In this article we will discuss about:- 1. Occurrence of Claviceps Purpurea 2. Distribution of Claviceps Purpurea 3. Primary Infection Caused 4. Hyphal Structure 5. Differentiation of Sclerotium 6. Dispersal of Sclerotia 7. Germination of Sclerotium 8. Reproduction 9. Host-Parasite Relationship 10. Control of the Disease 11. Economic Importance.
Contents:
- Occurrence of Claviceps Purpurea
- Distribution of Claviceps Purpurea
- Primary Infection Caused by Claviceps Purpurea
- Hyphal Structure of Claviceps Purpurea
- Differentiation of Sclerotium
- Dispersal of Sclerotia
- Germination of Sclerotium
- Reproduction in Claviceps Purpurea
- Host-Parasite Relationship of Claviceps Purpurea
- Control of the Disease Caused by Claviceps Purpurea
- Economic Importance of Claviceps Purpurea
1. Occurrence of Claviceps Purpurea:
This fungus ergotizes certain susceptible wild grasses and cereals such as rye (Secale cereales) and barley and occasionally wheat. The infection takes place when the plants are blooming. On the rye plant C. purpurea is responsible for causing a disease known as ergot of rye.
2. Distribution of Claviceps Purpurea:
It is a disease of the temperate zone and occurs in abundance in Canada, U.S.A., Russia and many other countries of European continent. In India, it is of rare occurrence and has been reported from a few places. Pushkar Nath and Padwick (1941) and Kamat (1953) recorded the fungus in the hilly regions of North. Kulkarni (1942) reported the ergot disease from Bombay, and Karnataka on Jowar crop.
3. Primary Infection Caused by Claviceps Purpurea:
It takes place by the wind disseminated ascospores (Fig. 11.19 A). Stager (1912), however, reported that the conidia are resistant to cold and thus remain viable during winter. The surviving conidia possibly bring about infection in the following growing season. The consensus of opinion still favours the view that the primary source of inocculum are the ascospores liberated as a result of natural germination of sclerotia.
Infection takes place when the plants are blooming. Watkins and Littlefield (1976) reported that the incidence of ergot infection is greater at or before anthesis than after anthesis (time when anthers are first extruded). Both the stigma and ovary wall are the sites of infection. Penetration occurs via both. Colonization is intercellular.
Landing on the feathery stigmas or ovary wall of Secale cereale (Fig. 11.17), the ascospore germinates by several germ tubes (Fig. 11.19B). The germ tubes penetrate between the cells of the stigma. Their growth is directed towards the ovule. The intercellular hyphae grow downwards between the cells of the style and extend into the apical tissue of the ovary.
From there the hyphae grow around the ovule in the inner layers of the ovary wall and reach the tip of the rachilla (vascular tissue in the base of the ovary). The fungal hyphae do not extend into the rachilla but form a narrow interface with the vascular tissue. To the naked eye the interface often appears as a brown line.
4. Hyphal Structure of Claviceps Purpurea:
The intercellular hyphae are hyaline, septate and about 3 µm in dia. Within the hyphal wall is the plasma membrane. The latter encloses granular cytoplasm containing mitochondria, vesicles, small lipid globules and several nuclei.
Sphacelium of Claviceps Purpurea:
Within about a week’s time after infection, the hyphal mass increases to such an extent that only few cells of the ovary could be seen intact. The ovule is completely surrounded and penetrated by the proliferating hyphae but there is no penetration into the rachilla. Finally the ovule is destroyed and is replaced by a thick soft cottony hypha mass or mat (D) of nearly the same form as the floral cavity itself by the 10th day.
It is known as the sphacelium (Fig 11. 18). The latter replaces the ovarian tissue and consists of loose plectenchyma.
It is differentiated externally into 3 regions, namely:
(i) The basal foot region with a smooth, white surface located at the base of the host floret,
(ii) A central larger portion constituting the conidial stroma region of the sphacelium; and
(iii) The sparsely invaded ovarian apical or cap region covered with bristles.
Sporulation of Claviceps Purpurea:
The first sign of sporulation which provides external evidence of infection appears between 4-6 days after infection in the form of white ridges of conidiophores on the surface of the ovary.
