Classification of Uredinales | Fungi

In this article we will discuss about the classification of uredinales. 

Family Pucciniaceae:

This is the largest and most important family of the order Uredinales. Rust fungi included in this family infect numerous valuable agricultural plants and cause enormous loss to crops. Members of the family are manifold and the complexities, owing to polymorphism, heteroecism and biologic specialization are very great.

Some of the economically important rusts are: Uromyces appendiculatus is rust of beans (Phaseolus mungo and P. radiatus); Puccinia graminis f. sp. tritici is black stem rust of wheat (Triticum aestivum); Puccinia malvacearum is hollyhock rust, attacks malvaceous hosts; Gymnosporangium juniperi-virginianae is juniper apple, and cedar apple rust; and Gymnosporangium sabinae is juniper, and pear rust.

Teleutospores in the family are stalked, stalk sometimes short or evanescent. They are one to several-celled being borne singly on a pedicel, or in groups on either simple or several united to form a parasol-like head on a compound stalk. The teleu­tospores are subepidermal and covered or free, and erumpent or embedded in a gelati­nous matrix.

Sporidia are formed from promycelia produced as a result of the germi­nation of the teleutospores. Uredospores are solitary.

Genus puccinia:

This genus possesses spherical spermogonia with minute ostioles and aecia with peridia. It has uredospores which are unicellular, spherical ellipsoid to variously- shaped having rough wall with many germ pores. The teleutospores are separate, and produced in flat sori. They are pedicellate consisting of two superimposed cells each of which is provided with a germ pore. The germ pore of the upper cell is at the apex; whereas the lower cell has its germ pore placed immediately below the septum present between the two cells.

Some Indian species of Genus puccinia:

Puccinia graminis Pers.; P. graminis f. sp. poae (Pers.) Erikss.; P. graminis Pers. f. sp. tritici Erikss. and Henn.; P. malvacearum Mont.; P. recondita Rob. ex. Desm.; P. striiformis West.

Puccinia Graminis Pers:

Puccinia graminis, commonly known as rust fungus, produces one of the most important plant diseases designated as rust disease. It has a wide host range of germinaceou plants including wheat, oats, barley, rye, and numerous wild and cultivated grasses. It is most important economically on wheat, on which crop it has been a subject of most extensive study for centuries.

Special mention may be made of classical researches of most Anton de Bary and the Tulasne brothers who established the parasitism of the rust fungus. The researches of de Bary have repeatedly served as the basis of fundaments investigations in parasitism, cytology and biologic specialization. The fungus is heterotcious, parasitizes two unrelated species of hosts alternating from cereal or grass the barberry.

The life cycle of the fungus is extremely complicated involving five distinct stages, viz., 0, spermogonia bearing spermatia and receptive hyphae; I, aecia bearing aeciospores; II, uredia bearing uredospores; III, telia bearing teleutospores; IV promycelia bearing basidiospores. This is known as long-cycled or macrocyclic rust.

The somatic structure of the fungus consists of septate hyphae which penetrate the host tissues and cause destruction but do not become organized into structures of definite form. The hyphae branch intercellularly and produce small haustoria which penetrate the host cells.

They occur in two well-marked alternating phases, the moncy karyon (n) and the dikaryon (n+n). The fungal hyphae live in the stem and lead tissues of graminaceous hosts but do not produce any visible change until late in the season, particularly in the wheat plants, mostly when the wheat is in ear.

Elongated rusty red streaks or pustules appear on the stem, leaf sheaths and leaves particularly on the stem. They very soon burst breaking through host epidermis due to pressure of the developing spores from inside the host tissue. Groups of minute yellowish spores formed just beneath the host epidermis are exposed.

They are known as uredosori (sing, uredosorus) or uredia (Fig. 266). The spores are known as uredospores. These spores give external evidence of the presence of mycelium growing among the cells of the host tissue. The uredosori are rusty red from which the name ‘rust’ is given to the fungus and the disease it causes. The production of uredospores is spoken of as the ‘red rust’ stage of the disease.

The uredospores are dikaryotic (n+n), single-celled, generally oval in form, and stalked. They are rather thick-walled. The wall bears numerous short, spiny projections and four equatorially arranged germ pores. They are yellow to orange-red. The uredospores are disseminatec by wind.

They directly infect the graminaceous host, specially the wheat plant and are chiefly responsible for the rapid spread of the disease. These spores are cap able of germination within four or five days after they are shed, if sufficient moisture is present.

The uredospores germinate rapidly in water or moist air, sending out a germ tube. The germ tube does not enter in the host tissue directly, but the tip swells up into an elongated appressorium (p1. appressoria).

From this, a narrow branch passes through stomata and immediately swells up to form a substomatal vesicle. Soon a dikaryotic hypha (n + n) arises from one end and grows towards a neighbouring cell, into which it sends a haustorium.

