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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THE DISEASE—BROWN SPOT<br />
Sir John Woodhead reported in 1945<br />
that brown spot was the principal<br />
cause <strong>of</strong> the 1942 Bengal famine<br />
(cited in Ou 1985). Vidhyasekaran<br />
and Ramadoss (1973) reported that<br />
severe infections can reduce yield by<br />
20-40%. Abnormal soil conditions<br />
increase damage.<br />
Symptoms<br />
Conspicuous brown spots appear on<br />
the leaves. Spots measure 2-1 ×<br />
0.5 cm, are oval and evenly distributed.<br />
They are brown with gray or<br />
whitish centers on maturity<br />
(Fig. 14.2e). In severe infections,<br />
spots fuse and leaves wither. Spots<br />
also develop on glumes. When conditions<br />
favor fungal development, a<br />
velvety growth can be seen over the<br />
<strong>seed</strong>s (Fig. 14.2a), and the fungus<br />
may enter the glumes and leave<br />
blackish spots on the endosperm.<br />
Brown spot symptoms may appear<br />
on the leaf coleoptile<br />
(Fig. 14.2e), leaf sheaths, and panicle<br />
branches. Blackish lesions may be<br />
seen on young roots.<br />
Disease development<br />
Both lowland and upland ecosystems<br />
support brown spot development.<br />
Brown spot is <strong>seed</strong>borne. Seedling<br />
infection (<strong>seed</strong>ling blight) arises<br />
from infected <strong>seed</strong>s. Secondary infection,<br />
which appears at the<br />
posttillering stage, occurs through<br />
windborne spores (conidia).<br />
Stubble <strong>of</strong> the previous crop and<br />
collateral hosts, such as Leersia<br />
hexandra, Echinochlaa colona,<br />
Pennisetum typhoides, and Setoria<br />
italica may be sources <strong>of</strong> secondary<br />
inoculum.<br />
Conidia germinate at 25-30 °C and<br />
are infectious at 90-100%) humidity.<br />
Control<br />
Seed treatment methods effectively<br />
control primary infection <strong>of</strong> <strong>seed</strong>lings.<br />
Before sowing, treat <strong>seed</strong>s with<br />
hot water (53-54°C) for 10-12 min.<br />
This controls primary infection at the<br />
<strong>seed</strong>ling stage. Presoaking the <strong>seed</strong><br />
in cold water for 8 h increases<br />
effectivity <strong>of</strong> the treatment.<br />
Grise<strong>of</strong>ulvin, Nystatin,<br />
Aure<strong>of</strong>ungin, and similar antibiotics<br />
have been found effective in India in<br />
preventing primary <strong>seed</strong>ling infection.<br />
Secondary airborne infections<br />
may be controlled by spraying<br />
Hinosan and Dithane M-45.<br />
Proper agronomic practices such<br />
as crop rotation, field sanitation, balanced<br />
application <strong>of</strong> fertilizers,<br />
proper water management, and soil<br />
amendments can help control brown<br />
spot.<br />
Cercospora janseana<br />
Pathogen: Cercospora janseana (Racib.)<br />
O. Const.<br />
Teleomorph: Sphaerulina olyzina Hara<br />
(Etymology: from cercos, worm; spora,<br />
spore)<br />
Disease: narrow brown leaf spot<br />
Detection level: infrequently<br />
detected (1.15% <strong>of</strong> <strong>seed</strong>s tested),<br />
with low epidemic potential<br />
Where detected: infected <strong>seed</strong>s and leaf<br />
blades<br />
How detected: blotter or agar test methods;<br />
washing test<br />
Appearance: see Figure 14.3,<br />
Under a stereobinocular microscope,<br />
dark color or almost black, erect,<br />
solitary or grouped conidiophores<br />
bearing long, hyaline conidia can be<br />
seen (Fig. 143).<br />
After 5 d incubation at 25 °C, a<br />
colony on potato dextrose agar attains<br />
2.9 cm diam (Fig. 14.3b).<br />
Growth is compact, restricted, cream<br />
with hyaline margin, and reverse<br />
black. Hyphae are branched and<br />
septate. Conidiophores occur singly<br />
or in groups, arc olivaceous-brown<br />
to almost black, simple to sometimes<br />
branched, straight or flexuous, with<br />
or without geniculations, and vary in<br />
length. Conidia are single, <strong>of</strong>ten<br />
greatly variable in size and shape,<br />
mostly cylindrical, hyaline, sometimes<br />
subhyaline, smooth, and three<br />
or more septate, and measure 25-48<br />
× 4-6 nm depending on whether they<br />
are from the host or culture medium<br />
(Fig. 14.3c).<br />
14.3a. Habit character<br />
<strong>of</strong> Cercospora<br />
janseana on sterile<br />
glumes.<br />
b. C. janseana<br />
colony on PDA.<br />
c. Conidia <strong>of</strong><br />
C. janseana stained<br />
with lactophenol<br />
blue. d. Lesions<br />
caused by<br />
C. janseana.<br />
Fungal pathogens 77