Sphaerotheca
Q.10. Give an account of structure, reproduction and economic importance of Sphaerotheca. (2009)
Ans. Structure: -
It grows as an obligate ectoparasite on a wide variety of angiospermic hosts including cultivated plants. The disease which is caused by Sphaerotheca is popularly known as powdery mildew. The presence of the parasite is marked by conspicuous powdery patches upon the leaves, stems and floral parts of the host owing to the white colour of its mycelium and masses of white conidia. After some time the patches spreads up and coalesce, the leaves looking as if they had been dusted with flour. These fungi are common on gooseberry, hops, raspberry, strawberry and rose.
Vegetative Structure: -
The mycelium which forms a dense coating on the surface of the host consists of branched, septate hyphae whose cells are multinucleate. The mycelium is hyaline when young and grayish brown when old. The mycelium is usually superficial and at intervals it sends special absorbing organs called haustoria, into the epidermal cells of the host. The haustoria penetrate into the inferior of the cells of epidermis and thus they anchor the mycelium to the leaf and absorb food.
Reproduction: -
Asexual Reproduction: -
A few days after infection, several unbranched specialized hyphae arise defectly from the surface mycelium. These erect hyphae are called conidiophores. Each conidiophore cut off a basipetal chain of asexual spores, the conidia.
The conidia are more or less ellipsoidal, one celled uninucleate structure. Because of the formation of enormous number of chains of conidia the leaf assumes the apearence of being dusted with powder; hence Sphaerotheca is called a ‘powdery mildew’. The conidia are detached and scattered by the wind. As the conidia fall on a suitable host, germinate immediately by pitting out one to three germ tubes which form a new mycelium.
Sexual Reproduction: -
It occurs in late summer when conidial production slows down. The male and female sex organs the antheridia and ascogonia are developed at the ends of two neighbouring hyphae on the superficial mycelium. From these adjoining hyphae arise antheridial and ascogonial branches. Both the branches are uninucleate. One branch enlarges and becomes club shaped and is cut off by a transverse wall to form uninucleate ascogonium. The nucleus of the other branch which is usually slender divides into two daughter cells. The terminal cell becomes the uninucleate antheridium and the other the uninucleate stalk cell. The antheridium becomes closely applied tpo the ascogonium and grows up with it soon overlapping it and bending over its apex. Where the antheridium and ascogonium come in contact the common wall dissolves to form an opening through which the protoplast of the antheridium migrares into the ascogonium.
The ascogonium elongates and becomes divided by transverse septa to form a row of 3-5 cells. In this row the cell below the terminal one i.e penultimate cell, contains two nuclei while the others are uninucleate. The penultimate cell which contains two nuclei behaves as the ascus mother cell. The two nuclei in the young ascus fuse to form a diploid nucleus. The diploid nucleus divides by three divisions of which the one is reduction division to form eight haploid ascospores.
In the meantime the cell beneath the ascogonium gives riese to sterile hyphae which grow rapidly around the ascogonium completely enveloping the fertilized ascogonium. The outer cells of the envelope become thick walled and form a protective covering the peridium. The whole structure thus obtained is the ascus fruit called clesitothecium. From the inner cells of the peridium short branches filled with very dense protoplasm grow inwards and become closely applied to the ascus. These thin walled cells are called nurse cells and nourish the developing ascus. Some of the superficial cells of the peridium grow out to form long, unbranched filamentous appendages.
Cleistothecia appear as minute black dots in the superficial mycelium on the leaf surface. These are at first whitish but soon change to black at maturity. They are sub globose and are visible to naked eyes as irregularly scattered minut black dots in the mycelium on the surface of the infected leaf.
When germination takes palce the ascus absorbs water swells up and causes the bursting of cleistothecium. The ascospores are violently disseminated into the air. The ascospores on falling upon suitable host germinate on the mycelium.
Economic Importance: -
Sphaerotheca causes powdery mildew disease on a great variety of plants.