Infection is now complete and the dikaryotic mycelium (n+n) continues its growth within the host. After 10 days or 2 Weeks a new crop of uredospores is formed. Thus it is possible to have a considerable number of successive crops of uredospores in a single season, and from one or a few infected plants the disease may be spread over a large area.

The uredospores are capable of infecting only the graminaceous host and are responsible for repeating infection of the susceptible host.

Within a short time, further formation of uredospores ceases, and from the mycelium that has been producing uredospores, or from mycelia originating from uredo­spores, gradually black pustules or black sori appear on the same stem and leaves. By the rupture of the host epidermis black sori consisting of black masses of spores are exposed.

These black sori consist of dark-coloured spores known as teleutospores or teliospores. The sori are called teleutosori (sing, teleutosorus) or telia (Fig. 267).

During the transitional time between the development of uredosori and teleutosori both uredospores and teleutospores may be formed in the same’ sorus. Such a sorus is often known as mixed sorus. The teleutospores are stalked, thick-walled smooth, two-celled with superimposed cells, the top cell being rounded or blunted or pointed and thickened at the apex.

They are ellipsoid to oblong clavate being slightly constricted at the septum, dark-brown (black is in the mass).

Each constituent cell of the spore is furnished with a germ pore, situated apically in the top cell, and just below the septum in the lower cell. When young, each cell is dikaryotic (n+n), but karyogamy takes place as the spore matures. Unlike uredospores, the teleutospores are not capable of germination immediately they are formed, but must go through a period of rest of several months.

Hence they are not responsible for the rapid spread, but are rather resting spores to carry the fungus over to the following season. After resting in the open on dead host tissues or on the soil they germinate freely. But their survival in such a condition in the plains of India is rather difficult since the tempera­ture during March to June is considerably high.

The teleutospores perennate in the uninucleate diplophasic condition (2n) after karyogamy has occurred.

On germination, each cell of the teleutospore gives rise to an elongated germ tube­like structure called a promycelium (Fig. 265A), also known as basidium. Each cell of the teleutospore is also known as a probasidium.

Those who hold this designation, according to them, the probasidium germinates to form an epibasidium which is nothing but the promycelium and the teleutospore cell from which the epibasidium is produced is the hypobasidium. The diploid nucleus of the teleutospore cell then migrates in the promycelium where it undergoes meiosis producing four haploid nuclei (n).

The promycelium is then transversely partitioned to develop into a four-celled structure, cells having a single haploid nucleus one in each. Each cell of the promy­celium gives rise to a sterigma on which a single ellipsoid sporidium (p1. sporidia) (Fig. 265A) or basidiospore is formed. The haploid nuclei ultimately squeeze through the sterigmata into the sporidia (n).

The sporidia are obliquely set on the sterigmata. Two of the sporidia are of one strain and two are of the other, may be designated as plus strain and minus strain respectively. The sporidia are forcibly discharged at maturity by water droplet method. They are caught up by air currents, and if they alight on the leaves of barberry they will germinate. The sporidia do not germinate on wheat or other graminaceous hosts.

The sporidia germinate on the upper surface of the barberry leaf in presence of moisture. Each sporidium produces a germ tube which gains entrance into the host tissue directly through the epidermis (Fig. 268A), not by way of stoma. The germ tube then produces an extensive intercellular (Fig. 268B) monokaryotic mycelium (n) which depending on the sporidium, carries plus or minus factor in its nuclei.

After a short period of growth, the mycelium submerged below the surface on the upper side of the leaves develops flask-shaped structures called spermogonia (sing, spermogonium) (Fig. 269A), also known as pyenia (sing, pyenium). The spermogonia are mono­karyotic (n) in nuclear condition.

The small yellow or reddish spots that are visible on the upper surface of the infected leaves are the areas where spermogonia appear.

The spermogonia open to the surface of the leaves. The cavity of each spermogonium is lined with numerous spermatophores which cut off minute colourless rod-shaped or oval monokaryotic (n) spermatia (sing, spermatium) (Fig. 269A), also known as pyeniospores.

Besides these structures, receptive hyphae (monokaryotic) growing from the side walls and several periphyses project outward through the opening of each spermogonium.

Depending on the nature of sporidia carrying plus or minus factor, the corresponding spermogonia produced by them are also plus or minus, which may develop on the same leaf or on different leaves. Similarly the spermatia produced by the spermogonia will carry plus or minus factor. The mature spremogonia produce spermatia and a kind of sweet sticky liquid.

This liquid along with spermatia exuded out from the spermogonia attracts insects.

The insects unwittingly transfer the sper­matia in moving from one spermogonium to other and thereby the spermatia come in contact with the receptive hyphae, which extend out through the opening of each spermogonium.

By the time the spermogonia are being developed just beneath the upper epidermis the monokaryotic mycelium (n) also produces near the lower epi­dermis, opposite the spermogonia, a number of aecial primordia (Fig. 269A), also known as immature aecia. Each one may also be termed an aecium initial.

