STEM ROT OF RICE ( Sclerotium oryzae )
STEM ROT OF RICE ( Sclerotium oryzae )
STEM ROT OF
RICE - Sclerotium Oryzae Catte
Magnaporthe
salvinii (Teleomorph)
INTRODUCTION
Cattaneo
from Italy published the first description of stem rot or
sclerotial disease in 1876. Since then, reports
of it have come from a number of rice-growing nations, including Japan, the
United States, Ceylon, Burma, Vietnam, the Philippines, and China. In India it was first observed in Bengal in 1911. Butler later
documented the condition in full a year later, in 1918.
Since then, the majority of states that farm rice have reported it. Damage
varies depending on the tract and possibly depending on the season.
Magnaporthe salvinii is an ascomycete fungus that infects a plant in a polycyclic
manner. Sclerotium oryzae is a fungal pathogen that causes stem rot
disease in rice plants. This disease is a significant concern in rice
cultivation, leading to about 35% crop loss. The pathogen has different
morphological variations in its life cycle, including sclerotium, perithecium,
and conidium. Sclerotia are macroscopic balls of fungal tissue that persist for
a long time in unfavorable conditions. They are the chief infectious agents of
S. oryzae and are found scattered in old rice straw and on the soil after
harvest.
CAUSAL
ORGANISM : Sclerotium oryzae
Catt.,
(1879)
Kingdom: Fungi
Phylum:
Ascomycota
Class: Sordariomycetes
Subclass: Sordariomycetidae
Order: Magnaporthales
Family: Magnaporthaceae
Genus: Magnaporthe
Species: M. salvinii
TELEOMORPH: Magnaporthe
salvinii (Catt.) R.A. Krause & R.K. Webster, (1972)
PERFECT
STAGE: Leptosphaeria salvinii Catt.,
(1879)
CONIDIAL
STAGE: Helminthosporium sigmoideum var. irregulare Cralley
& Tullis, (1935)
The genetic
connection between S. oryzae and L. salvinii, an ascomycetous
fungus whose conidial stage is H. sigmoideum, was shown by Tullis in the
United States in 1953.
SYMPTOMS
a)
The disease appears in the late stages of plant development. The principal
indication of the disease is an excess of late tillers at the stem's base,
which turns discoloured and rotting in severe cases.
b)
The emergence of tiny, black, irregular lesions on the outer leaf
sheath at the water line is the first symptom of
the disease.
c)
The discoloration extends up and down
from the water line as the sickness progresses.
d)
The interior of the culm may be
filled with grey fungal mycelium when the infected plants are cut apart and
examined.
e)
Numerous sclerotia might be
discovered inside the culm or implanted in the host tissue.
f)
The plants that are harmed produce
ears with lighter grains. Seedlings are also impacted, resulting in leaf
dryness. On the withering older basal leaves,
masses of sclerotial bodies can be detected.
g)
In the main field, infected plants
collapse owing to stem base weakness, and as the illness progresses, the basal
stem up to around 2-3 inches becomes dark green,
and the tissues within may rot.
PATHOGEN
a)
The mycelium in the sclerotial stage
is composed of extensively branching white to greyish hyphae that are 4-8 µ thick and produce sclerotia
that are nearly spherical, smooth, black, and shiny, measuring around 200-350µ in
diameter and visible to the naked eye as black spots.
b)
Sclerotium oryzae is phobic, buoyant, and uniform in size
c)
Perithecia at the ideal stage are
globose, each capped with a short cylindrical beak, 350-400
µ, in diameter, with clavate asci
bearing 8 ascospores, the spores measuring around 60 x
9 µ, are three septate, and pale yellow
in colour.
d)
Conidiophores are erect and nodular
in the conidial stage, with 8-10 septa bearing falcate to sigmoid, three
septate, pale olivaceous conidia measuring 55-65 x
11-14 µ.
e)
The fungus's imperfect stage can be
found both inter- and intracellularly in the host tissues.
f)
Morphology of the sclerotia can be
used to distinguish between S. oryzae and S. oryzae var.
irregulare.
