2. CARICA PAPAYA:
The papaya is a large tree with a single stem growing from 5
to 10m tall.
The leaves are large,
50–70 cm in diameter.
Papaya is the fruit of the Carica
papaya plant.
Papayas can be used as a food,
a cooking aid and in traditional
medicine.
The stem and bark may be
used in rope production.
3. BACTERIAL DISEASE OF PAPAYA:
BACTERIAL WILT:
CAUSATIVE AGENT:
Ralstonia solanacearum
Ralstonia solanacearum is an aerobic non-sporing,
Gram-negative plant pathogenic bacterium.
R. solanacearum is soil-borne and motile with a polar
flagellar tuft.
It colonises the xylem, causing bacterial wilt in a very
wide range of host plants.
6. SYMPTOMS
Wilting begins with lower leaves and petioles and
works its way up the plant.
Wilted leaves with chlorotic, wedge-shaped areas or
chlorotic and/or necrotic leaf margins.
No leaf spots are evident.
Eventually entire plant collapses on the medium
White runny ooze from cut stems.
8. DISEASE CYCLE:
High temperatures and high soil moisture generally
favors Ralstonia solanacearum.
The bacteria can survive for a long time in water (up to 40 years
at 20–25 °C in pure water) and the bacterial population is reduce
d in extreme conditions (temperature, pH, salts, e.g.).
DISPERSAL
R. solanacearum causes wilting at high population and disperses
in several routes. The large number of R. solanacearum can shed
from roots of symptomatic and non-symptomatic plants. Beside
that, bacterial ooze (which is usually used as a sign for detection)
on plant surfaces, can enter the surrounding soil or water.
9. INFECTION:
R. solanacearum usually enter the plant via a wound.
Natural wounds as well as unnatural ones, would become
entry sites for Ralstonia solanacearum. The bacteria get
access to the wounds partially by flagellar-mediated
swimming motility.
After invading a susceptible host, R. solanacearum
multiplies and moves systematically within the plant before
bacterial wilt symptoms occur. When the pathogen gets
into the xylems through natural openings or wounds,
tyloses may form to block the axial migration of bacteria
within the plant.
The primary factor contributing to wilting is probably
blocking of pit membranes in the petioles and leaves.
11. GENERAL MANAGEMENT:
Commercial chemicals have generally proven to be
ineffective in controlling the pathogen and are not
recommended as a means of control.
Using pathogen-free planting materials is a necessity.
Planting resistant cultivars minimizes the ill effects of
the pathogen.
12. VIRAL DISEASE OF
PAPAYA:
PAPAYA LETHAL YELLOWING:
CAUSATIVE AGENT:
Papaya lethal yellowing Virus
Papaya Lethal Yellowing Virus (PLYV) is an isometric viral
plant pathogen of the Tombusviridae family that causes
lethal yellowing disease.
The virus infects only C. papaya, and some other members
from the family Caricaceae.
PLYV consists of a capsid protein, a single strand of RNA,
and causes progressive leaf yellowing and greenish circular
spots on the fruits.
13. PAPAYA YELLOW LETHAL VIRUS:
Large numbers of isometric virus
particles can be observed by electron
microscopy in cytoplasm and
vacuoles of cells from leaves and
fruits of infected plants.
PLYV particles are isometric
and 30nm in diameter.
14. HOST:
Like the name implies, the host of PLYV is very
specific.
Inoculation studies indicate that the Caricaceae was
the only family of plants affected by the virus.
15. SYMPTOMS:
Symptoms of PLYV are very
straightforward.
The signs of the virus begin to
become visible when the leaves
at the top of the plant begin
turning yellow and eventually
falling off.
This usually occurs in the upper
third of the canopy and death of the
plant is usually soon after the leaves
wilt and die.
17. DISEASE CYCLE:
PLYV was first recognized in the 1980's, and has always
been restricted to a relatively small geographical area.
It is more likely that the virus was present in wild hosts
and jumped to papaya plants once this crop became widely
cultivated in that area.
While no known biological vector is know for PLYV is
known, it is known to be spread by human actions,
including contaminated hands, agricultural tools, soil, and
contaminated water.
The virus can be detected on the surface of seeds of
infected fruits, but it is not detected in the embryo or in
seeds harvested from infected roots.
18. MANAGEMENT:
There are various ways that PLYV can be controlled.
1. Virus-free plantlets:
A large reason for the spread of
PLYV is the use of infected
Papaya plantlets for Papaya growth.
In order to prevent the further
spread of PLYV, it is helpful to
start by growing new Papaya in
orchards far away from PLYV
infected orchard.
19. MANAGEMENT:
2. Eradication of virus-infected plants:
Any plants that show symptoms of PLYV or are suspected of having the
virus should be removed from growing orchards to prevent viral
transmission.
3. Disinfecting agricultural tools:
Just as infected plants can spread
the virus, infected agricultural tools
can too. The recommended method
for disinfecting trimming or harvesting
tools is to immerse them in 10%
commercial household bleach solution
for five minutes.
Also clean properly the shoes and
tyres of the tractors.
20. FUNGAL DISEASE OF
PAPAYA:
DAMPING OFF:
CAUSATIVE AGENT:
Pythium aphanidermatum
''Pythium aphanidermatum'' is a soil borne plant pathogen.
Pythium is a genus in the class Oomycetes, which are also
known as water molds. Also, they reproduce asexually with
motile biflagellate zoospores that require water to move
towards and infect a host. Sexually, they reproduce with
structures called antheridium, oogonium, and oospores.
22. HOSTS:
Pyhtium aphanidermatum has a wide host range, and
is of economic concern to beets, peppers,
chrysanthemum, cucurbits, cotton and turf grasses,
however, because P. aphanidermatum requires warmer
temperatures. It is a major cause of root rot in papaya
production in subtropical areas.
23. SYMPTOMS:
Pythium aphanidermatum is responsible for pre and
post emergence damping off.
Pre-emergence damping off is when the seed is
infected prior to germination. This can result in poor
or no germination, and is observable as a browning or
rotting of the seed.
Post emergence damping off takes place after
germination and results in a thinning, water-soaked
stem near the plant collar, which eventaully causes the
collapse of the plant.
24. SYMPTOMS:
P. aphanidermatum can also cause root rot.
Symptoms of root rot include stunted growth,
chlorotic leaves, leaf drop, and wilting.
25. DISEASE CYCLE
Pythium aphanidermatum survives in the soil as oospores, hyphae
and/or sporangia.
Oospores can produce a germ tube and infect the plant directly,
or, if the environment is favorable the oospore may produce
sporangia, which in turn produce motile, biflagellate zoospores
that swim to the host plant, encyst, and germinate. This infection
can occur on seeds, which can rot, or produce a weak seedling. If it
infects the roots of a seedling, the mycelium will grow throughout
the plant tissue, releasing digestive enzymes which break down
plant cell walls allowing the pathogen to absorb the nutrients,
effectively killing the plant overtime.
P. aphanidermatum is a polycyclic disease. A polycyclic plant
pathogen has several life cycles during a season.
26. MANAGEMENT:
Several cultural management methods can be effective in
avoiding disease caused by Phythium aphanidermatum.
The pathogen thrives in a moist environment, so it is
important to prevent an excessive amount of moisture from
building up in the plant media.
Irrigation that is too frequent and usage of soil that has
poor drainage increase its growth.
In addition, poor ventilation and insufficient exposure to
sunlight can cause the plants themselves to accumulate
moisture, potentially spreading disease.
Several chemical types can be used to manage the
pathogen including acylalanines, thiadiazoles, carbamates,
cinnamic acid derivatives.
