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 fungi belong to order- Uerdinales and are found in various forms; these are obligate parasites which infect most groups of vascular plants, ferns, monocots, dicots and Gymnosperms.
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 mycelium is well-developed and consists of septate and intercellular hyphae, containing small and branched haustoria.
The rust life 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 life cycle and some are autoecious and require a single host to complete their life cycle.
These cause rust disease to economically important plants such as wheat, barley, oats, etc.
The classical example of Rust fungi is Puccinia graminis, causing black stem rust on wheat and other cereals- whose life cycle is discussed below.
They are heteroecious and require two hosts to complete their life cycle- wheat (primary host) and barberry (secondary host).
The mycelium is in two stages- monokaryotic mycelium and dikaryotic mycelium. The dikaryotic mycelium phase is the important phase of the rust life cycle since few rust species can survive in this condition indefinitely. The dikaryotic mycelium is present in Triticum vulgare.
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Leaves of the primary hosts are infected by the aeciospores which are dikaryotic. These aeciospores form the dikaryotic mycelium 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.
Under unfavourable conditions, uredinia are replaced by telia which bear teliospores. Teliospores of Puccinia are stalked, two-celled, thick-walled and dark brown to black in colour. Both cells of the teliospores contain two haploid nuclei of opposite stains (+.-) and they fuse forming a diploid nucleus. Under favourable conditions, these teliospores undergo meiosis and germinate, forming the promycelium and haploid basidiospores. Basidiospores now infect the secondary host Berberis vulgaris (Barberry).
Basidiospores germinate on the adaxial surface of Barberry leaves and form haploid, monokaryotic mycelium. The mycelium forms flask-shaped fructification called spermatogonium containing uninucleate spermatia. The spermatia are present near the spermatogonial ostiole containing sugary liquid. 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 fertilization occurs. The primary mycelium travels to the abaxial surface of the leaf and forms a proto-aecium which remains undeveloped until fertilisation is completed.
Post fertilisation, 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 life cycle.
Smut fungi belong to the order- Ustilanginales and contain about 1500 species; which are not always obligate parasites. They majorly infect the reproductive organs of grasses and herbaceous plants.
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 teliospores which are black in colour.
The smut fungi have two types of mycelia monokaryotic which grow saprotrophically and are non-infectious on plants. The dikaryotic mycelial 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 mycelium 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
The Life cycle of Smut fungi can be divided into a monokaryotic phase (non-infectious on plants) and a dikaryotic phase (infections on plants).
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 teliospores which are uninucleate diploid, thick-walled and black-coloured.
Once matured, the spores are released by wind or are attached to the seeds of the infected hosts.
Under favourable conditions, these teliospores undergo meiosis and form haploid cells. These then germinate and form the promycelium, which bears basidiospores. These are haploid and numerous in number produced by direct budding of the promycelium. Two types of basidiospore are formed, containing + and - strains which are unable to infect the plant host.
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 life cycle.
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Rusts and Smuts belong to Division- Eumycota, Subdivision- Basidiomycotina.
They contain two types of mycelium- monokaryotic and dikaryotic.
They contain well-developed basidia which contain basidiospores.
The teliospores of rust and smut fungi are equivalent to each other i.e. they perform nuclear fission and give rise to promycelium upon germination.
Basidiospores are formed outside the basidium.
Well-defined sex-organs are lacking.
| Sr. No. | Rust | Smut |
|---|---|---|
| 1. | Belongs to order-Uredinales | Belongs to order-Ustilaginales |
| 2. | Rusty, reddish, yellow-orange coloured spores produced in host leaves, and stems. | Black dusty masses of spores are produced in host leaves and stems. |
| 3. | They are heteroecious-requires two hosts to complete its life cycle. | They require a single host. |
| 4. | These are obligate parasites. | These are obligate parasites but with an additional free-living saprotrophic (yeast) phase. |
| 5. | They infect dicots, monocots, ferns, and gymnosperms. | They infect majorly the reproductive organs of grasses. |
| 6. | They show 5 different types of spores-Uredospore, Teliospores, Basidiospores and Aeciospores. | They show only 2 types of spores-Teliospores, Basidiospores. |
| 7. | 4 haploid basidiospores are formed | Many haploid basidiospores are formed. |
| 8. | The basidiospores germinate by the formation of mycelium which then infects the leaf of the alternate host. | The basidiospores germinate and follow a saprotrophic (yeast-like) lifestyle. |
| 9. | Haustoria are present and differentiated into the narrow neck and wider haustorial body. | Haustoria are absent or take the form of simple intracellular hyphae which penetrate the plasmalemma. |
| 10. | Eg-Puccinia graminis-Stem rust of wheat, Gymnosporangium juniperi-virginianae Cedar-apple rust | Eg- Ustilago nuda-Loose smut, Tilletia tritici -Covered smut |
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 mycelium and a haustoria. The life cycle of rusts and smuts revolve around the formation of various types of spores and their genetic constitution. Most of the basidiomycetes are heterothallic-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.
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.
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