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

× 90-120 nm, with a distinctly protruding
apical papilla. The ostiole is
lined on the inside by hyaline
periphyses. The thin smooth wall of
9-18 nm thick is composed of two
layers. The outer wall of 3-5 layers is
made of light brown colored, angular,
somewhat elongated
pseudoparenchymatic cells up to
6 nm long and 2-3 nm wide. The inner
rather thin wall is almost hyaline
with thin-walled compressed cells
which usually disappear as the asci
mature. Asci cylindrical to cylindricclavate,
thin-walled, 8-spored,
unitunicate, 40-85 × 8-12 nm with a
distinct amyloid apical structure.
Ascospores obliquely distichous to
tristichous, fusoid, straight to
slightly curved, hyaline, 3-5 mostly
3 septate, not or slightly constricted
at the septum, 14-23 (30) × 3.5-4.5
(7.5) nm. Paraphyses filiform,
hyaline."
THE DISEASE—LEAF SCALD
Leaf scald is common in rice-growing
countries. It has been reported to
cause considerable damage in Latin
America and West Africa.
Symptoms
Symptoms appear on mature leaves
as zonate lesions starting on leaf tips
or edges (Fig. 14.7d). Lesions are
parallel to oblong, with light brown
halos. On mature leaves, lesions vary
from 1 to 5 cm in length and from 0.5
to 1 cm in breadth. Coalescing lesions
may blight the greater part of
the leaf blade and extend to 25 cm
long. Zonations become indistinct
with age.
Kwon et al (1973) in Korea observed
typical leaf symptoms, and
reddish-brown, small spots on the
leaves and long elliptical or rectangular,
purplish-black necrotic spots
on the leaf sheaths and panicle
necks. Spots enlarged and became
bright purplish-brown or light gray.
Microdochium oryzae can also
cause coleoptile decay and root rot
(De Gutierrez 1960).
IRRI has found salmon-red islands
of M. oryzae conidia during
rice seed health testing.
Disease development
Leaf scald develops in all rice ecosystems.
Infection initiates through the stomata
(Naito et al 1975). M. oryzae has
been isolated from dry plant litter,
leaf tissue, and seeds (Boratynski
1979), which may be the source of
primary inoculum. The weed
Echinochloa crus-galli has been found
infected by M. oryzae (Singh and
Gupta 1980), and thus may be a potent
source of primary and secondary
inoculum.
Control
Seed treatment with carbendazim
and thiram 12 h before sowing is effective.
Spray treatment with
thiophenyl-methyl significantly arrests
disease incidence (Swain et al
1990). Benomyl and mancozeb slurry
treatment, both at 0.3% by seed
weight, effectively eradicates seed
infection.
Nakataea sigmoidea
Pathogen: Nakataea sigmoidea (Cav.)
Hara (Hara 1918)
(Etymology: after Nakata, a scientist;
from Latin sigma, refers to the shape
of the conidium)
Disease: stem rot
Detection level: infrequently detected
(0-1%), with low epidemic potential
Where detected: blotter test; during
dry seed inspection, sclerotia may be
found with the seeds
How detected: infected seeds, panicle
branches, sclerotia found in infected
stems
Appearance: see Figure 14.8.
Under a stereobinocular microscope,
infected seeds on a blotter show
blackish, erect conidiophores with
three-septate, slightly curved,
fusiform conidia, borne singly on
pointed sterigmata (Fig. 14.8a).
Conidia measure 9.9-14.2 × 29-49 nm
(Fig. l4.8b). N. sigmoidea is the
conidial state of Magnaporthe salvinii.
14.8a. Habit
character of
Nakataea
sigmoidea.
b. Conidia of
N. sigmoidea.
c. Stem rot lesions
on rice tillers
(courtesy of S.
Merca).
d. Sclerotia formed
inside an infected
leaf sheath.
82 Rice seed health testing manual