Certain hyphae from the mycelial mat (stroma of sphacelium) grow outwards, emerge between the ovarian epidermal cells, rupture the cuticle and produce a compact, palisade-like superficial layer of phialidic conidiophores. At its tip each comdiophore bears a succession of small, spore-like structures, the conidia.
They are abstricted in a basipetal succession. The conidia are small, hyaline, uninucleate structures spherical to oblong in form. The earlier mycologists mistook these conidia as an imperfect fungus Sphacelia segetum. So the conidial stage of the fungus came to be known as the sphacelial stage in the life cycle.
It is characterised by the freely sporulating conidiophores all over the external surface of the ovary and internal pockets (D). At about this stage a sticky, dense yellow, sugary fluid termed the honey dew begains to exude between the glumes thereby making the infected florets conspicuous.
For this reason some mycologists prefer to call the sphacelial stage as the honey dew stage. Conidia accumulate in the sugary exudate and ooze from the florets in drops of honey dew by the 6th or 7th day (Luttrell, 1980). Insects are attracted to the honey drops.
While feeding upon honey they pick up the conidia and carry them to other healthy florets on the same or another rye plant to cause secondary infections. In this way the disease spreads during the growing season. Abundance of sugar in the honey dew makes it an excellent substrate for microbial growth.
According to Cunfer (1976) the honey dew contains a factor(s) which inhibits the germination of conidia of certain fungi that come in contact with the sphacelial stage. It is, however, ineffective against the conidia of those fungi (Fusarium heterosporum) which are capable of colonizing sphacelial stage and prevent maturation of ergot sclerotia.
According to some mycologists, honey is secreted by the hyphae and according to others by the flower nectary. The latest view is that profuse exudation of honey dew is the result of the fungal parasite tapping the host’s vascular system which remains uninvaded.
The production of conidia is profuse from 7 day to 11 day and ceases by 13 day. By the 10th day after infection of the ovary, all the host tissues, to which the parasite has access, are penetrated by the hyphae. The path of infection is complete. There are no further invasion of the host tissues. Further development of the fungus takes place by the development of the sclerotial stage.
5. Differentiation of Sclerotium:
Milovidov (1954) observed that sclerotial cells are formed by transformation of sphacelial cells. The process starts at the base of the sphacelium. Shaw and Mantle (1980) concurring with this view reported that the sphacelial tissue has the capacity to differentiate into sclerotial form.
As the transition starts, the fungal growth at the base of the sphacelium forms hyphae mainly of the sclerotial type and not of sphacelial type. As the conidial production ceases and growing season is about to be over, hyphal differentiation into sclerotial form becomes evident in the lower part of the sphacelium.
According to Luttrell (1980), the hyphae across the top of foot region of the sphacelium assume a vertical orientation and form a palisade-like layer. These hyphae differ from the adjacent sphacelial tissue in being short, bulbous, wide and frequently septate.
They are composed of cylindrical multinucleate, lipid rich cells containing small vacuoles and tend to form prosenchyma. This palisade-like layer of short hyphae is termed the generative layer.
According to Luttrell, the generative layer delimits the foot and gives rise to the tissues of the ergot sclerotium. Hyphal tip growth of the generative layer results in increase in length of the developing sclerotium.
Branching and septation of the resulting sclerotial hyphae and lipid deposition in the sclerotial cells determine its compact plectenchymatous nature. As the differentiation of sclerotial tissue proceeds from below upwards it pushes up beneath and within the conidial stroma.
Consequently the remnants of the conidial stroma are sloughed from its surface. The increase in sclerotium length pushes the conidial stroma (loosely packed sphacelial hyphae) upward with the sparsely infected bristly ovarian cap at its top. By this time the sclerotium is conspicuously developed.
It protrudes through the tips of the florets with the sphacelial cap consisting mainly of the remains of the ovary cell walls, stigmas, remnants of the ovule and degenerating sphacelial hyphae, still persisting at its apex. The host tissues are no longer alive.
The sclerotium is considerably firmer than the sphacelium which consists of loosely packed hyphae. The hyphae sound the periphery of the sclerotium darken and become coalesced to form the rind.
The sclerotium which has destroyed the ovarian tissue and replaced the grain lies between the glumes of the spikelet. It protrudes out of the glumes as a dark spur on the spike (Fig. 11.19 H). It is much longer and broader in size than the grain which it has replaced. The mature ears of rye bear both the sclerotia and the healthy rye grains.
