Cercospora
Q.20. Write short note on Cercospora. (2006, 09)
Related Question -
Q. Mention the systematic position of cercospora giving suitable reason. (2006, 09) Ans. Cercospora: -
Systematic Position -
Division - Mycota
Sub-division - Eumycotina
Form-class - Deuteromycetes
Form-order - Moniliales
Form-family - Dematiaceae
Form-genus - Cercospora
This genus causes leaf spot disese of higher plants. The well known leaf spot or tikka disease of groundnut.
The mycelium of C. personata is internal and consists of strictly intercellular, septate and branched hyphae. The hyphae obtain their nutrition by sending branched haustoria into the spongy mesophyll cells of the host.
At the time of sporulation the hyphae become segregated to form small stromata beneath the epidermis, usually on both surfaces of the infected leaf.
Asexual Reproduction: -
It reproduces asexually by means of conidia. The conidiophores are dark, continuous or one to two septate with characteristic knee shaped bending and arise in tufts from a stroma lying in the sub stomatal cavity. The conidia are generally borne singly at the terminal end of these conidiophores.
The conidia are long,cylindrical usually hyaline and multiseptate. The conidia produce scars as they fall away .When the conditions are suitable each conidium germinates into new mycelium.
Disease and its Symptoms: -
The tikka disease is the serious disease of groundnuts. Symptoms of the disease first appear as pale spots on the leaves when the plants are one to two months old. Later lesions also appear on the stem. Leaves infected by C.arachidicola show circular to irregular spots that are surrounded by a yellow halo from the very beginning. On the upper surface the necrotic areas are reddish brown t o black,while on the lower surface they are light brown. The spots formed by C. personata are smaller , circular and dark brown to black in colour.Hence the spots are not surrounded by the yellow halo. Due to these spots the leaf weakens and ultimately dries up, falling prematurely on the ground. Consequently fewer and smaller nuts are formed.
Q.21. Differentiate between Cleistothecia and Perithecia. (2016)
Ans. Cleistothecia: -
It is a more or less globose ascocarp which has no natural opening. The cleistothecial wall, the peridium may or may or may not be covered without growths called the appendages. Asci in a cleistothecium may be arranged in a hymenial layer or they may be scattered throughout the interior. Asci and ascospores are liberated by the disintegration of the wall of cleistothecium.
Perithecia: -
Perithecia here the spore bearing body is not open as apothecia but is in a chamber with only a small opening to the outside world, this opening is called an ‘ostiole’. Perithecia are often buried almost completely in the substrate on which the lichen is growing. Both these structures release ascospores - spores produced within an ascus (a sack).
Q.22. Give the main characters and systematic position of Aspergillus.
Ans. Aspergillus: -
Systematic Position -
Division - Mycota
Sub-divislon - Eumycotina
Class - Ascomycetes
Sub class - Euascomycetidae
Series - Plectomycetes
Order - Plectascales or Aspergillales
Family - Aspergillacea
Genus - Aspergillus
Characters: -
The plant body of Aspergillus consists of profusely branched mycelium. Mycelium consists of a loosely interwoven mass of .much branched. septate hyphae. The hyphae ramify in and on the surface of the substratum and serve to absorb food material. The hyphae are septate, with multinucleate cells. Each hyphal cell contains several granulated and vacuolated cytoplasm, several nuclei and oil globules.
Q.23. Mention the systematic position of Aspergillus giving suitable reasons. (2015)
Ans. Systematic Position: -
Division - Mycota
Sub-division - Eumycotina
Class - Ascomycetes
Sub class - Euascomycetidae
Series - Plectomycetes
Order - Plectascales or Aspergillales
Family - Aspergillaceae
Genus - Aspergillus.
Eumycotina: - Eumycotina (eu = true, myketes = fungus) are the true fungi. They are heterotrophic organisms and differ from the slime moulds (myxomycetes), in lacking plasmodial stages and phagotrophic nutrition.
Eumycotina were divided into four classes according to the septation of the mycelium and on the basis of characteristic features of reproduction -
1. Class - phycomycetes
2. Class - Ascomycetes
3. Class - Basidiomycetes
4. Class - Deuteromycetes.
Ascomycetes: -
Except in a few cases in which the plant body is unicellular (yeast), the majority of species from hyphae, which unlike those of phycomycetes, are septate, i.e., cross walls are present. Vegetative hyphae are monokaryotic.
