Ex. No. 5                                                               Study of Oomycetes fungi

Domain: Eukarya  Kingdom: Chromista / Straminopila

Division: Oomycota

Class: Oomycetes                                             Oomycetes /Straminopila (Water mould fungi)

 

    Order: Pythiales                                             Peronosporales                                                                       Sclerosporales

	Sclerosporaceae

 

(Family)Pythiaceae                                (Family) Albuginaceae         (Family) Peronosporaceae                                                 

Damping – off                                               White rust of Amaranthus             Downy mildew        cereals Downy mildew

Late blight of Potato

Pythiaceae

Disease name	Causal Organism	Symptoms
Damping – off In ginger – Rhizome rot In Papaya – Foot rot	Pythium aphanidermatum	1.       Pre emergence damping – off : Failure of seed germination 2.       Post emergence damping – off : Dark brown lesions/ spots are formed on the collar region of the stem and affected tissues are soon become rotten condition
Previously Pythiaceae family, at present in Peronosporaceae family Late blight of potato Citrus gummosis, Cardamom Azhukal, Pepper Quick wilt	Phytophthora infestans Phytophthora sp.	Brown to black coloured water soaked lesions are formed on the tip or margin of the leaves and corresponding lower surface of the leaves, white coloured mycelial growth are appear. Potato tubers show dry rot symptoms.

Albuginaceae

Disease name	Causal Organism	Symptoms
White rust or White blisters of Amaranthus	Albugo bliti /                       Albugo candida	White rust pustules appear on the lower/abaxial  surface of the leaves. It produces knob shaped Haustoria. (Obligate parasite)

Peronosporaceae

Downy mildew (Obligate parasite / Biotroph) : White colour mycelial growth is appear on the lower/ abaxial surface of the leaves and corresponding upper/ adaxial surface they can produce chlorotic/ yellow spots are appear

Disease name	Causal Organism		Symptoms
Downy mildew of grapes	Plasmopara viticola		Fruit infection: Infected berries become harden, leathery and wrinkled (Mummified)
Downy mildew of cumbu or Green ear of cumbu	Sclerospora graminicola		Flowers are malformed into green leaf like structures
Downy mildew of sorghum or Sorghum leaf shredding	Sclerospora sorghi		The leaves are converted into thread like structures
Downy mildew of maize or Crazy top of Maize	Peronosclerospora sorghi		Malformed flower buds are called as Crazy top
Lettuce downy mildew	Bremia lactucae
Downy mildew of cucurbits		Pseudoperonospora cunbensis
Tobacco blue mould		Peronospora tabacina
Downy mildew of ornamental plants (aster)		Basidiophora endospora
Downy mildew of Impatiens		Bremiella sphaerosperma

Life cycle of Pythium aphanidermatum                        

Taxonomic position

Kingdom: Fungi

Division: Eumycota

Class: Oomycetes

Order: Peronosporales

Family: Pythiaceae

Genus: Pythium

Species: aphanidermatum

Disease name: Damping – off / Seedling rot

Anamorphic stage:

1.       The hyphae are both intercellular and intracellular and there is no Haustoria are produced.

2.       The sporangia are globose and are produced terminal or intercalary on the hyphae.

3.       The sproangiophore consists of sporangia with a bubble like structure called as vesicle.

4.       On maturity the vesicular wall is burst out like a soap bubble and the zoospores are released.

5.       The zoospores are asexual spore, kidney shaped and has two lateral flagella. (Whiplash & tinsel).

6.       After, Zoospores comes to rest, encyst and germinates by germ tube.

Teleomorphic stage:

1.       Paragynous antheridium, the Oogonium is globose with a multinucleate oosphere surroundly by a layer of periplasm and the antheridium is a club shaped.

2.       Upon gametangial contact, a fertilization tube develops fertilization tube develops and penetrates the oogonial wall and the periplasm,

3.       In the mean time, meiosis has taken place in both the gametangia. The male nucleus now passes through the tube into the oosphere, approaches the female nucleus, unites and forms the zygote.

4.       Oosphere develops into a thick walled oospore and it germinates after undergoing a rest period.

Life cycle of Phytophthora infestans                            

Taxonomic position

Kingdom: Fungi

Division: Eumycota

Class: Oomycetes

Order: Peronosporales

Family: Pythiaceae

Genus: Phytophthora

Species: infestans

Disease name: Late blight of potato

Mitosporic stage:

1.       The Sporangiophores are symboidally branched and hyaline multinucleate lemon shaped papillate sporangia are produced.

2.       In the presence of water the sporangium germinates either by a germ tube or zoospore formation.

3.       After the zoospores are comes to rest they encyst and germinates produces by germ tube, produces an Appressorium.

4.       After penetrating the germ tube develops into a profusely branched, hyaline, Coenocytic, intercellular mycelium and haustoria are produced. 

5.       Low temperature favours zoospore production and high temperature favours germ tube formation.

Meiosporic stage:

1.       heterothallic fungus, the oogonia penetrates ad grow through the antheridium and form a globose structure above the antheridium (Amphigynous antheridia)

2.       Both gametes are multinucleate are initially, but they mature only a single nucleus is left that probably undergoes meiosis before fertilization.

3.       Migration of the male nucleus occurs through the oogonial wall and fusion takes place and after a rest period of several weeks the oospore germinates and produces sporangium.

Sporangiophore characters of Peronosporaceae  

1.       Basidiophora

2.       Sclerospora

3.       Plasmopara

4.       Peronospora & Pesudoperonospora

5.       Bremia

6.       Bremiella

Basidiophora: The Sporangiophore is club shaped with swollen head over which the sporangia are borne.                               Ex. Basidiophora entospora

Sclerospora: The Sporangiophore is long, stout with upright branches bearing sporangia at the tips.

Ex. Sclerospora graminicola

Plasmopara : Sporangiophore branched at right angles with blunt tips. Ex. Plasmopara viticola

Peronospora & Pseudoperonospora : The sporangiophores are dichotomously branched on which sporangia are borne singly. Ex. Peronospora destructor & Pseudoperonospora cunbensis

Bremia: They are similar to Peronospora except the tip of branches are cup shape with four sterigmata from which sporangia are borne. Ex. Bremia lactucae

Bremiella : Tip of its branches inflated into bulbous apophysis. Ex. Bremiella sp.