· Sphaerrotheca humuli causes powdery mildew of cucurbits
· Sphaerotheca morseuave causes powdery mildew of goose berry
· Sphaerotheca humuli causes a powder mildew of hops which is responsible for the great loss to the hop growing industry.
· Sphaerotheca pannosa attack roses.
Symptoms of the Disease: -
The disease appears as tiny, white powdery patches upon the leaves, stems and floral parts of the host. After sometime the patches spread up and coalesce the leaves looking as if they had been dusted with flour.
Control Measures: -
· Use of resistant varieties
· Dusting of the crops with sulphur and karathane.
Q.11. Give the name and symptoms of any disease caused by Sphaerotheca. With the help of labeled diagrams only explain its life cycle. (2008)
Ans. Sphaerotheca causes powdery mildew disease on a great variety of plants.
Fig. Sphaerotheca sp. Sexual reproduction. A-somatic hypha. B-antheridium and ascogonium in contact. C-the antheridium nucleus has migrated into the ascogonium through an opening. D-male and female nuclei in ascogonium. (Plasmogamy). E-division of ascogonium into a row of ascogonial cells, the penultimate cell of which is binucleate. F-the nuclei of the penultimate cell have fused to become the ascus. G-young cleistothecium, showing three layers of cells surrounding the young ascus. H-ruptured cleistothecium showing liberation of an ascus. I-germinating ascospore.
- Sphaerrotheca humuli causes powdery mildew of cucurbits
- Sphaerotheca morseuave causes powdery mildew of goose berry
- Sphaerotheca humuli causes a powder mildew of hops which is responsible for the great loss to the hop growing industry.
- Sphaerotheca pannosa attack roses.
Symptoms of the Disease: -
The disease appears as tiny, white powdery patches upon the leaves, stems and floral parts of the host. After sometime the patches spread up and coalesce the leaves looking as if they had been dusted with flour.
Control Measures: -
- Use of resistant varieties
- Dusting of the crops with sulphur and karathane.
Q.12. Write short note on Sphaerotheca. (2006)
Ans. Sphaerotheca: -
It grows as an obligate ectoparasite on a wide variety of angiospermic hosts including cultivated plants. The disease which is caused by Sphaerotheca is popularly known as powdery mildew. The presence of the parasite is marked by conspicuous powdery patches upon the leaves, stems and floral parts of the host owing to the whit colour of its mycelium and masses of white conidia. After some time the patches spreads up and coalesce, the leaves looking as if they had been dusted with flour. These fungi are common on gooseberry, hops, raspberry, strawberry and rose.
Vegetative Structure: -
The mycelium which forms a dense coating on the surface of the host consists of branched, septate hyphae whose cells are multinucleate. The mycelium is hyaline when young and grayish brown when old. The mycelium is usually superficial and at intervals it sends special absorbing organs called haustoria, into the epidermal cells of the host. The haustoria penetrate into the inferior of the cells of epidermis and thus they anchor the mycelium to the leaf and absorb food.
Q.13. Give an illustrated account of structure and development of apothecium in Peziza? (2007)
Ans. Sexual Reproduction in Peziza: -
In Peziza the typical sexual reproduction with the help of sexual organs is absent. Here the sexual process consists in the fusion of two nuclei within the vegetative cell. This type of sexual act is called somatogamy.
The mycelium forms a tangled mass of hyphae. In the centre of the tangled hyphal mass some cells to possess nuclei which fuse in pairs. These pairs of nuclei are called dikaryons. The binucleate cells give rise to a number of ascogenous hyphae. From the tips of ascogenous hyphae the asci and ascospores are formed in the same manner typical of all other Ascomycetes. The tip cell of the ascogenous hyphae bends over to form a hook or crozier which is binucleate. Its two nuclei divide simultaneously to form four nuclei. Septa are then laid down delimiting a uninucleate terminal cell, a binucleate penultimate cell distinct in each hook. This binucleate penultimate cell behaves as ascus mother cell. No hook or crozier formation takes place in P.vesiculosa. The ascogenous hyphae with the ascus mother cells constitute the dikaryophase in the life cycle of Peziza. The paired nuclei of the ascus mother cell now fuse resulting in the formation of a diploid nucleus the synkaryon. The diploid nucleus undergoes nuclear division, the first two being meiotic and eight haploid nuclei are formed around which the ascospores are organized.
