A manual of rice seed health testing - IRRI books - International Rice ...

THE DISEASE—SHEATH BLIGHT
Sheath blight, a widespread disease,
occurs in almost all rice-growing regions.
Higher tillerage of the modern
varieties increases humidity around
the plants, predisposing them to
sheath blight infection.
Loss records vary depending on
cultivars, plant growth stage, and
environment. Yield losses range
from 22 to 40% (Gangopadhyay and
Chakrabarti 1982). In the tropics, total
crop failure has been reported
due to death of all the leaves in susceptible
cultivars (Ou 1985).
Symptoms
Symptoms become apparent at
tillering or flowering stage. Spots or
lesions first develop near the water
(in lowland fields) or soil (in upland
fields). Spots initially appear on the
leaf sheath. Spots may be oval or ellipsoidal,
and measure 1-3 cm long.
Lesions on the leaf blade are usually
irregular and banded with green,
brown, and orange coloration. Lesions
are grayish white in the center
with brown margins. At advanced
stages, when the flag leaf is infected
(Fig. 14.10e), panicle exsertion is affected.
Some cultivars raised under
conditions favoring disease development
may later develop spots extending
up to the leaf blades. Leaves
with spots eventually die.
Sclerotia form on the spots.
Sclerotia are usually large, 4-5 mm in
diameter, white when young, turn
brown or purplish brown at maturity,
and fall off easily. Sclerotia can
be seen between the leaf sheath and
the culm.
14.10a. Rhizoctonia
solani showing characteristic
branching
pattern on agar stained
with lactophenol blue.
b. Young R. solani
colonies on agar.
C. R. solani colonies turn
brown with age.
d. Sclerotia of
R. solani germinating on
agar medium.
e. Sheath blight lesions
affecting flag leaves and
panicles (courtesy of
S. Merca).
Disease development
Both upland and lowland ecosystems
support the disease. Incidence
is higher in drained ricefields than in
upland fields.
Infection normally occurs through
sclerotia which survive in the soil for
a long time, depending on the temperature
and moisture levels. In the
tropics, infected straw, stubble,
weeds, and sclerotia cause primary
infection. Sclerotia cannot withstand
dry soil for more than 21 mo. Surviving
sclerotia come to the water surface
during agronomic operations.
They infect healthy plants upon contact.
The number of sclerotia in the
soil determines the intensity of primary
infection. Mycelia enter host
tissue through stomata or directly
penetrate the cuticle. Mycelia proliferate
throughout the host. Mycelia of
younger lesions actively cause secondary
infections.
High humidity and temperature
(up to 40 °C) aid sheath blight spread
and development.
Younger plants contract sheath
blight at tillering and flowering
stages. Upward development takes
place after the heading stage under
favorable conditions.
Close planting helps spread
sheath blight by increasing humidity
and temperature of the plants'
microenvironment.
High doses of nitrogenous fertilizer
cause susceptibility. Potassium
induces some resistance.
Control
The disease can be controlled
through
cultural practices such as green
manuring with Seshania aculeata, soil
solarization, and deep plowing to
bury infested plant residues into the
soil (Dath 1979, Grinstein et al 1979,
Papavizas and Lewis 1979);
treating seed with thiram 75% ai
and quintozene 23.2% at 1 g/kg of
seed (Marcos 1975);
applying organo-arsenic compounds,
such as methylarsene sulphide
and methylarsine bisdimethyl
dithiocarbamate, effective at about
50 ppm with only two applications:
Fungal pathogens 85