DISEASES
CYCLE
a)
During the off-season, the fungus's
sclerotia thrive in soil and stubble and serve
as the principal source of inoculum.
b)
Sclerotia are present in the upper
5-8 cm of soil in the field and float on the surface of water when the fields
are flooded.
c)
Sclerotium oryzae
is a necrotrophic pathogen, which means it
applies brute force during invasion by secreting potent tissue-degrading
enzymes or toxins.
d)
They are carried from field to field
by irrigation or flood water, accounting for secondary disease dissemination.
e)
They sprout quickly in the presence
of enough soil moisture. Sclerotia survive in dry soil for several months
before becoming active in the following cropping season.
f)
When conidia are generated, they
contribute to the spread of the disease.
FACTORS
INFLUENCING DISEASE
a.
This is a weak
pathogen that only becomes active under favourable conditions.
b.
Severe outbreaks of the illness have
been linked to plant weakness caused by physiological imbalance, unusual
dryness, or insect attack, specifically stem borer incidence. Plant lodging
hastens disease transmission.
c.
While several writers have reported
success in infecting rice plants with the fungus through artificial
inoculation, the mere presence of sclerotial bodies in soil does not appear to
induce any meaningful incidence of the disease in the field.
d.
Application of nitrogen and phosphorus
fertilizers to soil, either individually or in combination, increases
the disease intensity. Potash effectively counters the harmful effects of
nitrogen and phosphorus.
INTEGRATED DISEASE MANAGEMENT SPECIFIC TO
BIOCONTROL
a)
Controlling the disease is
challenging since sclerotia can live in the soil
for lengthy periods of time.
b)
Burning the stubble left in the
contaminated field is beneficial but difficult to implement, especially when
rice is grown in rainy conditions.
c)
Draining the irrigation water and
leaving the soil to dry appears to be the greatest approach for controlling the
disease when done carefully. Furthermore, soaking the soil with 0.1 per
cent Ceresan after draining the water three times during the growing
season in the main field has been shown to effectively control the disease.
d)
Balanced manuring aids in the control
of the disease. Irrigation water should never be directed from an affected
field to an uninfested field.
e)
Stem rot in rice can be controlled
through the use of biological control agents. Some important biocontrol agents
for stem rot include Pseudomonas fluorescens, Pseudomonas aeruginosa, Bacillus subtilis, and
Bacillus pumilus.
f)
These agents can be applied through
direct inoculation, such as dipping seeds in culture or aerial spraying, or by
using solid-phase inoculants.
g)
Additionally, formulations of these
biocontrol agents, such as those containing methyl
cellulose and talc, have shown satisfactory results in suppressing stem
rot.
h)
However, it is important to note that
commercially available biocontrol agents for rice disease control are currently
limited.
i)
One such method is the use of
microbe-based biofertilizers, which can control the pathogenic activity of S.
oryzae by exploiting, competing, or exhibiting antibiosis against the fungus
j)
The combined use of butachlor mix in soil and propiconazole
spray at 400 ppm and 100 ppm,
respectively, after pathogen inoculation followed by introduction of T. viride and P. fluorescens,
just as disease symptoms appeared, was found to be highly effective in
controlling disease spread and even beneficial in promoting rice plant growth.
k)
Biocontrol using Trichoderma
is mentioned as an alternative and cost-effective method for controlling stem
rot. This approach can help reduce the reliance on fungicides, which can have
negative environmental and health effects
l)
The document mentions the use of various fungicides
for stem rot management, such as hexaconazole,
propiconazole, butachlor, and carbendazim. These fungicides can be
applied at different stages of the disease to reduce its incidence and
severity.
m)
Inorganic fungicides like potassium fertilizers, benzoic acid, and naphthalene
acetic acid have been found to induce resistance against stem rot in
rice plants. However, the negative impacts of these chemical substances on the
environment and other organisms are also acknowledged.
n)
Cultural practices like proper management of rice residues,
crop rotation, and polyethylene mulching of soil have been suggested to reduce
the viability and growth of Sclerotium oryzae, the causal agent of stem
rot
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