The sclerotium matures in about 4 weeks time. The mature sclerotium is a hard pseudoparenchymatous dark-purple or blackish, usually a curved structure (Fig. 11.19 G). Beneath the dark outer coat or rind, which is protective and thus prevents too much loss of moisture, is the hyaline (colourless), tissue consisting primarily of compactly arranged isodiametric storage cells.
The medullary region consists of a loose hyphal conducting tissue forming the central strand. From the latter radiate narrow veins irregularly toward the periphery. The sclerotia are produced in late summer and fall to the ground in autumn. They are resistant to freezing and thus possess a long viability enabling the fungus to over winter in this condition.
Sclerotia are, thus, the structures of survival of ergot fungi and the source of primary inoculum for infection. However, Cunifer and Seckenger (1977) reported that sclerotia which mature on plants and fall to the ground at crop maturity cannot survive until the following spring. Six months is the longest surviving period for any sclerotia.
6. Dispersal of Sclerotia:
The sclerotia reach maturity at about the same time as the healthy grains in other ovaries on the spike. Many of the mature sclerotia fall to the ground (G). Some become intermingled with the grains during harvesting operations. They are returned to the field at the time of fresh sowing and may be sown together with the grain.
7. Germination of Sclerotium:
The sclerotia which have fallen to the ground hibernate during winter. The following spring they germinate under suitable moisture conditions.
About half a dozen or even more small pink or dark purple, drum stick-like outgrowths grow from its surface and turn towards light (A). Each of these is about 10-12 mm long and is called a stroma (pi. stromata). The stroma consists of a slender stalk ending in a flattered spherical or capitate head. Both the stalk and the stromatal head are made up of compactly interwoven hyphae.
8. Reproduction in Claviceps Purpurea:
Sexual Reproduction:
(a) Sex organs (Fig. 11.21):
Embedded in the pseudoparenchymatous tissue of each stromatal head are a number of small, flask-shaped cavities arranged in a peripheral layer (Fig. 11.22 B). Some authors prefer to call them the perithecial cavities. Each cavity contains both kinds of sex organs (Fig. 11.21 A). They are developed from the terminal cells of lateral branches arising from the same hypha at the base of the perithecial cavity.
The fertile hyphae have richer protoplasmic contents. The male gametangium is called the antheridium and female ascogonium. Both are multinucleate at maturity. The ascogonium is relatively stouter and broader than the antheridium which is slender and elongated. There may be one or more antheridia arising from the base of the ascogonium.
(b) Plasmogamy:
Towards maturity the ascogonium develops a small, lateral papilla-like outgrowth which becomes applied to the nearest antheridium. Dissolution of the intervening, walls occurs in the region of contact (Fig. 11.21B). Killian (1919) reported the migration of the male nuclei into the ascogonium.
Development of Ascocarp (Perithecium):
Plasmogamy is followed by the development of ascogenous hyphae from the ascogonium. The ascogenous hyphae (Fig. 11.21 C) recurve terminally to form croziers as in the other Ascomycetes. The resultant binculeate crook or penultimate cell of each ascogenous hypha is destined to become an elongate ascus.
It is known as the ascus mother cell. The two nuclei in the ascus mother cell fuse. The cell with fusion nucleus is the young ascus. It elongates considerably to form an elongated, narrow tubular ascus (Fig. 11.22 C). The diploid nucleus at the same time undergoes meiosis to form four haploid nuclei.
Each of the latter again divides mitotically. A portion of cytoplasm gathers round each of the eight haploid nuclei. Each unincleate protoplast becomes enveloped by a wall. The eight cells thus formed in each ascus mature into ascospores. The ascospores are long, thread-like, hyaline structures.
They lie parallel to one another in the ascus cavity (Fig. 11.22D). While these changes are taking place a soft thin, perithecial wall (peridium) grows up around the sexual apparatus to form a definite fructification called the perithecium (Fig. 11.22C).
