospores rupture the peridium by exerting a pressure on it. Thus, the aeciospores are liberated. They are unicellular, polyhedral, thin walled, bi-nucleate and orange yellow coloured.
Germination of Aeciospores:
The aeciospores are disseminated by wind. They are capable of immediate germination but cannot infect barberry plants. Falling on suitable host i.e., what leaf they germinate by producing a germ tube or primary hyphae.
The further development of the germ tube is similar as described in the uredinal stage and ultimately the dikaryotic mycelium is produced. This is the mycelium which produces the uredospore’s and later the teleutospores on wheat. In this way, the life cycle of Puccinia graminis is completed.
