Fungi

 ALBUGO (CYSTOPUS)

                                                              

Occurrence: 

·         Most of the species of Albugo are obligate parasites occurring as endoparasite in higher plants.

·         They usually infect the member of the families’ cruciferae, convolvulaceae, compositae, amaranthaceae.

·         There are 30 species is found all over the world also species are present in India.

 

Symptoms:  

·         The fungus insects mainly the aerial parts of the plant such as petioles, leaves, stem, flowers and fruits.

·         The early symptoms of the disease are the formation of whit shiny irregular, pustules (spot) on the lower surface of the leaves.

·         Later on the pustules enlarge in size and fuses with each other to form large pustules. The host epidermis is ruptured and white powdery mass comes out at maturity.

·         The infected part of the form hypertrophied or distortion (eg- in leaf, stem). Infected flower become sterile.

Vegetative structure:

·         The mycelium of Albugo is well branched, aseptate and caerocytic.

·         The hyphal wall is composed of chitin and the cytoplasm is granular and contains many nuclei, vacuoles, oil globules and glycogen.

·         The fungus grows in intracellular space of the host.

·         Tissue and absorbs food material with the help of knob like haustoria.

·         The haustorium is differentiated into haustoria head and a slender neck like stalk.

·         Head of haustorium filled with dense cytoplasm and having all cell organells except nuclei.

·         The base of haustorium surrounded by cellular like sheath form by extension of host cell wall.

Reproduction:

 Albugo reproduces by asexual and sexual means.

1.      Asexual reproduction-

·         Asexually it is reproduced by multinucleate structure known as conidia sporangia or zoosporangia. It development are as follows.

a)      Formation of conidiophores or sporangiophore-

·         After infecting host tissue intercellular mycelium collects beneath the host epidermis.

·         The tips of these hyphae develop into short erect, thick walled and club-shaped structure called conidiophores or sporangiophore.

·         The aerial end of the conidiophores is multinucleate thin walled and densely cytoplasmic present just beneath of epidermis.

b)     Formation of conidia or sporangia or zoosporangia-

·         Conidia or sporangia develop in long basipetal chain at the tip of sporangiophore or conidiophores.

·         During the formation of the mycelium a deep constriction appears below the swollen end of the conidiophores which come out from the periphery and move toward the centre of the tip of sporangiophore.

·         At the last constriction develop into transverse wall and small bud like structure develops into a spherical multinucleate conidium or sporangium.

·         This process repeated for several time and each other with the help of disjunctor.

·         This newly formed conidia excreted the pressure on the wall of epidermis and ruptured the pressure on the wall and they come out from host surface as white powdery mass. Hence the disease caused by this fungus is known as white rust.

c)      Germination of conidia-

·         Conidia are smooth multinucleate and spore structure measuring 13-18µm in diameter.

·          Conidia are dispersed form the pustules by shrub and drying of the disjunctor.

·         They get below by wind or washed away by rain. They get germinate after 2-3 hours on suitable substratum under moist condition.

·         Multinucleate conidia divide into number of segment.

·         Each segment metamorphosed into reniform biflagellate zoospore.

·         These newly formed zoospores germinate and form a germ tube which penetrate through epidermis and develops into a fresh mycelium.

Sexual reproduction:  

In Albugo sexual reproduction is oogamous and male and female sex organs are known as antheridium and oogonium respectively. They develop in intracellular space quite deep into host tissue develop from male branch and female branch respectively.

Oogonium:

·         During oogonial development the apex of the hyphal branch swell considerably and becomes globular by accumulating cytoplasm and nuclei.

·         The swollen apex with 6-12 nuclei is separated from main hypha by the formation of a transverse septum.

·         The cytoplasm of the young oogonium is uniformly vacuolated and nuclei are evenly distributed throughout.

·         But protoplasm of oogonium differentiated into outer more vacuolated and nuclei present periplasm and central densely cytoplasmic ooplasm.

·         In between ooplasm and periplasm a membrane is form.

·         The ooplasm represent the egg. There are only one nucleus is present at the maturity of egg.

Antheridium:

·         Antheridia are elongated, club shape and multinucleate structure.

·         They develop on male hyphae which are placed very close to the oogonium.

·         The tips of the male hyphae accumulate cytoplasm and nuclei and swollen antheridial tip is soon cut off by transverse septum.

·         There are many nuclei in antheridia but only a single remain function and remaining get degenerate.

Fertilization:

·         Fertilization takes place by gametangial contact.

·         The wall of antheridium and oogonium become thin at the region of their contact.

·         The antheridium form a slender fertilization tube that penetrates the oogonial wall and through this tube plasmogamy take place also male nuclei fuses with that of female nuclei in it.

·         Fertilized egg secretes a thick wall around and knows it known as oospore. The wall of oospore differentiated into two layer outer layer known as exine while inner wall known as intine.

 

 

 

Germination of oospore-

·         The nuclei in the oospore divide meiotically after that mitotically and formed many haploid nuclei.

·         The protoplasm is also divide in number of segments in each segment one nuclei is present.

·         Each segment get metamorphosed into uninucleate reniform and biflagellate zoospore.

·          The zoospore germinate by absorbing water and form germ tube and this germ tube later on form mycelium of thallus.