Write a short essay on the symptom, transmission and control of plant diseases caused by Puccinia.

Ans. Puccinia: The Rest Fungi : The genus includes about 1800 cosmopolitan species which are obligate parasites and are extremely host specific. There are two types of species: (i) Autoecious, which complete their life cycle on a single host. (ii) Heteroecious, which complete their life cycle on two hosts P. graminis, Puccinia causes ‘rust’ diseases of several economically portant plants like wheat, barely, oats etc. The wheat rusts are of great economic importance. Three types of wheat rusts are reported from our country.

1. Yellow rust caused by Puccinia glumarum (Striiformis).

In India it is limited to the northern and eastern parts. It appears by January. The uredosori are confined chiefly on the leaves. The uredosori are subepidermal and remain covered for a much longer time than in the other rust. When they finally break through, the yellow uredospores are shed and dispersed by wind. The teleutosori appear towards the maturity of host. The teleutosori are compact black spots, arranged in rows like the uredosori.

They do not break through the epidermis but remain as flat crusts.

2. Brown rust caused by puccinia triticina.

In India it is prevalent in northern and eastern parts appearing mostly by January. Brown rust is mainly confined to leaves. Heavy infections retard photosynthesis and therefore the leaves turn yellow and wither before the plants are actually mature. Uredosori break out on leaves of orange or brown specks. They are grouped in small clusters or are irregularly scattered. Each uredosori is round to oblong in shape and burst early and shed the uredospores. Teleutosori are rarely seen. When found, these are oval to linear, black and covered by the epidermis.

3. Black rust caused by Puccinia graminis, symptoms on wheat are produced on stem, leaves, leaf sheath, glumes and culms as brown pustules. They are known as uredia or uredosori and release a powdery mass of brown uredospores. Uredospores are capable for reinfecting the wheat. Telia or teleutosori may arise from the fresh infection of uredospores later in the season. Telia are black in colour and releases black mass of teleutospores on maturation. The grains are shrivelled and are much lighter in weight and, therefore, yield is reduced to a considerable extent.

The pycnia and aecia of this rust occur on barberries, the alternate host. The pycina occurs on the upper surface of the leaves whereas the aecia on the under surface.

Life Cycle of Black Rust of Wheat : P. graminis, the black rust of stem or rust of wheat is the most well known. It requires two hosts to complete its life cycle (i) wheat as primary host and (ii) barberry as an alternate host. This rust robs the host food for its own purpose affecting their metabolic activities adversely. Transpiration occurs in excess due to rupturing epidermis thus causing dying of the host tissues. It is macrocyclic fungus and reproduces by producing five different types of spores at different stages in its life cycle. The mycelium within the host tissue, comprises septate and branched intercellular hyphae which send haustoria into the cells for obtaining nourishment.

1. Uredo Stage: First visible evidence of this rust is in the form of reddish brown pustules on stems and leaves by late february. Each pustule is pale sorus called uredosporus in which uredospores develop. Due to the pressure of the developing uredospores, the epidermis breaks and the uredospores are liberated. The uredospores are single celled stalked, thickwalled oval spores provided with 4 germ spores. They contain enough reserve food to support them during long distance dissemination. The uredospores are capable of immediate germination on reaching new wheat plants. Through several successive infections during the season they spread disease and fungus to various places. At the close of wheat season, the uredosori start producing teliospores besides uredospores and finally convert into teliosori producing only teliospores. Teliosori are also produced independently from the mycelium produced by uredospores of late infections.

2. Teleuto Stage: Teliosori (teliospore) are black, and elongated pustules, producing teliospores. They are darkbrown, stalked, bicelled, spindle-shaped structure. The apex is rounded or pointed. They possess thick smooth wall. Each cell is binucleate provided with a germ pore. It is a resting spore which represents the sexual parts of puccinia. In each cell, the two nuclei fuse and a diploid nucleus is formed. At this stage it undergoes a resting period, period attached to wheat straw or in soil.

3. Basidial Stage : On return of favourable condition in spring, the teliospores germinate to form curved promycelium. The diploid nucleus now divides meiotically and forms four haploid nuclei. Septa are formed to separate them into four cells in the promycelium. Each cell produces a single basidiospore borne asymmetrically on a fine sterigma. Two of the basidiospores are of one strain and the two of opposite strain. The basidiospores are thin-walled spores which are discharged by an explosive mechanism, and disseminated by wind.

4. Spermatogonial Stage: The basidiospores can infect only berberry leaf or young twig. The basidiospores germinate by giving out a germ tube which directly penetrates the epidermis. Inside, an intercellular monokaryotic mycelium (+ or – strain) depending on the basidiospore is formed, which gets nutrition through haustoria. Several basidiospores may infect the same leaf and each basidiospore forms a primary mycelium.

In due course each diseased spot develops a flask-shaped pycnium (or spermatogonia) which opens on the upper surface of leaf by a minute pore, the ostiole which is guarded by sterile hyphae called periphyses. The pycnium wall is lined internally by a layer of very fine elongate hyphae called spermatophores. These cut off a succession of small spermatia, which ooze out through ostiole. In addition to spermatophores receptive hyphae (flexuous hyphae) arise from the lateral wall of pycnium. These hyphae are seen amidst periphysis but project out much further beyond them.

During course of pycnium formation, some of the hyphae of each mating type form protocide by reaching near the lower surface of leaf. It is first globose mass of hyphae but soon its cells towards the upper side differentiate into a palisade like layer called basal cells. The cells facing the lower epidermis are called displacement cells. Development of aecidia depends upon the intermingling of spermaita of opposite strains. Spermatia of opposite stains are exchanged (Spermatization) chiefly through the agency of insects.

5. Aecidial Stage: Spermatia fuse on coming on contact with the receptive hyphae of opposite strains. The common wall dissolves and the male spermatial nucleus enters the female receptive hypha. The male nucleus by mitosis forms a second male nucleus, which moves to the next cell. The male nuclei produced by successive mitotic divisions pass down from the cells of primary mycelium binucleate (dikaryotization). Dikaryotization is followed by the formation of aecidiospores. The binucleate cells of the chain are alternately large and small. The former maturing into an aecidiospore but the later remains sterile and is called intercalary cell or disjunctor cell. Some of the basal cells lying on the periphery of protoaecium mature into a protective layer peridium which surrounds the entire spore it elongates pushing through the lower epidermis of the barberry leaf followed by its breaking up. The aecidium assumes a bell shaped forms.

6. Germination of Aecidiospore: In mature aecidium the intercalary cells disintegrate separating the aecidiospores free from each other facilitating their scattering by wind. The aecidiospore walls are thick and each spore possesses a few germ pores through which hyphal tubes come out on germination.

The aecidiospores return the rust to the wheat plant because they are incapable of infecting barberry which produced them.

On wheat, the aecidiospores germinate and form germ tubes which penetrate the host tissue through stomata. The spores are binucleate and responsible for the establishment of a dikaryotic mycelium within wheat. This mycelium produces uredospores.

The best way to control leaf rust is to grow resistant varieties. In most parts of the country leaf rust has been effectively controlled because of the widespread use of wheat varieties with resistance to this disease.

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