The most common types of asexual spores are the conidia. Sexual reproduction takes place generally by fusion of the cytoplasm of a male cell with that of a female within a characteristic sac-like structure, the ascus.
In most species, the asci are embedded within or upon special fruiting bodies formed by the sterile hyphae. Such a fruiting body is called an ascocarp.
Alexopoulos classified the class Ascomycetes into three subclasses -
(i) Hemiascomycetidae (ii) Euascomycetidae (iii) Loculoascomycetidae
Euascomycetidae: -
The sub-class Euascomycetidae include all the Ascomycetes in which the asci are unitunicate and are formed on ascogenous hyphae which develop within or on a definite fruit body or ascocarp.
On the basis of the structure of the ascocarp and the form and distribution of the asci, the sub-class Euascomycetidae falls into following four series (Alexopoulos, 1962).
Series (1) Plectomycetes
Series (2) Pyrenomycetes
Series (3) Discomycetes
Series (4) Laboulbeniomycetes.
Plectomycetes: -
In the members of Plectomycetes, the asci are globose or broadly club-shaped and are irregularly arranged within the fruit body (ascocarp). They have no natural opening but liberation of ascospores takes place by weathering of wall (peridium) of the ascocarp. The ascocarp is either completely closed (Cleistothecium), or has an opening by an ostiole.
The series Plectomycetes include three orders viz., Eurotiales (Aspergillales or Plectascales), Onygenales and Microascales.
Order - Eurotiales (Aspergillales): -
It includes about 50 genera. They are commonly called the black, blue or green molds.
The asexual reproduction takes place by chains of conidia which are produced in enormous numbers at the tips of certain hyphae. The cleistothecia are produced without the formation of a stroma. The asci are formed at the ends of ascogenous hyphae of varying lengths and are scattered irregularly throughout the interior of the cleistothecium rather than being oriented in a layer of hymenium. The ascospores are set free by decay of breaking of the peridium.
Alexopoulos (1962) recognises three families: Ascosphaeriaceace, Gymnoascaceae and Eurotiaceae.
Family - Eurotiaceae Or Aspergillaceae: -
The members of this family are probably the most widespread true, fungi, many of which are of great economic importance. The family contains saprophytic forms in which the conidial stage is much more prominent than the ascus stage. Members of this family are characterised by sub-aerial and sessile ascocarps with well developed peridia. The asci are globose and usually eight spored.
The two most important genera of the family are Aspergillus and Penicillium.
Aspergillus -
The plant body of Aspergillus consist of profusely branched mycelium. Mycelium consists of a loosely interwoven mass of much branched. septate hyphae. The hyphae ramify in and on the surface of the substratum and serve to absorb food material. The hyphae are septate, with multinucleate cells. Each hyphal cell contains several granulated and vacuolated cytoplasm, several nuclei and oil globules.
Q.24. Differentiate between the following-
(a) Oospore and Zygospore (2010, 11, 12)
(b) Rust and Smut
(c) Pycnidium and Acervulus
(d) Ascus and Basidium. (2007, 10)
Ans. (a) Oospore and Zygospore: -
An oospore is a thick-walled sexual resting spore that develops from a fertilized oosphere ( union of oogonium and antheridium) in some algae and fungi.Whereas a zygospore is a reproductive part of a fungus, a chlamydospore that is created by the nuclear fusion of haploid hyphae of different mating types.wheras A zygospore remains dormant while it waits for environmental cues, such as moisture, heat, or chemicals secreted by plants.
When the environment is favorable, the zygospore germinates, meiosis occurs, and a sporangium is produced at the end of a sporangiophore. The sporangium sheds spores.
A fungus that forms zygospores is called a zygomycete, indicating that the class is characterized by this evolutionary development
(b) Rust and Smut: -
Rusts and smuts are fungi belonging to the orders Urediniales (rusts) and Ustilaginales (smuts) which are basidiomycete fungi.
