TYPE: PYTHIUM

Occurrence

It is common in soil water. Its various species live as saprophyte. Some of them live on decaying plant as parasites. The parasitic stage attacks the roots and the seedlings. It causes disease Damping-off. Pyd him debaryanum causes this disease. Some species are parasitic on some freshwater algae.

General structure

The mycelium of Pythium consists of coenocytic, unseptate and branched hyphae. The cell wall is composed of cellulose. Hyphae contain a large mass of protoplasm. This protoplast has large number of nuclei, fat globule and glycogen. The growth of parasitic hyphae is both intercellular and intracellular. They are without any hlustoria. Disease cycle: The infected hyphae enter into plant through the ston ata or cell wall. The mycelium remains in cortical cells (host plant) during early attack. The host plant collapses at this point. It falls on ground. It is called dumping off disease. Later hypae spread to other parts of the plant through vascular bundle. It absorbs the fooe of plant. The plant dies and Pythium becomes saprophytic. It releases its enzymes and decomposes the plant body.

Fig: Pythiuin showing effect on host plant seedling.

Reproduction

Asexual Reproduction

(a) Zoospore formation: Asexual reproduction takes place by zoospores. Zoospores are kidney-shaped. They have two lateral flagella on the concave side Zoospores are produced in the globose sporangia. A papilla is formed on the tip of sporangium. A membranous vesicle comes out through this papilla. This

vesicle contains content of sporangium. Its cytoplasm divides ;nto uninucleate parts. Each part changes into a biflagellate zoospore. The zoospores escape by the rupturing of the vesicular wall. Each zoospore germinates to produce a new mycelium.

(b) Conidia formation: Sometimes, the sporangia detach from the ayphae in dry conditions. Now these are called

• :onidiosporangia. It forms conidia in dry conditions.

(c) kplanospore formation: Chlamydospores also produced on certain hyphae. These spores germinate to form long hyphae. This hypha develops sporangium. Zoospores are produced in this worangi um.

Fig: Asexual reproduction in lithium.

Sexual Reproduction

Sexual reproduaion is oogainous. Oogonia and antheridia are produced. They are very close to each other. They are often present on the same hypha. Antheridium is present just below the oogonium. Both the sex organs are cut off from the hyphae by the formation of septa.

(a) Oogonium: Each mature oogonium is globose. It contains a single oosphere. The oosphere is surrounded by a layer of the cytoplasm called periplasm. Young oosphere is multinucleate.

dui later only one nucleus persists. Others migrate to the periplasm and disintegrate there.

(b) fitntheridium: Each mature antheridium is club shaped. The antheridium is multinucleate in the beginning. But later on it lecomes uninucleate.

Fig: A-c development of sex^. organs, D-F germination of zygote, D. fortntdion ofgerm tube. Flormation of sporangium

Ferilization: Fertilization takes place a fertilization tube. This tube ariscs from antheridial wall and penetrates the periplasm of ooganium. The male nucleus from antheridium passes through this fertilization tube and enters the oosphere. Two nuclei fuse to form zygotic nucleus. Fertilized egg secretes a thick wall and changes into an oospore. Periplasm disintegrates and forms the oospore wall. Sometimes, unfertilized eggs develop into oospore by parthenogenesis.

Germination of oospore: Oospore germinates after a period of jest. It m Ay produce mycelium directly. Or it may produce a short hypha. This hypha develops a sporangium at its tip. High temperature favours mycelium formation. But lov, temperature favours the production of zoospores. Reduction division takes place bcfore the germination of oospore.

KarYoganlY

Fig: Life cycle of Pythium

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