The asci stand side by side and are intermixed with numerous slender sterile hyphae known as paraphyses. The asci and paraphyses become enveloped by numerous vegetative hyphae to form the characteristic cupshaped fruiting body which is termed the apothecium.
The apothecia of Peziza are saucer to cup shaped structures and vary from2 cm to several inches in diameter. They may be sessile or may have a short stalk. The apothecia are generally drab in colour but some may be brilliantly coloured the apothecium consists consists of three parts-
(a) Hymenium (b) Hypothecium (c) Excipulum.
The upper most layers are the hymenium lining the inside of the cup. It consists of elongated asci and sterile hyphae or paraphyses. Just below the hymenium there is second layer called layer called the hypothecium. It consists of light coloured hyphae running parallel to the hymenial layer and from which asci and paraphyses arise. The excipulum which is the third layer forms the larger part of the basal portion of the apothecium. It is made up pseudoparenchymatous tissue or interwoven hyphae.
Each ascus is elongated, cylindrical structure and contains 8 ascospores. The ascospores are arranged obliquely in a row within the ascus. Ascospores are ellipsoidal and smooth walled. They are discharged through an apical pore of the ascus only under damp conditions. On germination these gives rise to hyphae.
Q.14. Give an illustrated account of ascocarp of peziza. (2005)
Ans. Ascocarp of Peziza: -
In Peziza the typical sexual reproduction with the help of sexual organs is absent. Here the sexual process consists of the fusion of two nulei within the vegetative cell. This type of sexual act is called somatogamy.
The mycelium forms a tangled mass of hyphae. In the centre of the tangled hyphal mass some cells to possess nuclei which fuse in pairs. These pairs of nuclei are called dikaryons. The binucleate cells give rise to a number of ascogenous hyphae. From the tips of ascogenous hyphae the asci and ascospores are formed in the same manner typical of all other Ascomycetes. The tip cell of the ascogenous hyphae bends over to form a hook or crozier which is binucleate. Its two nuclei divide simultaneously to form four nuclei. Septa are then laid down delimiting a uninucleate terminal cell, a binucleate penultimate cell is distinct in each hook. This binucleate penultimate cell behaves as ascus mother cell .No hook or crozier formation takes place in P.vesiculosa. The ascogenous hyphae with the ascus mother cells constitute the dikaryophase in the life cycle of Peziza. The paired nuclei of the ascus mother cell now fuse resulting in the formation of a diploid nucleus the synkaryon. The diploid nucleus undergoes nuclear division, the first two being meiotic and eight haploid nuclei are formed around which the ascospores are organized.
The asci stand side by side and are intermixed with numerous slender sterile hyphae known as paraphyses. The asci and paraphyses become enveloped by numerous vegetative hyphae to form the characteristic cupshaped fruiting body which is termed the apothecium.
The apothecia of Peziza are saucer to cup shaped structures and vary from2 cm to several inches in diameter. They may be sessile or may have a short stalk. The apothecia are generally drab in colour but some may be brilliantly coloured the apothecium consists consists of three parts-
(a) Hymenium
(b) Hypothecium
(c) Excipulum.
The upper most layer is the hymenium lining the inside of the cup.It consists of elongated asci and sterile hyphae or paraphyses. Just below the hymenium there is second layer called the hypothecium. It consists of light coloured hyphae running parallel to the hymenial layer and from which asci and paraphyses arise. The excipulum which is the third layer forms the larger part of the basal portion of the apothecium. It is made up pseudoparenchymatous tissue or interwoven hyphae.
Each ascus is elongated, cylindrical structure and contains 8 ascospores. The ascospores are arranged obliquely in a row within the ascus. Ascospores are ellipsoidal and smooth walled. They are discharged through an apical pore of the ascus only under damp conditions. On germination these gives rise to hyphae.