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Albugo

(1) Albugo candida is an obligate parasitic fungus that infects plants in the crucifer family like mustard and causes white rust disease. It has both asexual and sexual reproduction stages. (2) During asexual reproduction, sporangiophores produce chains of sporangia on the leaf surface that release zoospores or germinate directly to infect other leaves. (3) During sexual reproduction in unfavorable conditions, antheridia and oogonia are produced deeper in host tissues and undergo fertilization to form thick-walled oospores that overwinter in soil or plant debris.

Science◦
(CYSTOPUS) Classification: Kingdom : Mycota / Fungi Division : Eumycotina Subdivision: Mastigomycotina Class : Oomycetes Order : Peranosporales Family : Albuginaceae Genus : Albugo By: Dr P B MALLIKHARJUNA GFGC YELAHANKA BANGALORE - 64 Infected mustard leaf with white pustules GFGC Yelahanka 1 3/11/2022 Ploidy state is controversial : Diploid / Haploid
Introduction: Albugo is an obligate parasitic fungal genus. It consists of about 45 species. All these will infect higher plants and cause leaf spot /blister diseases. Some of these are ;  Albugo candida: infects the crucifers/ mustard family and cause white rust disease (misnomer) ,a leaf spot disease in more than 300 species.  Albugo ipomae-panduranae: infects the plants belonging to the family Convolvulaceae  Albugo bliti: infects the Amarantheceae members.  Albugo occidentale: infects spinach- (Spinacea oleracea, phalak)  Albugo portulacea: infects Portulaca oleracea  Albugo oleracea: infects the Asteraceae members GFGC Yelahanka 2 3/11/2022
 Albugo species can be identified based on the host specificity, oospore ornamentation and conidial characteristics.  Albugo candida is the common and widely distributed fungus, which infects the crucifer members such as mustard, cabbage, radish, horseradish and Capsella bursa-pastoris (Shepard purse).  It causes white rust/blister disease especially on the leaves.  This fungus causes two types of symptoms. The primary symptoms will occur on the leaf surface in the host plants, indicating the predominant phase/stage of the fungus called the asexual stage.  Whereas, the systemic (infection) symptoms may appear at later stage as the hypertrophied and hyperplasia structures, appears as tumours on the entire aerial parts of the plants (such as stem, leaves, petioles and veins) indicating the sexual stage of Albugo. GFGC Yelahanka 3 3/11/2022
Habit and Habitat: • Albugo candida is an obligate parasitic plant pathogen that infects primarily the leaves and finally spreads into the different aerial parts, especially, the cortical tissues of the petiole , stem and floral parts.  It is an aseptate mycelial form and highly branched, siphon like mycelial fungus.  It will grow intercellular in the host tissues and absorbs the nutrients from the host by producing the characteristic knob or button shaped haustoria.  Further, the mycelium is coenocytic (multinucleate) due to the no cross septa formation in the hypha. The cytoplasm with all cell organelles mainly confined to the peripheral region. Its wall is made of cellulose.  Albugo will undergo both the asexual and sexual reproduction soon after establishing in the host vegetatively. GFGC Yelahanka 4 3/11/2022
The ultrastructure of haustorium Nuclei GFGC Yelahanka 5 3/11/2022
Asexual reproduction: is the most predominant type of reproduction of Albugo, that occurs during favourable conditions.  It occurs due to the formation of sporangiospores  Some of the mycelial hyphae terminal tips are going to accumulate as a palisade like structures just below the lower epidermis of leaf.  These hyphae tips will protrude vertically as the club shaped conidiophores/sporangiophores due to the accumulation of more nutrients and nuclei. Further, they are with thick lateral walls and thin apex.  The terminal tip of each sporangiophore produces a bulged structure, which metamorphoses into a sporangium. The developmental stages of sporangia GFGC Yelahanka 6 3/11/2022
VS of mustard leaf showing the asexual reproduction of Albugo In the permanent slide under microscope micrograph GFGC Yelahanka 7 3/11/2022
 Both these structures create pressure over the leaf lower epidermis, and hence ruptures, so as the tuft of club shaped sporangiophores along with the sporangial chains is being exposed.  The tip of sporangiophore is continuously produce sporangia, and hence these are arranged in the form of a chain in an basipetal manner i.e, the earliest (older) spore at the tip and young one at the base.  All the spores/conidia are colourless and multinucleate. Further, the young spores are spheroidal while the matured spores are hexagonal in shape.  In-between these conidia, there will be hygroscopic intercalary discs called disjunctors, which help in the spore dispersal.  The matured sporangia/conidia are get detached from the conidial chain and there by germinate on the same leaf or different host plant. GFGC Yelahanka 8 3/11/2022
 The germination of these sporangia is of two types based on availability of water/ moisture content.  Usually under favourable conditions (sufficient water) each sporangium will produce 8 zoospores due to cytokinesis.  Each zoospore is small, motile, biflagellate (heterokont), naked and uni-nucleate structure.  Whereas, under unfavourable conditions rarely the sporangia directly germinates and establishes the daughter mycelia.  The entire asexual reproduction is completed within 10 days. The germination of Sporangium Zoospore GFGC Yelahanka 9 3/11/2022
Sexual reproduction: in Albugo occurs during unfavourable conditions like the end of host season.  This process occurs in the host plant deeper tissues like the petiolar or the stem cortex, which is indicated by the hypertrophied (tumour) structure.  Sexual reproduction in Albugo is the Oogamous and gametangial contact. Further, it is a eucarpic, and the homothalial or heterothalial fungus.  Antheridium is male sex organ, which is a tubular or club shaped structure. Further, it is a long slender and multinucleate (6-8) structure, rich with nutrients.  Oogonium is the female reproductive structure. It is a large and conspicuous globular structure with distinct Periplasm and Ooplasm. GFGC Yelahanka 10 3/11/2022
 The Ooplasm is present at the centre with dense cytoplasm and egg nucleus and other nuclei.  Whereas, the periplasm is a vacuolated cytoplasm that surrounds the ooplasm.  The antheridium and oogonium usually produced by the two different individual mycelia and lay side-by-side (Perigynous) or the antheridium surrounds the oogonium.  The fertilization takes place due to the gametangial contact. Both sex organs undergo meiosis and produce haploid nuclei.  The antheridium makes a contact with the oogonium at receptive papilla. Further, the antheridium penetrates into the oogonium by producing the fertilization tube.  Meanwhile, some visible cytological changes occurs in the Oogonium. Two distinct regions develops namely the vacuolated periplasm and the multinucleated dense cytoplasm in the centre called Ooplasm. GFGC Yelahanka 11 3/11/2022
 The centre of the ooplasm is denser and multinucleated structure generally referred as the coenocentrum.  The tip of the fertilization tube ruptures and the functional male nucleus and some amount of cytoplasm discharged into the oogonium.  The male nucleus fuses with the one egg nucleus called Oogamy and produces a diploid oospore (zygote). The sequential events occurring in the sexual reproduction of Albugo Gametangial contact GFGC Yelahanka 12 3/11/2022
 The oospore is a diploid, resting spore with trilayer thick wall, dark brown in colour, spheroidal and highly resistant structure, usually found in the soil or infected plant debris.  During favourable conditions each oospore will undergo mitotic divisions in order to produce 32 to 64 zoospores (reniform and biflagellate).  These spores germinate on the suitable host plant and develops into individual Albugo. Resting Oospore GFGC Yelahanka 13 3/11/2022
The life cycle of Albugo Asexual Sexual Diplobiontic life cycle GFGC Yelahanka 14 3/11/2022