The rusts have complicated life cycles which involve the infection of two different plant species. The most well-known members of these groups are wheat rust (Puccinia graminis tritici) and corn smut (Ustilago myadis). Rust fungi attack plants such as ferns, gymnosperms, and flowering plants. When a wheat plant is infested by Puccinia graminis tritici, the infestation may become obvious during the summer growing season when rust colored growth appears on the stems of infected plants. Fungal hyphae are composed of groups of spore generating structures (sporangia) called uredinia that rupture the stem and become visible. It is the spores released from the uredinia (called urediniospores) that infect new wheat plants and spread the disease. In the fall, Puccinia produces black sporangia (called telia) and the infected wheat plants have distinct black patches on their stems. Spores from the telia (called teliospores) do not attack other wheat plants but instead infect barberry plants. Teliospores which land on barberry leaves germinate and form small cup-shaped structures called spermagonia. Each spermagonium produces long filaments called receptive hyphae which extend above the spermagonium and spermatia, which are sexual gametes. The spermagonium also produces a nectar-like substance which is attractive to flies. Spermatia are mixed with this nectar and flies transfer the spermatia from adjacent spermagonia as they feed. New fungal mycelia, resulting from the union of the spermatia with the receptive hyphae of spermagonia of different genetic strains, grow on the underside of the barberry leaf. There, the mycelium produces a larger bell-shaped sporangium called an aecium, which generates aeciospores which in turn infect new wheat plants.
Smut fungi differ from rust fungi in several ways. While rust fungi require two different hosts to complete their life cycle, smut fungi may complete their life cycle on only one host, which is always a flowering plant. Another difference between rust and smut fungi is seen in the way that they infect their host plants. Infections from rust fungi are localized to that part of the plant close to where a germinated urediniospore, aeciospore, or teliospore becomes established. Smut fungi spread to infest the entire plant from a single initial infection site, often targeting specific organs. This is exemplified by the smut fungus Ustilago violacea which attacks plants of the genus Silene. Ustilago violacea infests the entire plant but its presence within the plant is only apparent where mycelia grow within the anthers of the plant. There, hyphae divide to become teliospores and these take the place of pollen grains. Pollinating insects then carry the teliospores from infected Silene plants to uninfected ones. Teliospores mature along with the Silene flower and fall to the ground along with seeds of the host Silene plant. When the seeds germinate, the smut fungus teliospores germinate along with them and immediately infect the Silene seedlings. Ustilago myadis, is a well-known smut fungus that infects corn, where its immature teliospores are enclosed in sacs which replace the kernels of corn. When these sacs burst, U. myadis spores are released and cling to normal corn kernels. When these kernels are planted, teliospores are planted along with them, infecting new corn plants when they germinate.
Rust and smut fungi are both of great economic importance due to their destruction of cash crops. An effort to eliminate Puccinia graminis tritici by the eradication of barberry was not successful. This rust fungus is now controlled by selection for genetically resistant wheat plants, but rust fungi frequently mutate and override wheat resistance, so an ongoing genetic selection program for wheat is necessary. Another economically important rust fungus is Gymnosporangium juniperus-virginiae, which has as its two plant hosts the common juniper (Juniperus virginianus) and the domestic apple, and other species of the rose family. This fungus produces large orange colored spore-generating structures on juniper trees, which then infect apple trees, causing the tree to produce deformed and unmarketable apples. The best way to avoid ruined apples is to keep apple trees away from juniper trees and to remove all infected juniper trees in the area.
One way that humans have reduced infection of corn by smut has been to wash away any clinging fungal spores from the kernels of corn. In the southwestern United States and in Mexico immature corn smut sacs are fried and eaten as a delicacy.
(c) Pycnidium and Acervulus: -
Pycnidium (pl. pycnidia) is an asexual fruiting body produced by mitosporic fungi in the form order Sphaeropsidales (Deuteromycota, Coelomycetes). It is often spherical or inversely pearshaped (obpyriform) and its internal cavity is lined with conidiophores. When ripe, an opening generally appears at the top, through which the pycnidiospores escape.
Acervulus (pl. acervuli) is a small asexual fruiting body that erupts through the epidermis of host plants parasitised by mitosporic fungi of the form order Melanconiales (Deuteromycota, Coelomycetes). It has the form of a small cushion at the bottom of which short crowded conidiophores are formed. The spores escape through an opening at the top.
(d) Ascus and Basidium: -
A small, specialized club-shaped structure typically bearing four basidiospores at the tips of minute projections. The basidium is unique to basidiomycetes and distinguishes them from other kinds of fungi. A membranous, often club-shaped structure in which typically eight ascospores are formed through sexual reproduction of ascomycetes.
Fungi with sporophores (fruiting bodies) large enough to be readily visible will usually belong to one of two main groups. The Basidiomycetes or the Ascomycetes. The main difference between these two groups is in the way in which they produce their microscopic spores.
In the Basidiomycetes, the spores are produced externally, on the end of specialised cells called basidia.
In Ascomycetes, spores are produced internally, inside a sac called an ascus.
Asci and basidia are both microscopic structures.
