2. Albugo (Cystopus)
White rust or white blister
diseases
Kingdom- Mycota (Fungi)
Division- Eumycota
Sub-Division- Mastigomycotina
Class- Oomycetes
Order- Albuginales
Family- Albuginaceae
Genus- Albugo sp.
3. • This organism causes white rust or white blister diseases in above-
ground plant tissues.
• While these organisms affect many types of plants, the destructive
aspect of infection is limited to a few agricultural crops, including :
beets, cabbages, cauliflower, mustards, radish, spinach, sweet
potatoes, turnips etc.
• Symptoms of white rust caused by Albugo typically include yellow
lesions on the upper leaf surface and white pustules on the underside
of the leaf.
• The pathogen is spread by wind, water, and insects.
• Management includes use of resistant cultivars,
proper irrigation practices, crop rotation, sanitation, and chemical
control. White rust is an important economic disease, causing severe
crop losses if not controlled.
5. • White rust pathogens create chlorotic (yellowed)
lesions and sometimes galls on the upper leaf
surface and there are corresponding white blister-
like dispersal pustules of sporangia on the underside
of the leaf.
• Species of the Albuginaceae deform the branches
and flower parts of many host species. Increase in
the size of the cells (hypertrophy) and organs takes
place.
• Proliferation of the lateral buds, discoloration of
flowers, malformation of floral parts and sterile
gynoecium.
6. 1.White rust is an obligate parasite.(needs a living host
to grow and reproduce).
2.The Albuginaceae reproduce by producing both sexual
spores (called oospores) and asexual spores
(called sporangia) in a many-stage (polycyclic) disease
cycle.
3.White rust is an economically important foliar disease,
causing substantial yield losses and eventual death of
various crops.
7. Structure-
Thallus is eucarpic and mycelial. Hyphae are intercellular, coenocytic,
aseptate and profusely branched(Fig. 2 B). Cell wall is composed of
fungal cellulose. The protoplasm contains a large number of nuclei
distributed in the cytoplasm.
• Reserve food material is in the form of oil drops and glycogen bodies.
Some mycelium is intracellular in the form of knob-like haustoria for
the absorption of food material from the host cells.
Haustoria
8. Reproduction-
1)Asexual Reproduction:
• The asexual reproduction takes place by conidia, condiosporangia or
zoosporangia.
• They are produced on the sporangiophores. Under suitable conditions
the mycelium grows and branches rapidly. After attaining a certain age
of maturity, it produces a dense mat like growth just beneath the
epidermis of the host. (Fig. 2 D). These hyphae produce, at right
angles to the epidermis are short, thick walled, un-branched and club
shaped. These are the sporangiophores or conidiophores.
• They form a solid, palisade like layer beneath the epidermis (Fig. 2 A-
D).They are thick walled on lateral sides and thin walled at tip. The
sporangiophores contain dense cytoplasm and about a dozen nuclei.
After reaching a certain stage of maturity, the apical portion of
sporangiophore gets swollen and is ready to cut off a sporangium or
conidium (Fig. 2E).
9. The sporangia are produced at the tip by abstraction method. A
Deeping constriction appears below the swollen end (fig 2. F)
and results in the formation of first sporangium. A second
sporangium is similarly formed from the tip just beneath the
previous one (Fig. 2 G).
This process is repeated several times. The new nuclei
migrates from mycelium to cytoplasm and are used in the
formation of another sporangium or conidium. Thus along
chain of sporangia or conidia is formed above each
sporangiophore in basipetal succession. (youngest at the base
and oldest at the tip) (Fig. 2 H). The sporangia or conidia are
spherical, smooth, hyaline and multinucleate structures. The
walls between them fuse to form a gelatinous disc-like
structure called disjunctor or separation disc or intercalary
disc. (Fig. 2 G).
10. It tends to hold the sporangia together. The continued growth
and production of sporangia exerts a pressure upon the
enveloping epidermis. Which is firstly raised up but finally
ruptured exposing the underlying sours containing white
powdery dust of multinucleate sporangia or conidia (Fig. 2 A,
3).
The separation discs are dissolved by water, and the sporangia
are set free. They are blown away in the air by wind or washed
away by rain water under suitable environmental conditions
and falling on a suitable host, sporangia germinates with in 2
or 3 hours. The sporangia germinate directly or indirectly
depending on temperature conditions.
1. Direct Germination:At high temperature and comparative
dry conditions the sporangium germinates directly. It gives
rise to a germ tube which in-fact the host tissue through stoma
or through an injury in the epidermis (Fig. 2 I, P).
11. 2. Indirect Germination:
In the presence of moisture and low temperature (10°C) the sporangium
germinates indirectly i.e., it behaves like zoosporangium and produces
zoospores. It absorbs water, swells up, and its contents divide by
cleaving into 5-8 polyhedral parts (Fig. 2 J) depending upon the nuclei
present in it. Each part later on rounds up and metamorphoses into
zoospore (Fig. 2 K, L). A papilla is developed on one side which later
burst and liberates the zoospores.
Zoospore: The zoospores are uninucleate, slightly concavo-convex and
biflagellate. The flagella are attached laterally near the vacuole. Of the
two flagella one is of whiplash type and the other tinsel type (Fig. 2M).
