In this article we will discuss about:- 1. Occurrence of Pythium 2. Predisposing Factors of Pythium Debaryanum 3. Mode of Infection 4. Symptoms 5. Economic Importance 6. Salient Features.
Occurrence of Pythium:
P. debaryanum is the classic example of the most common species of Pythium which causes the widespread “damping off” disease of tobacco, tomato, mustard, chillies and cress seedlings. The fungus lives saprophytically in the moist, humus soil and attacks the seedlings at the soil level. Thereafter it lives as a parasite.
Mostly it occurs in tobacco seed beds but it also grows readily on decaying animal and vegetable material floating in water. A luxuriant growth of this fungus can be easily obtained by germinating tobacco, cress or mustard seeds in moist soil or on moist blotting paper under a bell jar.
Predisposing Factors of Pythium:
The three predisposing causes for this disease to appear are:
(i) Excessive moisture or dampness and presence of too much decaying vegetable matter in the soil. These factors promote growth of the fungus mycelium and production of zoospores.
(ii) Over-crowding of the seedlings. It increases susceptibility by tending to delay thickening and lignification of the cell walls. The tissue with thin walls of the cells is particularly vulnerable.
(iii) Insufficient exposure to air and light. Poor illumination promotes fungal growth.
Mode of Infection:
The mycelium appears in the moist substratum. The fungal hyphae attack the young seedlings below the soil line on the hypocotyl (Fig. 6.17 B) or the region of the upper tap root.
The infecting hypha may gain entrance into the host tissue in either of the following two ways:
(i) Through a stoma.
(ii) By boring through the walls of the epidermal cells probably by enzyme action. Once within the host the hypha grows vigorously and branches to form the mycelium. The hyphae mostly remain confined to the intercellular spaces but have the power of penetrating into the cells.
Symptoms of Pythium:
The “damping-off” disease affects the young seedlings in patches in the seed bed. Once the fungus has gained firm hold there is rapid killing of cells and collapse of the tissue of the hypocotyl. The infected tissue appears soft and water-soaked. The damage may be complete before emergence of the seedling above ground.
This is pre-emergence phase of the disease. The young seedlings are killed before they come out of the soil. The plumule and radicle undergo complete rotting after coming out of the seed. In many cases the disease may continue even after the emergence of the seedling.
This is known as the post-emergence phase. The infected seedlings in the patch soon tumble over on to the ground as if pinched off (Fig. 6.17 B). This is due to the fact that at the point of infection the tissues of the host are softened and killed. The infected stem becomes killed at the base. The damage by the parasite is so much that the seedlings can no longer stand upright. They collapse at soil level and fall over.
From the point of attack the disease spreads in ever widening circles. The fallen seedlings, at first, lose their colour and soon rot away. Now the fungus continues to live saprophytically upon the dead remains. It spreads by external hyphae and zoospores which attach other seedlings nearby. In this way the disease spreads rapidly.
Economic Importance of Pythium:
Most of the species are destructive parasites of our economic plants.
They cause the following serious diseases, sometimes resulting in a huge loss:
1. “Damping off” of tobacco and chillies. The causal agent is P. debaryanum.
2. “Rhizome rot” of ginger. It is caused by P. myriotylum and P aphanidermatum.
3. “Soft rot” of Papaya. It was reported by Subramanium in 1920. The causal agent is P. aphanidermatum.
4. “Fruit rot” of Hibiscus esculentus was reported by Bala Krishnan (1948). The pathogen is P. indicum.
5. “Rhizome and root rot” of turmeric was reported by Rama Krishna and Sowmmi (1954). The causal agent is P. graminicolum.
‘Root rots’ and ‘damping off’ are the serious and extremely destructive diseases caused by species of Pythium. They are responsible for poor germination and thin stand of seedlings in field crops and also of ornamentals, vegetable plants and forest trees in nurseries.
Some species bring about decay and decomposition of plant remains in the soil.
Salient Features of Pythium:
1. P debaryanum parasitizes seedlings of many vascular plants and causes a destructive disease known as “damping off”.
2. The mycelium is well developed, freely branched and coenocytic. It is located within the tissue of the host. Some hyphae may lie outside the host.
3. The hyphae are both intracellular and intercellular. No haustoria are formed.
4. Asexually it reproduces by means of biflagellate zoospores produced in globose to oval, multinucleate sporangia which are borne on somatic hyphae terminally and singly. In some species the sporangia are intercalary in position. No sporangiophores are organised.
5. Usually the sporangia remain attached to the hyphae bearing them in the damp weather and produce zoospores.
6. The protoplast of sporangium is discharged through an exit tube into a vesicle where differentiation of zoospores takes place. The vesicle wall bursts and the zoospores are set free.
7. The liberated biflagellate, reniform zoospore after a short period of activity comes to rest, withdraws its flagella and rounds off. It then secretes a wall around it. In the encysted condition it germinates to produce a germ tube which infects a fresh seedling.
8. In dry atmosphere the sporangia get detached and are dispersed by wind. They function as conidia and each germinates directly by a germ tube.
9. Asexual reproduction also takes place by the formation of gemmae and chlamydospores.
10. Sexual reproduction is oogamous and the fungus is homothallic. The antheridia and oogonia are produced near each other, the oogonium at the end of a hypha and the antheridium just below it on a branch arising from the stalk of the oogonium. Both are terminal in position.
11. They are multinucleate at first but become uninucleate towards maturity. They are brought into contact by curvature of the hyphae bearing them.
12. Fertilisation is brought about by means of a fertilisation tube developed on the antheridium. It penetrates the oogonial wall through a pore formed at the point of gametangial contact and introduces a male nucleus with a certain amount of cytoplasm into an egg.
13. The male nucleus fuses with the egg nucleus. The resultant thick and smooth walled oospore enters upon a period of rest. It is aplerotic.
14. Prior to germination the oospore nucleus undergoes zygotic meiosis but Mrs. Eva Sansome (1963) reported that meiosis in P. debaryanum takes place in the oogonium and antheridium at the time of gamete formation or differentiation of sexual nuclei. The somatic phase, according to this view, is diploid. The consensus of opinion, at present, favours this view.
15. At high temperatures the outer coat of the oospore bursts and the contents surrounded by the inner wall grow into a germ tube which develops into a new mycelium.
16. At lower temperatures the germ tube grows to a limited extent and swells at its tip to form a vesicle into which migrates the oospore protoplast and gets differentiated into motile meiospores called the zoomeiospores.
17. The liberated zoomeiospore withdraws its two flagella, rounds off and secretes a wall. It then germinates to form a germ tube which develops into the mycelium.