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Puccinia graminis tritici belongs to the species of Puccinia. Puccinia is a parasite of many cereals such as wheat, barley, rye, oats, etc and causes serious damage to crops. More than 700 species of Puccinia have been reported so far all around the world. Most common of these is the Puccinia graminis tritici parasitizing wheat.
Puccinia graminis tritici is a heteroclous, i. e., requires two alternate hosts to complete its life cycle; and macrocyclic, i. e., produce more than one type of spores during its life cycle, rust. The primary host is Triticium aestivum (wheat), upon which three types of spores, the urediniospores, teliospores and basidiospores are produced, and diakaryotic phase of the life cycle is also confined to it; whereas the alternate host is the Berberis vulgar is (barberry bush), upon which pycniospores and aeciospores are produced, and the haplophase is confined to it.
Plant Body____ Mycelium: The mycelium consists of intercellular septate hyphae with spherical haustoria. The septa contain simple pores. The mycelium is dikaryotic mycelium in wheat and monolaryotic in berberry bush.
Life Cycle ____ Reproduction:
The life cycle is completed in about one year s time and is characterized by formation of five different types of spores. These spores are produced in definite sequence.
Infection of Wheat: A symptom of infection on wheat leaves is the appearance of streak-like brick-red pustules, uredinia, between the veins in early summer. The uredinia are mainly distributed on the internodes and leaf sheaths.
Uredinia and Urediniospores:
The uredinia contain stalked, one-celled, binucleate, yellowish, spiny, oval shaped urediniospores. The wall of the spores has four green pores. The pressure from the developing spores causes a break in the host epidermis to form uredinia. The urediniospores are detached by wind and blow to fresh wheat leaves. They germinate by formation of germ tube which penetrates the leaf through a stoma. The tip of the germ tube expands to form a vesicle from which branches aise to give to intercellular mycelium and haustoria. Within about 7-21 days of infection, a new crop of urediniospores is formed which spread the infection further. Therefore, this stage is the repeating stage of the rust is responsible for spread of this disease.
Telia and Teliospores:
When the wheat is nearing maturity, a second kind of spores may be visible along with the urediniospores. These spores are dark-red, thick-walled, and two celled. They are called teliospores. The individual cells are binucleate at first, but the nuclei unite later to form a diploid nucleus.
Finally the formation of urediniodpores is stopped and pustules form teliospores only. These pustules are called telia and these appear as black raised streaks along leaf-sheaths and stems of infected plants. These presences of telia provide the disease its name, the black stem rust of wheat. The developing teliospores exert pressure on epidermis which ruptures to expose the spores.
Overwintering: The teliospores represent overwintering stage. These remain dormant during winter and germinate after a period of maturation following spring (April or May). The teliospores survive the winter on wheat stubble.
Basidia and Basidiospores:
Each cell of the teliospore produces a curved, four-celled metabasidium. The diploid nuclei of teliospores divide by meiosis to produce four haploid nuclei. A septum is formed after each division of meiosis, separating the four nuclei into four cells. A short sterigma develops form each cell of the probasidium, on the tip of which a small basidiospore is formed Each basdiospore is small, ellipsoid, elongate, unicellular, uninucleate and haploid. The basidiospores are of two types, teo of + strain and two of – strain, and projected from the basidium. The basidiospores are unable to infect wheat.
Infection of Barberry Leaves:  The basidiospores infect young leaves of the alternate host, the Berberis vulgaris (barberry). The infection of a Berberis leaf by a single basidiospore results in the formation of a haploid mycelium which appears a yellowish, circular pustule.
Pycina and Pycniospores:
A few days after the infection, the hyphae of the fungus near the upper epidermis develop several flask shaped structures called pycnia  (spermogonia) upper surface of the leaf penetrating the epidermis. These open to the surface of the leaf through a pore.
The mouth of the flask is lined with a tuft of unbranched tapering, pointed, orange-coloured hairs, the periphyses, and among periphyses are thin-walled, branched hyphae, the flexous hyphae (receptive hyphae). The body of the pycnium is lined with tapering cells which give rise to minute, oval or spherical, hyaline, uninucleate  pycniospores.
Dikaryotization: The pycniospores may be of + or – mating types. The pycniospores (spermatia) are exuded out in small droplets of nectar. The sweet-scented nectar attracts insects which feed on nectar and during the process the pycniospores stick to their mouth parts. When these insects visit another pycnium they rub their mouth parts against the flexuous hyphae to get rid of the pycniospores and the spores are transferred to the flexuous hyphae. A small ferm tube arise from the pycniospore is transferred to the flexuous hyphae, divides and migrate to the original haploid pustule. This is dikaryotization.
Aecia and Aeciospores:
While the development of the pycnium is taking place, the mycelium of the fungus penetrates the entire leaf, and lnot-like structures of uninucleate cells are formed from the hyphae present near the lower epidermis. These are proto-acecium or aecial primordia. The pycnial nucleai travel down to the cells of aecial primordial to make these cells binucleate. These dikaryons give rise to chains of cells made up of alternating long and short cells. The longer cells enlarge and become aeciospores, but the short cells become crushed.
While the aeciospores are developing, the cells surrounding them differentiate into a layer of cells called peridium. The outer walls of these cells are thick and fibrous. The aeciospores chains and peridium burst through the lower epidermis of the Berberis leaf. The spores are visible as orange coloured cells enclosed in white-coloured cup like peridium. The cup like sori are called aecia and several of these are clustred together beneath a pustule, so that this stage is popularly known as cluster-cup stage.
The aeciospores are violently discharged from the end of the spore chain and are carried to wheat leaves where they germinate to form uredinia. The aeciospores are also capable of infecting Berberis leaves.
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