In this article we will discuss about Pythium. After reading this article you will learn about: 1. Habit and Habitat of Pythium 2. Symptoms of Pythium 3. Vegetative Structure 4. Reproduction 5. Diseases Caused 6. Control Measures.
- Habit and Habitat of Pythium
- Symptoms of Pythium
- Vegetative Structure of Pythium
- Reproduction in Pythium
- Diseases Caused by Pythium
- Control Measures of Pythium
1. Habit and Habitat of Pythium:
The genus is represented by about 92 species which are cosmopolitan in distribution. The species may be aquatic, terrestrial or amphibious in habitat. About 28 species of Pythium have been reported from India. The fungus is facultative parasite and lives saprophytically on the moist humus in soil and attacks seedlings at the soil level. Thereafter, it lives as parasite.
It also grows readily on floating vegetable matter and decaying animals. “Damping off”, “Soft rot”, “Wheat rot”, or the “foot rot” of the seedlings are common diseases caused by the fungus. P. debaryanum causes “damping off” disease of tobacco, mustard, chilies and cress seedlings. Excessive moisture, presence of too much decaying matter in the soil, over- crowding of the seedlings and insufficient exposure to air and light are the predisposing causes for this disease to appear.
2. Symptoms of Pythium:
Young seedlings are affected near the ground level. Initially no external symptoms of its presence are noticeable on the host, but after it has gained a firm hold, the seedlings become pale green and show a weak, shriveled place of girdle of brown colour on the stem near the surface of the soil.
At this point the cortical cells are killed making the tissue weak. The infected tissues appear soft and water soaked. Infected seedlings collapse. The upper part of seedling bends or topples over in a characteristic manner (Fig. 1). This peculiar symptom of disease is known as “damping off”.
The disease occurs in two stages:
1. Pre-Emergence Phase:
Young seedlings are killed before they come out of the soil.
2. Post-Emergence Phase:
Toppling over the infected seedlings after they come out of the soil surface.
3. Vegetative Structure of Pythium:
The mycelium appears as white, fluffy mass. It is well developed, branched, inter- or intracellular (Fig. 1A B), multinucleate and without septa (coenocytic). Haustoria are not produced. Food material is absorbed hyphal walls. Septa appear only in association with the sex organs formation. Cell wall lacks chitin and instead it is composed of glucan and cellulose.
Cell wall encloses vacuolated cytoplasm, mitochondria, a nuclei, endoplasmic reticulum, ribosomes, oil globules and glycogen in the form of reserve food mater. The branched hyphae are very much similar to the filaments of Vaucheria, a member of Chlorophvceae.
4. Reproduction in Pythium:
The fungus reproduces by asexual and sexual methods.
(i) Asexual Reproduction:
Asexual reproduction takes place by the formation of sporangia. Sporangia may be terminal (Fig. 3 A, B) or intercalary in position and are produced on the aerial hyphae. There are no specialised sporangiophores to produce sporangia. The terminal or intercalary position of a hypha enlarges and becomes spherical and starts off function as sporangial initial.
It gets cut off from rest of the mycelium by a transverse septum. Sporangia are multinucleate and may be filamentous, tubular or globes in shape. Sporangia germinate indirectly or directly. Indirect germination. Under favourable conditions (high humidity) sporangia behave as zoosporangia. And produce zoospores. Sporangium germinates by germ tube or by forming a hypha. Both terminate into a thin walled vesicle (Fig. 3 C).
The cleavage of cytoplasm to form zoospores begins within the sporangium but completes within the vesicle (Fig. 3 D). Eight-to-twenty zoospores are formed within the vesicle. The ericle wall bursts like a soap bubble and the zoospores move in all directions (Fig. 3 E).
The zoospores or Reni form, uninucleate and biflagellate (Fig. 3 E). One flagellum is of tinsel type and die other is of whiplash type (Fig. 3 E). Flagella are inserted laterally in the depression. The zoospores swim only for a short time and come to rest.
They retract flagella, round off and encyst by secreting a delicate wall around sporangial proliferation i.e., formation of secondary sporangia from the primary one (e.g., P. proliferum Fig. 3. F, G) and again formation of zoospores from the encysted zoospores has also been reported. Germ tube develops from the encysted zoospore which infects the host by entering through the stomata or by directly renetrating the epidermal cells of the host.
