Classification of Ustilaginales: 2 Families

The following points highlight the two familes under which ustilaginales has been classified. The families are: 1. Ustilaginageae 2. Tilletiaceae.

Family # 1. Ustilaginageae:

In the Ustilaginaceae the infection may be localized or systemic. All the three types of infection as already indicated are encountered in this family. The teleuto­spore masses or sori may be developed in leaves, stems, or inflorescences. They are covered by a thin membrane which may be delicate or firm.

The spores may be buried within the host tissue or sometimes a central columella may be present along with the spores. The teleutospores are single or united into spore balls. They germinate by means of septate promycelia bearing terminal or lateral sporidia, rarely, a single sporidium, conjugation tubes, or infection hyphae.

Genus Ustilago:

Sori are produced in various parts of the host forming at maturity dusty, dark- coloured spore masses which may be naked or covered. Teleutospores are single. Spores on germination produce septate promycelia. Cells of the promycelia may pro­duce sporidia, infection hyphae, or conjugation tubes formed terminally or laterally near the septa.

The sporidia may germinate directly into infection hyphae or go on budding indefinitely.

Some Indian species of Genus Ustilago:

Ustilago avenac (Pers.) Rostr.; U. hordei (Pers.) Lagerh.; U. maydis (DG.) Gda.; U. nuda (Jens.) Rostr., U. nuda var. tritici Schaf.

Ustilago Avenae (Pers.) Rost:

It is the causative fungus of loose smut of oats which is sometimes known as naked smut or black head. This fungus does not attack any other cereal crop. It infects only the panicle, where the individual flowers are replaced by sori of black powdery masses of teleutospores (smut spores). The sorus is covered temporarily with a delicate membrane which ruptures to expose the spores.

The teleutospores are dark-brown to black with fine echinulations.

These spores produced on diseased heads are dis­persed by the wind. Many of them lodge on healthy heads and are carried beneath the lemma and palea of the young kernels. Some of the spores germinate immediately there and remain inactive until the seeds are sown the following year.

Other spores do not germinate immediately but remain on the seeds, in the grooves or between the kernels and the glumes, until the seeds are sown.

On germination the spores produce septate promycelia from the cells of which oblong to elongate, hyaline, uninucleate sporidia are developed (Fig. 274A & B). Karyogamy and meiosis take place during spore germination and each cell of the promycelium contains one haploid nucleus. The sporidia increase in number by budding producing secondary sporidia (Fig. 274G).

The sporidia of compatible haploid lines fuse in pairs and the nucleus of one passes into the other to initiate dikaryotic condition.

The fungus infects the oat seedlings and after penetration it grows intracellularly at first in mesocotyl, coleoptile, and first leaf. It then becomes intercellular, advancing with the growing tip, ultimately entering the flower primordia and by heading time replaces the oat spikelets with black masses of spores. The spores, when shed, lodge on the grain. They germinate as the grain germinates and infect the seedlings.

Usually all the heads on an infected plant are smutted, but sometimes one or two are normal. Generally, too, all the spikelets of a panicle are diseased, but occasionally the upper ones appear healthy. Diseased plants cannot be told from healthy ones before heading out, they are usually shorter than healthy plants.

Ustilago Nuda (Jens.) Rostr:

This fungus causes loose smut of wheat and barley. It occurs on wheat and barley and a few wild grasses destroying heads. But it does not attack any of the other grain crops. The characteristic dusty-black appearance of the diseased heads is produced by the teleutospores’ (chlamydospores) that are formed in sori principally in the position of the grains.

Each sorus is enclosed in a delicate, silvery membrane which ruptures early to expose the finely echinulate and dark-coloured teleutospores. The spores are scattered mostly by air current during the flowering time or during harvest, to healthy heads. Those that alight on the heads lie dormant on the surface or become lodged under the glumes.

Spores may live over in the soil under dry conditions. Again if tem­perature and moisture are favourable to the fungus, the spores germinate at once by the formation of a promycelium of one to four cells (Fig. 274D) during which karyo­gamy and meiosis take place.

No sporidia are produced. The promycelial cells are haploid, and fusion between compatible cells takes place by conjugation tubes (Fig. 274E & E). The conjugated cells then produce slender dikaryotic infection threads (hyphae) which enter in the host (young ovary) through the young tissue at the base of the ovary. The infection threads become established in the pericarp or in the embryo­nic tissues of the grains.

Usually the infection threads remain dormant in the scutellum, until the grains are sown in the following growing season. In cases where the spores remain on the surface of the grains, the spores germinate along with the seeds. The fungus grows through the first leaf into the growing point of each young seedling.

The fungus perennates as dormant infection threads (hyphae) in seeds rather than as teleutospores on the surface of the seeds. In any case, the fungus grows with the growing point of the wheat and barley plants and ultimately the grains are replaced by sori of spores that are developed from the dikaryotic mycelium. The chlamydospores of U. nuda are short-lived, rarely survive more than a few days under normal conditions.

Ustilago Maydis (Dc.) Cda:

It is the causal organism of corn (maize) smut disease. It produces sori on any part of the host forming large, irregular galls. These galls are at first protected by a white membrane composed of host tissue and semi-gelatinized fungus mycelium. The white membrane remains intact until a late stage, when it breaks and exposes the pow­dery black mass of teleutospores.

