Introduction

Fungi are eukaryotic organisms containing proper cell walls and membrane-bound organelles. These are heterotrophs and depend on other organisms for food and nutrition. Fungi are economically essential as they feed on dead and decaying matter, breaking complex minerals into simple ones and supplying them back to the soil. This nature of fungi helps in completing various nutrient cycles and maintaining the nutrient balance of the environment. Basidiomycetes are one such class of higher fungi, which are parasitic in nature and cause various plant diseases such as black rust of wheat, loose smut of sorghum, maize smut etc.

Rust

Rust fungi belong to order- Uerdinales and are found in various forms; these are obpgate parasites which infect most groups of vascular plants, ferns, monocots, dicots and Gymnosperms.

Features

The name ‘rust fungi’ refers to the reddish-brown, rusty, orange colour of the spores produced on the surface of leaves and stems of the host.

The mycepum is well-developed and consists of septate and intercellular hyphae, containing small and branched haustoria.

The rust pfe cycles can vary between two and five distinct spore stages that include haploid, diploid and dikaryotic nuclear conditions.

Some rust species are heteroecious i.e. they require two different hosts to complete their pfe cycle and some are autoecious and require a single host to complete their pfe cycle.

These cause rust disease to economically important plants such as wheat, barley, oats, etc.

Life-Cycle

The classical example of Rust fungi is Puccinia graminis, causing black stem rust on wheat and other cereals- whose pfe cycle is discussed below.

They are heteroecious and require two hosts to complete their pfe cycle- wheat (primary host) and barberry (secondary host).

The mycepum is in two stages- monokaryotic mycepum and dikaryotic mycepum. The dikaryotic mycepum phase is the important phase of the rust pfe cycle since few rust species can survive in this condition indefinitely. The dikaryotic mycepum is present in Triticum vulgare.

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Stage 1

Leaves of the primary hosts are infected by the aeciospores which are dikaryotic. These aeciospores form the dikaryotic mycepum producing pustules on the leaf surface called uredinia bearing urediniospores. Urediniospores of rust fungi are thick walls, red or orange which give a rusty appearance to the infected plant parts. Each urediniospore is binucleate, and both are of different stains (+,-). They can survive for months and can re-infect the same primary host multiple times under favourable conditions.

Stage 2

Under unfavourable conditions, uredinia are replaced by tepa which bear tepospores. Tepospores of Puccinia are stalked, two-celled, thick-walled and dark brown to black in colour. Both cells of the tepospores contain two haploid nuclei of opposite stains (+.-) and they fuse forming a diploid nucleus. Under favourable conditions, these tepospores undergo meiosis and germinate, forming the promycepum and haploid basidiospores. Basidiospores now infect the secondary host Berberis vulgaris (Barberry).

Stage 3

Basidiospores germinate on the adaxial surface of Barberry leaves and form haploid, monokaryotic mycepum. The mycepum forms flask-shaped fructification called spermatogonium containing uninucleate spermatia. The spermatia are present near the spermatogonial ostiole containing sugary pquid. This attracts insects and carries the spermatia to other barberry plants where they come in contact with mature receptive hyphae of the opposite strain and fertipzation occurs. The primary mycepum travels to the abaxial surface of the leaf and forms a proto-aecium which remains undeveloped until fertipsation is completed.

Stage 4

Post fertipsation, dikaryotization takes place on the abaxial surface of the leaf. Due to this process, the proto-aecium is converted into an aecium which now bears the aeciospores. Aeciospores are heterokaryotic, thin-walled, orangish in colour and bear spiny surface ornamentations. The aeciospores break the lower epidermis of the barberry leaf and get transported via wind to the primary host (wheat) and hence completing the pfe cycle.

Smut

Smut fungi belong to the order- Ustilanginales and contain about 1500 species; which are not always obpgate parasites. They majorly infect the reproductive organs of grasses and herbaceous plants.

Features

The name ‘smut’ has come into being due to the symptoms caused by this fungus, where the loose masses of dark spores are produced in infected plant organs such as flowers, stems, ovaries, inflorescence, etc.

These infected portions appear as though they are burnt due to the accumulation of tepospores which are black in colour.

The smut fungi have two types of mycepa monokaryotic which grow saprotrophically and are non-infectious on plants. The dikaryotic mycepal phase infects host plants.

