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Fungi
Next only to bacteria in abundance in soil, fungi dominate all soils and possess filamentous mycelium composed of individual hyphae. The hyphae may be uni-, bi- or multinucleate and non-septate (without cross alls) or septate.
Asexual propagation by the production of spores or conidia takes place mitotically either with or without interception by a well-defined sexual cycle involving gametic fusion and subsequent production of spores through meiotic or reduction division.
The size, shape and colour of conidia or spores and the physiological characteristics of cultures in artificial as well as natural substrates provide valuable taxonomic criteria in the classification of fungal isolates into well-defined genera and species.
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All the environmental factors which influence the distribution of bacteria and actinomycetes also influence the fungal flora of soil. The quality and quantity of organic matter present in soil have a direct bearing on fungal numbers in soil since most fungi are heterotrophic in nutrition. Fungi are dominant in acid soils because acidic environment is not conducive for the existence of either bacteria or actinomyctes, resulting in the monopoly of fungi for utilization of native substrates in soil. They are also
| A. Streptomyces |
B. S Roseochromogenus |
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| C. S Coelicolor |
D. Phase Contrast Photomicrograph |
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present in neutral or alkaline soils and some can tolerate pH beyond 9.0. Arable soils contain abundant fungi since they are strictly aerobic and excess of soil moisture decreases their numbers. Isolation of fungi from different horizons of soil profiles shows that these organisms exhibit selective preference for various depths of soil.
| A. General View of Mycelium and Sporphores |
B. General View of Mycelium of Actinomycetes |
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Those fungi which are common in lower depths are rarely encountered on the surface of soils which may be explained on the basis of the availability of organic matter and the ratio between oxygen and carbon dioxide in the soil atmosphere at varying depths.
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Seasonal fluctuations in fungal numbers are not uncommon. Farm practices including crop rotation and fertilizer or pesticide applications influence the nature and dominance of fungal species.
Broadly speaking, fungi are classified into Phycomycetes, Ascomycetes, Basidiomycetes and Fungi Imperfecti.
Many fungi which are commonly isolated from' soil come under the class Fungi Imperfecti by virtue of the fact that they produce abundant asexual spores and lack sexual stages. Members of this class are distinguished by their septate mycelium and a structure called conidiophore from which conidia or spores are continuously produced.
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The other three classes of fungi have both sexual and asexual means of reproduction. Phycomycetous members possess non-septate and unicellular mycelia and have sacs called sporangia containing spores. A specialized sporangium known as ascus is characteristic of Ascomycetes which contains a definite number of ascospores usually numbering 4 or 8. Unlike Phycomycetes, Ascomycetes have septate mycelia.
As a rule, the members of Basidiomycetes (which are characterized by specialized reproductive structure known as basidium, producing basidiospores) are difficult to isolate from soil on agar plates for the simple reason that the nutritional requirements of many of the basidiomycetes are exacting and the conventional method of soil-dilution plating is inadequate to isolate them in pure culture.
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Many of the wood-rotting fungi such as Polyporus and ectotrophic mycorrhizal fungi (example, Boletus) which inhabit the root region of forest trees come under soil Basidiomycetes. They need B-vitamins and special growth factors contained in root exudates for growth in laboratory media.
Nevertheless, Basidiomycetes are usually present in soil in a mycelial state and can be recognized by the fructifications or fruit bodies which they produce on the surface of soil or decaying wood. Suitable baits can be buried in soils and Basidiomycetes may be selectively isolated by such baiting techniques.
Many of the Basidiomycetes are capable of utilizing cellulose and in this respect they appear to be good colonizers of forest soils.
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The following are genera of fungi which are most commonly encountered in soils and which can be isolated by conventional methods. Acrostalagmus, Aspergillus, Botrytis, Cephalosporium, Gliocladium, Monilia, Penidllium, Scopulariopsis, Spicaria, richoderma, Trichothecium, Verticillium, Alternaria, Cladosporium, Pullularia, Cylindrocarpon and Fusarium (Fungi Imperfecti); Absidia, Cunninghamella, Mortierella, Mucor, Rhizopus, Zygorynchus and Pythium (Phycomycetes); Chaetomium (Ascomycetes) and Rhizoctonia (Mycelia sterilia, which fail to produce reproductive structures).
Many soil yeasts belonging to true Ascomycetes such as Saccharomyces and those belonging to Fungi Imperfecti such as Candida can also be isolated on acidified media (pH 4.0). Their numbers in soil are low and their significance in soil is poorly understood.
One of the primary functions of filamentous fungi in soil is to degrade organic matter and help in soil aggregation. Besides this property, certain species of Alternaria, Aspergillus, Cladosporium, Dematium, Gliocladium, Helminthosporium, Humicola and Metarhizium produce substances similar to humic substances in soil and hence may be important in the maintenance of soil organic matter.
Some of the fungi capable of forming ectotrophic associations on the root system of forest trees such as pine belonging to the genera Boletus and Lactarius help in the mobilization of soil phosphorus and nitrogen into plants. In many instances, establishment of new forests becomes difficult unless mycorrhizal fungi are artificially introduced into soil by inoculation.
| 1. Rhizopus A. Habit B. Sporangiophores C. Sporangiospores D. Zygospore |
2. Mucor A. Habit B. Sporangiophore C. Sporangiospores D. Zygospore |
3. Zygorhynchus A. Habit B. Sporangiophore C. Sporangiospores D. Zygospore |
4. Pythium A. Sporangia B. Zoospore Discharge C. Zoospore D. Oogonium |
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| 5. Chaetomium A. Perithecium B. Ascus C .Ascospores |
6. Cephalosporium A. Conidiophores B. Conidia |
7. Trichoderma A. Conidiophores B. Conidia |
8. Aspergillus A. Habit B. Phialides C. Hullen Cellen D. Ascus E. Habit F. Conidial Head G. Conidia H. Ascospores |
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| 9. Penicillium A. Conidiophore B. C. Conidial Heads D. Cleistothecium E. Ascus F. Ascospores |
10. Verticillium Conidiophore and Conidia |
11. Cladosporium A. Conidiophore B. Conidia |
12. Helminthosporium Conidiophores and Conidia |
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| 13. Fusarium A. Conidial Head B. Microconidia C. Macroconidia D. Chlamdospores |
14. Cylindrocarpon Conidiophores and Conidia |
15. Rhizoctonia A. Hypha B. Basidium and Spores C. Sclerotial Hyphae |
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