The spores are unicellular, ellipsoidal to subglobose, black with medium-thick epispore being heavily echinulate. The teleutospores germinate and produce septate promycelia, when karyogamy and meiosis take place. Each cell of the promycelium produces continuously ovate sporidia. The sporidia are released directly in air.

Ustilago maydis is heterothallic. The sporidia produced from a teleutos­pore belong to two groups (+) and (—). The sporidia.bud profusely in a yeast-like fashion under suitable organic medium.

The sporidia are readily carried by the wind, and, when they fall upon any young tissue of maize plant, under suitable conditions of moisture, they germinate sending out germ tubes which penetrate the epidermal cells and thus infect the plant. Each germ tube develops into a mycelium which is either or (—).

The dikaryotic condition may be initiated early or late, and, according to the circumstances, may arise from fusion of spores (+) and (—), fusion of hyphae from different mycelia (+) and (—) which come in contact with each other. The dikaryotic mycelium stimulates enlargement (hypertrophy) and increase in number (hyperplasia) of host cells resulting in the formation of a gall.

The mycelium grows through or between the cells of the host tissue, and branches profusely.

Most of the hyphal cells of the gall are transformed into teleutospores. The teleutospores are capable of germination as soon as mature, if conditions are favourable. They may remain viable for two or more years. Life cycle of Ustilago maydis is presented in Figure 275.

Family # 2. Tilletiaceae:

Teleutospores are single or combined in sori of dusty, erumpent spore masses or permanently embedded in the host tissues. During spore germination, karyogamy, meiosis and a short promycelium development take place. The promycelium is simple’ usually non-septate up to the time of formation of sporidia. Large number of sporidia even up to 128 or more are produced in clusters at the apex of the promycelium (Fig 274J & K).

The sporidia are elongate with or without tapering appendages and each one bears a single haploid nucleus. They may germinate while still attached to the promycelium with or without fusing in pairs by conjugation tube and produce similar or dissimilar secondary sporidia (Fig. 274 I) or germinate directly into infection threads.

Genus Tilletia:

Sori develop in various parts of the host, but are usually in the ovaries, forming dusty spore masses. The releutospores are formed from the dikaryotic phyphae which later disappear more or less completely through gelatinization. Germination of teleuto­spore is by a short non-septate promycelium which bears a terminal cluster of elongate sporidia (Fig. 274G & H).

The sporidia fusing in pairs give rise to conidia with or without the formation of mycelium.

Some Indian species of Genus Tilletia:

Tilletia ajrekari Mundk.; T. caries (DC.) Tul.; T. eleusines Syd.; T. foetida (Wallr.) Liro; T. koeteriae Mundk.

Tilletia Caries (DC.):

TUL This is the causal organism of the disease bunt, or stinking smut of wheat. A characteristic fishy odour is given off by the teleutospores of the fungus formed in the infected heads. Black powdery teleutospores occupy the entire kernel within the pericarp being transformed into a spore ball.

The spore balls of infected heads are shattered during threshing, and teleutospores thus liberated lodge on healthy kernels of wheat.

The teleutospore wall has reticulations ranging from minute shallow meshes to deep indentations. The teleutospores measure 15 to 23 µ in diameter. The young teleuto­spore is binucleate, karyogamy is in mature teleutospore. The diploid nucleus by meiosis and mitosis gives rise to haploid nuclei which pass into sporidia borne on pro­mycelium, each sporidium having one haploid nucleus.

Planted in the soil with the wheat kernel itself, the teleutospores germinate when soil conditions become favourable.

By the time a wheat sprout has emerged, the spores germinate by the production of promycelia at the apex of which filiform hyaline sporidia are produced 8 to 16 in number. The sporidia fuse in pairs while already attached with the promycelium, to form characteristic H-shaped structures. The fused sporidia germinate producing dikaryotic mycelium on which sickle-shaped hyaline dikaryotic conidia are borne.

These conidia on germination produce dikaryotic mycelia which cause infection of the young wheat plants. After gaining entrance in the host, the mycelium continues to grow as an internal parasite. From the infection point, the mycelium approaches the growing point and follows the development of its host, sending its branches into each spikelet and finally into the growing ovules.

Eventually, the wheat kernels transform into spore balls.

Soil can become infested with teleutospores when spore balls shatter off before or during harvest or when spore balls are planted with wheat sowing. Life cycle of Tilletia caries is presented in Figure 276.

Genus Neovossia:

Sori in the ovaries form cavities in the tissues and even sometimes destroy the entire grain. The sori rupture and the dusty spore masses are exposed. The teleutospores are produced from the dikaryotic hyphae. Germination of teleutospores is by the formation of a promycelium producing numerous sporidia, number being even up to 128 or more.

The sporidia cluster terminally on a promycelium (Fig, 274J & K). They germinate without conjugation giving rise to mycelia which produce conidia.

Some Indian species of Genus Neovossia:

Neovossia barclayana Bref.; N. brachypodii (Mundk.) Mundk. and Thirum.; N. horrida (Takahashi) Padwick and Khan; N. indica (Mitra) Mundk.