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

THE DISEASE—STEM ROT
Stem rot was first reported in 1879.
Since then, it has been reported in
rice-growing countries in Europe,
Africa, South America, and Asia.
Lesions develop on stems and
cause decay of the leaf sheath and
culm. The weakened tillers lodge,
thus contributing to grain yield loss
and to poor grain milling quality.
Estimated losses due to stem rot
range from 18 to 80% of yield.
Symptoms
The disease usually develops on
older rice crops where sclerotia initiate
a small, blackish irregular lesion
on the leaf sheath near the waterline
(Fig. 14.8c). The lesion advances and
penetrates the inner leaf sheath.
Here it causes the leaf sheath to partially
or entirely rot, and the infection
penetrates the culm. Brownishblack
lesions may develop in one or
two internodes causing the stem to
collapse and lodge. Sclerotia are usually
formed inside thc affected leaf
sheath (Fig. 14.8d) and culm but
some may be found outside the leaf
sheath. The disease continues to develop
as the crop matures. At maturity,
examination of infected tillers
and panicles reveals sclerotia in the
culm; conidiophores and conidia on
the leaf sheath; and sclerotia,
conidia, and conidiophores on the
panicle and spikelet.
Disease development
Sclerotia of M. salvinii, which serve
as the primary source of inoculum,
survive in the stubble and soil surface
for 190 d or for 133 d buried in
the soil (Park and Bertus 1932).
Sclerotia stay in the upper 2-3 inches
of soil and float to the surface of the
water during land preparation.
These sclerotia later come in contact
with the rice leaf sheath and germinate
to form appressoria or infection
cushions and initiate lesions on leaf
sheaths. Stem rot progresses to infect
the inner leaf sheaths and culm.
Wounds from lodging or insects
directly increase the disease incidence.
Artificial lodging has caused
the disease to spread. Kobari (1961)
reported 2-3 times more stem rot on
rice plants with stem borers than
those free from stem borers.
Control
Burning rice stubble in the field
minimizes inoculum levels in the
field. Deep plowing using a
moldboard reduces inoculum potential
by burying a large percentage of
sclerotia. Proper use of fertilizers,
avoiding excess nitrogen availability,
and increasing potassium tend to
reduce the damage.
Although many fungicides are
effective, chemical control has not
been used against stem rot. Resistant
and nonlodging varieties are the preferred
disease controls.
Pyricularia oryzae
Pathogen: Pyricularia oryzae Cav.
(Etymology: from pirum, pear shape,
describing the spores)
Disease: blast
Detection level: infrequently detected
(1.4% of seeds observed), with high
epidemic potential
Where detected: infected seeds, panicles,
nodes, leaves
How detected: blotter or agar plate
methods, washing test
Appearance: see Figure 14.9.
Under a stereobinocular microscope,
infected seeds on a blotter exhibit a
fine, grayish growth of erect
conidiophores bearing conidia
mostly on their sterile glumes
(Fig. 14.9a). Cladosporium appears
similar, but it has bigger dark brown
to almost black conidiophores and
smaller conidia.
14.9a. Habit character of
P. oryzae on the embryonal
end of seed showing grayish
colony growth on sterile
glumes. b. P. oryzae colony
on potato dextrose agar.
c. P. oryzae conidia stained
with lactophenol blue.
d. Blast symptoms—spindleshaped
spots with brown or
reddish brown margins, ashy
centers, and pointed ends.
e. Node blast. Leaf sheaths
have been removed to show
infected node. f. Blast
symptoms—rotten neck
infection at panicle base.
Fungal pathogens 83