After swimming for some time in water, they settle down on the host.
They retract their flagella, secrete a wall and undergo a period of
encystment (Fig. 2 N). On germination, they put out a short germ tube
which enters the host through stomata (Fig. 2 O, P) or again infects the
healthy plants.
12.
13. Sexual Reproduction:
It takes place when the growing season comes to an end. The
mycelium penetrates into the deeper tissues of the host. The
sexual reproduction is highly oogamous type. The antheridium
and oogonium develops deeper in the host tissue in close
association within the intercellular spaces.
Its formation is externally indicated by hypertrophy. The
antheridium and oogonium are formed near each other on
hyphal branches. They are terminal in position, however,
intercalary oogonia also occur, though rarely.
Antheridium:
It is elongated and club shaped structure. It is multinucleate
(6-12 nuclei) but only one nucleus remains functional at the
time of fertilization in C. candidus. It is paragynous i.e.,
laterally attached to the oogonium (Fig. 4 A-C). It is separated
by a cross wall from the rest of the male hyphae.
14. Oogonium:
It is spherical and multinucleate containing as many as 65 to 115 nuclei.
All nuclei are evenly distributed throughout the cytoplasm (Fig. 4 A-C).
As the oogonium reaches towards the maturity the contents of the
oogonium get organised into an outer peripheral region of periplasm and
the inner dense central region of ooplasm or oosphere or the egg (Fig. 4
D-G). The ooplasm and periplasm are separated by a plasma membrane.
The nuclei in the oogonium divides mitotically. The first mitotic division
takes place before the organization of the periplasm and oosplasm (Fig. 4
E). After the organization, all the nuclei of the ooplasm, except one,
migrate to the periplasm forming a ring and undergo second mitotic
division. They divide in such a manner that one pole of each spindle is in
ooplasm and the other in the periplasm (Fig. 4 E).
At the end of the division one daughter nucleus of each spindle goes to
the oosplasm and other in periplasm (Fig. 4 F). However, at the time of
maturity, all nuclei disintegrate, except single functional nucleus (Fig. 4
G). On the basis of functional nuclei in ooplasm,” Albugo is divided into
following groups:
15. Group I:
The number of functional egg nucleus in ooplasm is
one. It is represented by C. Candidus.
Group II:
The number of functional eggs in ooplasm is many. It
is in many species.
It carries a single male nucleus. Its tip ruptures to
discharge the male nucleus near the female nucleus.
Ultimately the male nucleus fuses with the female
nucleus (karyogamy).
16. Fertilization:
Before fertilization a deeply staining mass of
cytoplasm, (Fig. 4 H) appears almost in the centre of
the ooplasm. This is called coenocentrum. It persists
only up to the time of fertilization. The functional
female nucleus attracted towards it and becomes
attached to a point near it.
The oogonium develops a papilla like out growth at
the point of contact with the antheridium. This is
called as receptive papilla (Fig. 4 G). Soon it
disappears, and the antheridium develops a
fertilization tube.It penetrates through receptive
papilla, oogonial wall and periplasm and finally
reaches upto the ooplasm (Fig. 4 H, L).
17. Oospore:
The oospore alongwith the fusion nucleus is called
oospore (Fig. 4 J). In C. candidus, one male functional
nucleus fuses with one female functional nucleus. So,
the oospore is uninucleate..
The same number of functional male nuclei are
discharged by the fertilization tube. Both male and
female nuclei fuse, and the oospore produced in these
species in multinucleate. Such oospore is called a
compound oospore.
The oospore on maturity secretes a two to three
layered wall (Fig. 4 J-L). The outer layer is thick,
warty or tuberculated and represents the exospore. The
inner layer is thin and culled the endospore.
18. Germination of oospore:
With the secretion of the wall, the zygotic nucleus divides
repeatedly to form about 32 nuclei. The first division is
meiotic. At this stage the oospore undergoes a long period of
rest until unfavorable conditions are over. Meanwhile its host
tissues disintegrate leaving the oospore free. After a long
period of rest the oospore germinates. Its nuclei divide
mitotically and large number of nuclei are produced.
A small amount of cytoplasm gathers around each nucleus.
Protoplasm undergoes segmentation and each segment later on
rounds up and metamorphoses into a zoomeiospre or zoospore
(Fig. 4 O). The exospore is ruptured and the endospore comes
out as a thin vesicle (Fig. 4 M). The zoospores move out into
the thin vesicle which soon perishes to liberate the zoospores.
19. However, Vanterpool (1959) reported that oospore forms a
short exit or germ tube which ends in a thin vesicle. According
to Stevens (1899), Sansome and Sansome (1974), the thallus
of Albugo is diploid and the meiosis occurs in gametangla i.e.,
Antheridia and oogomia. Zygotic nucleus divides only
mitotically and not meiotically.
Germination of Zoospore:
The zoospores are reniform (kidney shaped) and biflagellate.
Of the two Safe flagella, long one is of whiplash type and
short one is of tinsel type (Fig. 4 O). The zoospores after
swimming for sometime encyst and germinate by a germ tube
which reinfects the host plant (Fig. 4 O, P, Q).