Under un-favourable conditions (dry conditions), the sporangia behave as conidia and are called conidiosporangia. It does not produce zoospores but germinates directly to form a germ tube.
(ii) Sexual Reproduction:
Sexual reproduction is oogamous and occurs under un-favourable conditions at the end of the growing season of the fungus. Pythium is homothallic i.e., male and female sex organs develop in close proximity of each other either on the same or different hypha embedded in the host tissue. However, some species are eterothallic e.g., P. heterothalicum, P. sylvaticum. Male and female sex organs are called anthcridia and oogonia, respectively.
Each antheridium is club shaped, paragynous (present to the side of the archegonium) and multinucleate. It arises as branch of the stalk of the oogonium (monoclinous) or on separate hypha (diclinous). Male branch gets slightly inflated at the tips.
A transverse septum appears between the inflated portion and rest of the hypha (Fig. 4 A-D). The inflated portion is called antheridium. On maturity all antheridial nuclei except one degenerate and the surviving nucleus is called functional nucleus (Fig. 4 E). Many antheridia 1-6) are sometimes seen in association with one oogonium.
It is a globose, spherical, smooth walled structure. It develops at the tip of the hypha (terminal) (Fig. 4, A-D). Sometimes it is also intercalary in position. The tip of the hypha swells up and it soon cuts off from the main hypha by the formation of a septum.
On maturity its protoplasm is differentiated into outer thin spongy portion lining the oogonial wall, the periplasm and the inner, granular denser rounded portion, the ooplasm or oosphere (Fig. 4 E). The ooplasm contains a single female nucleus, while the other nuclei within the periplasm degenerate.
As antheridium matures, it bends towards the oogonium (gametangial contact) to which it applies itself closely. The wall of the antheridium and oogonium dissolves at the point of contact. Antheridium developed a fine fertilization tube or penetration tube which penetrates the periplasm and reaches the ooplasm (fig. F,G). The functional male nucleus enters the oogonium through fertilization tube and fuses with the femalenecleus and forms a diploid zygotic nucleus (Fig. 4 I).
The zygote nucleus secretes a thick double wall and called oospore. (Fig. 4 I). The outer wall Is thick and is called exine while the inner wall is thin and is called intine. The oospore is a resting spore. It is filled with plenty of reserve food material.
Germination of oospore:
Oospore germinates in the presence of moisture. The exine ruptures and intine comes out in the form of germ tube. The zygotic nucleus divides, the first division is meiosis. However, according to Sansome (1961, 1963) meiosis occurs in the oogonia and antheridia of P. debaryanum. If the temperature is high (28°C) the germ tube directly develops into mycelium (Fig. 5A-E).
However, if the temperature is low (10-17°C) and enough moisture is available the contents of the oospore migrate into the sporangium through the germ tube. The sporangium behaves like a zoosporangium and produces zoospores as in asexual reproduction (Fig. 5a-e). Sometimes (for example, P. anandrum) zoospores are directly formed inside the oospore. They are later on liberated in a vesicle (Fig. 5. 1-5). Sometimes zoosporangium gets detached and behaves like conidium.
5. Some Diseases Caused by Pythium:
6. Control Measures of Pythium:
1. Proper drainage system of the field should be maintained.
2. Water logging in the soil and excessive watering should be avoided.
3. Adequate ventilation should be provided.
4. Too thick sowing on the seed beds should be avoided.
5. Soil should be sterilised by using steam, dry heat or chemicals such as formaldehyde diluted in water (1: 50) and Bordeaux mixture (half gallon per square foot).
6. Soil can be fumigated with Chloropicrin or Methyl bromide.
7. De-trenching soil with 0.2% Fytolan, 0.5% Perenox and 0.2% flit is very effective method to control the damping off disease.
8. For pre-emergence phase of disease, the seeds should be treated with Biltox-50 and Arasan.
9. Chemicals like Chloroneb (1, 4-dichloro 2-5 dimethoxy-benzene) can be used in seedlings of pepper, bean, tomato etc.