Classification of chromista and description about important diseases especially Pythium and Phytophthora and their general characters and life cycles will give us an detail description and idea about diseases comes under this group

1. CHROMISTA
N.H. SHANKAR REDDY
2nd Ph.D. Plant Pathology
Annamalai University

3. Oomycota
Genera characters of Oomycota
• Otherwise called as Egg fungi/ pseudofungi
• Cell wall is composed of cellulose + hydroxyproline
• Oomycota fungi can reproduce by means of both sexual and asexual reproduction
• Asexual reproduction by means of motile spores (zoospores) with two flagella and
sexual reproduction is by gametangial contact (male gametes, antheridium and
female gametes, oogoium) resulting the formation of thick walled resting spore
(oospore).
• Asexual spores – zoospores
• Sexual spores – oospores
• The special feature, zoospores is of biflagellate with anterior long tinsel and
posterior short whiplash
• The gametes are non-flagellate. The fusion of gametes results in the formation of
thick-walled oospores
• Eg – Late blight of potato - Phytophthora infestans
• Grapes downy mildew – Plasmopara viticola

4. Life cycle of damping off of vegetables –
Pythium aphanidermatum
Systemic classification :-
Domain – Eukarya
Kingdom – Chromista
Phylum – Oomycota
Class – Oomycetes
Order – Pythiales
Family – Pythiaceae
Genus – Pythium
Species – aphanidermatum

5. Symptoms
• Pre emergence damping off – seeds
and radicles rots before the
seedling emergence out of the soil
• Post emergence damping off –
newly emerging seedlings are killed
at ground level after emerge from
the soil

6. Asexual reproduction –
• Mycelium consists of coenocytic hyphae, hyphae is both intercellular
and intracellular. No haustoria are produced
• Pythium produces simple branched and globose zoosporangia
(produces at terminal or intercalary)
• Germination is either zoospores on germtube, sporangiospore consists
of sporangia with bubble like vesicle, the sporangial protoplast moves
rapidly through tube into vesicle and appears remains delimitation of
zoospores takes place
• Zoospore are released in a rocking motion and bounces on the vesicle
• The reniform zoospores are biflagellate with anterior tinsel and
posterior whiplash type, spores come in rest and encyst and germinate
by producing germ tube

7. Sexual reproduction
• Sexual reproduction is oogmous and takes place by gametangial contact
• The male sex organ is antheridium and female is oogonium
• Upon the gametangial contact, a fertilization tube develops and
penetrates the oogonial wall and enter the periplasm
• In the meantime meiosis takes place only functional nuclei remains,
other disintegrate.
• Now male nuclei passes to female nuclei (oosphere) through
fertilization tube, which forms zygote
• The oospore develop a thick wall around it self and germinate by using
germination tube (if favourable conditions occurs it germinates,
otherwise it undergoes resting period)

9. Life cycle of Phytophthora infestans
Systemic Clasification
Domain – Eukarya
Kingdom – Chromista
Phylum – Oomycota
Class – Oomycetes
Order – Pernosporales
Family – Pernosporaceae
Genus – Phytophthora
Species – infestans

10. • Symptoms – ‘Phytophthotra’ means
plant destroyer, water soaked light
brown lesions develop on the leaves and
later enlarges and cover the entire leaf,
petiole and stem. Rotten portion emits
characteristic odour

11. Asexual life cycle
• Sporangia are borne aerial sympodially branched sporangiophore
arise directly from internal mycelium
• Hyphae is inter and intracellular profusely branched and produced
finger like haustoria
• The sporangiophores are with characteristic swellings at the nodes of
indeterminate growth
• On falling suitable host the sporangia germinate (< 15o C) favours
the zoospore production, high temperature favours the germ tube
formation (> 20 o C)

12. Sexual life cycle
• Sexual reproduction takes place means of antheridia and oogonia
of opposite mating types
• Oogonia penetrates and go through the antheridia and form a
globose structure above the antheridia is called amphigynous
antheridia
• Migration of antheridium nucleus into oogonial wall by
fertilization tube
• The fertilized egg secretes a heavy wall around it self and become
oospore
• Oospore germinates by means of germ tube and produces zoospore
and produced new thalli.

14. Difference between Pythium and Phytophthora
Particulars Pythium Phytophthora
Meaning Pythium which means Decay Phytophthora which means Plant
destroyer
Family Pythiaceae Pernosporaceae
Order Pythiales Pernosporales
Symptoms Rooting of roots, slow growth, stunting Rotting of roots and stem, stunting,
discolouration and wilting
Parasitism Facultative parasite Facultative saprophyte
Haustoria and appresoria Absent Present
Sporangiophores Sporangiophores are not well-
distinguished from hyphae
Sporangiophores are well distinguished
from hyphae having sympodial branches
(zig zag )
Differentiation of
zoospores
Zoospores are differentiated in the vesicle
and release upon the rupture of vesicle
Zoospore are differentiated inside the
sporangia and released in an
disappearing vesicle at the sporangia
apex after maturation
Type of Antheridia Antheridia is of paragynous type Antheridia is of amphigynous type
Baiting technique Mustard is used for baiting technique Apple is used for baiting technique
Examples Pytium aphanidermatum (damping of
vegetables), Pythium debaryanum
Phytophthora infestans (late blight of
potato)