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