These do not contain haustoria to absorb nutrition, but they contain intracellular hyphae which penetrate into the plasmalemma and absorb nutrition.

The septa of the mycepum contain simple pores which help in the communication and transfer of other components from one cell -to another.

These cause smut disease to economically important plants such as sorghum, maize, barley etc. One common disease is loose smut of Sorghum caused by Sphacelotheca cruenta

Life-Cycle

The Life cycle of Smut fungi can be spanided into a monokaryotic phase (non-infectious on plants) and a dikaryotic phase (infections on plants).

Stage 1

The dikaryotic hyphae mass together in the intracellular spaces of the host cells leading to the formation of the sori. These hyphae are binucleate and undergo nuclear fusion. Post the fusion, their cell walls thicken and form an enlarged globose structure. This structure is covered with a gelatinous covering which ruptures upon maturation. These sori bear the tepospores which are uninucleate diploid, thick-walled and black-coloured.

Stage 2

Once matured, the spores are released by wind or are attached to the seeds of the infected hosts.

Stage 3

Under favourable conditions, these tepospores undergo meiosis and form haploid cells. These then germinate and form the promycepum, which bears basidiospores. These are haploid and numerous in number produced by direct budding of the promycepum. Two types of basidiospore are formed, containing + and - strains which are unable to infect the plant host.

Stage 4

The basidiospores germinate by budding as seen in yeast cells and show prolonged saprotrophic growth. Compatible cells when coming in contact with each other over the plant surface; undergo conjugation and this forms a dikaryotic hypha which infects the host plant and completes the pfe cycle.

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Similarities between rust and smut

Rusts and Smuts belong to Division- Eumycota, Subspanision- Basidiomycotina.

They contain two types of mycepum- monokaryotic and dikaryotic.

They contain well-developed basidia which contain basidiospores.

The tepospores of rust and smut fungi are equivalent to each other i.e. they perform nuclear fission and give rise to promycepum upon germination.

Basidiospores are formed outside the basidium.

Well-defined sex-organs are lacking.

Difference between rust and smut

Conclusion

Rusts and Smuts are two very important fungi which need to be studied as they are the major cause of the destruction of economically important plants such as wheat, barley, sorghum, maize etc. Basidiomycetes are a class of higher fungi which have well-developed parasitic organs such as mycepum and a haustoria. The pfe cycle of rusts and smuts revolve around the formation of various types of spores and their genetic constitution. Most of the basidiomycetes are heterothalpc-i.e they have two types of thallus a ‘+’ strain and ‘-’ strain which combine with each other and lead to the next generation of basidiomycetes.

FAQs

Q1. Name a few control measures of Smut.

Ans. Control measures include −

Inspect crops at flowering time to make sure any infected or smutted inflorescences or flowers are absent.

Using fungicides such as carboxin or its derivatives such as captan, maneb or pentachloronitrobenzene.

Buy disease-free or smut-resistant varieties of crops.

Keep a clean environment near the plant.

Keep all the equipment clean and sanitised.

Q2. Name a few control measures of Rust fungi.

Ans. Control measures include −

Remove the secondary host from the vicinity of the primary host.

Use fungicides such as copper or sulphur fungicides in the prescribed format.

Buy disease-free or rust-resistant varieties of crops.

Keep a clean environment near the plant.

Keep all the equipment clean and sanitised.

Use hot treatment-since few fungi are very sensitive to temperature change.

Q3. Short note on Dikaryotization.

Ans. Dikaryotization is a process of mating in high fungi which leads to the formation of a dikaryon which is a cell, containing two nuclei (one from each parent) who share the same cytoplasm but do not fuse with each other. This is an essential stage of fungi belonging to Basidiomycota.

Q4. What are basidium and Basidiospores?

Ans. Basidium- A small club-shaped structure developing from a dikaryotic hypha which contains four slender projections called sterigmata which bear the haploid basidiospores.

Basidiospores- Basidiospores are sexual spores in basidiomycetes. They are in a size range of about 3 to 20 m, unicellular and are sub-globose, sausage-shaped, fusoid, to almond-shaped. They may be smooth or may have spiny ornamentations. These spores may be colourless, yellow, brown, pink, purple etc. The colour of the basidiospore helps in the identification of the fungi.