Buller (1933) suggested for this structure the term proto-aecidium. The aecial primordium consists of an upper closely packed weft of hyphae, the basal Cells, and a lower group of larger parenchyma-like cells. Unless spermatization takes place by the contact of a plus spermatium with a minus receptive hypha, or vice versa, the aecial primordium develops no further, and is functionless.

During spermatization spermatial contents pass into the receptive hypha by a pofe developed in the wails at the point of contact of the spermatium and the receptive hypha. The’ spermatial nucleus passes down through the hypha from one cell to another establishing dikaryotic condition (n+n) and ultimately the hyphal cells of the aecial primordium become dikaryotized (n+n).

Within a few days after initiation of the dikaryotic condition, the aecial primordium is stimulated to further growth, and after about two days later, on the lower surface of the leaf, aecium (pi. aecia) (Fig. 270A) or cluster cup (n+n) is formed.

That spermatization between a plus spermatium and a minus recep­tive hypha or vice versa is a prerequisite to the development of aecium has been confirmatively established by Craigie (1927) and Buller (1950). The yound aecium is closed (Fig. 269B) and buried in the tissue of the leaf, but as growth proceeds it pushes through the leaf surface and opens, exposing the spores within.

The aecium is cup- shaped, composed of whitish hyphae, and the uppermost cells forming the rim of the cup, readily slough away, leaving a ragged margin. It has distinct peridium. Within the cup are the dikaryotic (n+n) aeciospores (Fig. 270A), yellow to orange in colour, extending in chains from specialized stalk cells at the base of the cup.

The cells of the chain producing aeciospores are alternately large and small, the former maturing into an aeciospore but the latter remains sterile and is called intercalary cell or disjunctor cell (Fig. 270B).

When mature, the aeciospores fall from the cup and are carried away by currents of air. The aeciospores are infectious only to graminaceous hosts. If they fall upon graminaceous hosts, and if suitable conditions of temperature and moisture prevail, they germinate by sending germ tubes which pass through stomata into the tissues of the stem and leaves and produce dikaryotic hyphae (n+n).

The dikaryotic hyphae in one to two weeks produce rusty red uredosori on the stem, leaf sheaths and basal part of the leaf blades. Thus the cycle is continued. Puccinia graminsi seldom does much damage to the barberry but is often very destructive to wheat and ether graminaceous hosts.

Aeciospores may infect graminaceous hosts hundreds of miles away from the barberry bushes where they are produced. The principle of heleroecism in Puccinia graminis was demonstrated by Anton de Bary. Although the sporidia cannot infect the graminaceous hosts, the uredospores may frequently sur­vive a mild winter and infect the wheat during the following spring.

In some parts of India, the uredospores survive throughout the whole year by infecting various graminaceous hosts which grow at different altitude in the hills and in the plains. As such it is evident that Puccinia graminis may be propagated without the aecial stage. Life cycle of Puccinia graminis is presented in Figure 271.

Biological specialization of Genus Puccinia:

That the aeciospores can bring about infection of graminaceous hosts has been demonstrated as early as 1816. Inoculations in the reverse order between one host and another were made in 1865. This study has pro­duced very interesting results. Puccinia graminis is a good morphological species within which there are biological forms or biological strains or subspecies.

These biological forms are morphologically identical but differ in physiological, biochemical and pathological characters. They are specialized in their nature of parasitism and are specific to particular host or group of hosts. Such a behaviour is known as biolo­gical specialization. Stakman and many others have added much to our know­ledge of biological specialization in Puccinia graminis.

The biological forms or subspecies of Puccinia graminis designated formae speciales abbreviated as f. sp., are named by rinomials, i.e., by adding to its binomial a third name after the chief host parasizing. These are genetically different strains with differing pathogenic qualities.

Eriksson and Henning have divided Puccinia graminis into fdllowing subspecies:

Puccinia graminis f. sp. tritici parasitizes Triticum aestivum, Hordeum vulgare and many wild grasses.

Puccinia graminis f. sp. avenae parasitizes Avena sativa and wild grasses.

Puccinia graminis f. sp. secalis parasitizes Sec ale cereale and wild grasses.

Puccinia graminis f. sp. agrostidis parasitizes Agrostis caninis and wild grasses. Puccinia graminis f. sp. airae parasitizes Aira caespitosa.

Puccinia graminis f. sp. bromi parasitizes species of Bromus and other wild grasses. Puccinia graminis f. sp. phleipratensis parasitizes Phleum pratense and wild grasses: Puccinia graminis f. sp. poae parasitizes Poa compressa and related species.

Within each of these biological forms or subspecies there are numerous physiologic races which differ in their nature of parasitism on various agronomic varieties’ of the host. But all these biological forms and physiologic races have the common barberry plant as their alternate host and whatever differences they exhibit are on the graminaceous hosts. They are believed to originate during growth upon the barberry plant.