Structure of the Perithecium (11.22 B-C):
The mature flask-shaped perithecia are deeply sunk in cavities or locules in the cortical area of the stroma (B). The perithecial wall is one or two cell layers thick with a short, narrow neck lined with periphyses and opening by an ostiole at the stromatic surface (B). The minute, ostiolate pore is situated at the tip of a small papilla. The presence of ostiolate papillae on the surface gives a slightly warty appearance to the stroma.
Within each perithecium lie several asci (C) which are narrow, elongated, somewhat curved tubular structures, each bearing a thick cap at its apex (D). They arise from the base of the perithecial cavity in a tuff and grow up to the ostiole.
From the sides of the inner wall of the perithecium arise the paraphyses which surround the asci. The paraphyses are thus lateral in origin. They do not intermingle with the asci but remain appressed to the perithecial wall. Lining the short neck and the ostiole are the periphyses.
Dehiscence of asci and dispersal of ascospores:
By the time the ascospores reach maturity the rye plants blossom in the fields. The mature asci, according to some authors, grow up in succession and extend out of the ostiolate pore.
Hygroscopic pressures develop within them causing the ascospores to be discharged with a considerable force through a pore in the thick ascal cap into the air for a distance of up to 50 mm. The filiform ascospores (Fig. 11.19A) are thus forcefully ejected, one by one, at short intervals.
They are shot out only when the perithecia are vertically upright. The direction is adjusted by the slow twist and turn movements of the stalk just below the stroma. These movements bring each perithecium in an upright position for a short while.
The discharged ascospores are borne by the air currents to the flowering ears of rye plants to start the life cycle afresh. Stager, however, claims that the ascospores are disseminated by insects. The ascospores are thus disseminated when the rye flowers are open with the feathery stigmas spread out.
9. Host-Parasite Relationship of Claviceps Purpurea:
Tulsane (1950) pointed that ergot of rye caused by C.purpurea is a replacement disease. The fungal parasite disintegrates the host tissues in the floral cavity to make room for the development of it sphacelial and sclerotial stages which constitute the replacement structures. It colonizes the floral tissue.
The physical presence of the intercellular hyphe of the parasite appear to cause only slight distortion of the host cells. While selectively colonizing the ovary, the parasite maintains a compatible relationship with the adjacent host cell tissue at the base of the ovary in the rachilla region as a source of water and nutrients.
10. Control of the Disease Caused by Claviceps Purpurea:
The chief methods of control ergot disease include:
(i) Burial of sclerotia by deep ploughing,
(ii) Sowing grasses before the formation of sclerotia,
(iii) Planting disease resistant or non-susceptible crops,
(iv) Prevention of heading in fence rows or other adjacent waste areas
(v) Sowing ergot free or 2-year old seeds for sclerotia cannot survive for more than 6 months
(vi) Killing the fungus in the sclerotia falling in the fields at the harvesting time by burning
(vii) Burning straw and stubble in and around the adjacent areas.
Hardison (1972) recommended the use of systemic and protectant fungicides applied over sclerotia at the soil surface to suppress perithecia formation. He found Phenyl 5, 6 dichloro-2-trinuoro-methyl-l-benzimidazole carboxylate useful in suppressing perithecia formation at 2 mg/92 cm2 of soil surface. Cardium chloride may provide an economic chemical cantrol.
11. Economic Importance of Claviceps Purpurea:
The fungus (Claviceps purpurea) is responsible for causing the ergot disease which to some extent lowers the yield of rye plant but the disease seldom is of much consequence.
However, the hard purple-black sclerotia (popularly called ergots) which replace the grain are highly poisonous. They contain a toxic substance which poisons and kills animals. It is fatal to man also. The sclerotia are collected or cultivated to extract an officially recognized drug known as ergotin.
It contains several important alkaloids. The best known are ergonovine, ergometrin and ergotamine. These alkaloids are useful in pregnancy and labour to promote expulsion of the foetus and thus are employed in the manufacture of a medicine which causes artificial abortion.
Consequently, cattle which graze on grassland infected with Claviceps abort because of the ingestion of sclerotia. Thus the cattlemen suffer enormous losses.
The ergot alkaloids are also utilized in controlling uterine haemorrhage (bleeding) after child birth and to check other uterine troubles. The animals feeding on sclerotia lying on the ground or ergot contaminated flour, suffer from a serious physiological disease termed ergotism. The severe attack of ergotism may even result in gangrene of toes and feet.