A manual of rice seed health testing - IRRI books - International Rice ...
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Pseudomonas fuscovaginae<br />
Pathogen: Pseudomonas fuscovaginae<br />
Miyajima, Tanii, and Akita<br />
(Etymology: from Greek pseudes,<br />
false; monas, unit; from Latin adj.<br />
fuscus, fuscous; from Latin fem. n.)<br />
vagina, sheath)<br />
Disease: bacterial sheath brown rot<br />
Detection level: uncertain, epidemic potential<br />
depends on temperature at<br />
panicle initiation<br />
Where detected: infected <strong>seed</strong>s, rotted<br />
sheaths and <strong>seed</strong>lings<br />
How detected: see Chapter 7.<br />
Appearance and characteristics: see<br />
Figure 15.2.<br />
See Chapter 7 and Figure 15.2a for a<br />
comparison <strong>of</strong> P. avenue, P. glumae,<br />
P. fuscovaginae, and P. syringae pv.<br />
syringae.<br />
THE DISEASE—BACTERIAL SHEATH<br />
BROWN ROT<br />
Bacterial sheath brown rot is now<br />
widely distributed in Latin America,<br />
Asia, the Burundi highlands <strong>of</strong> Central<br />
Africa, and Madagascar.<br />
Precise information regarding the<br />
loss caused by the disease is lacking,<br />
but it is known that appreciable<br />
losses have occurred in tropical and<br />
temperate South America.<br />
Pseudomonas fuscovaginae differs<br />
from other oxidase and arginine<br />
dehydrolase positive nonpathogenic<br />
fluorescent pseudomonads in that it<br />
does not produce 2-ketogluconate.<br />
However, it does produce acid from<br />
trehalose but not from inositol<br />
(Duveiller et al 1988).<br />
15.2a. Pseudomonas<br />
fuscovaginae on nutrient<br />
agar (blue background).<br />
b. Sheath brown rot<br />
symptoms on <strong>seed</strong>ling.<br />
c. Early symptoms <strong>of</strong><br />
infection at the booting<br />
stage (courtesy <strong>of</strong> K.<br />
Miyajima). d. Late<br />
symptoms <strong>of</strong> infection at<br />
the booting stage (courtesy<br />
<strong>of</strong> K. Miyajima).<br />
Symptoms<br />
Sheath brown rot symptoms appearing<br />
at <strong>seed</strong>ling and later stages include<br />
discoloration and rotting <strong>of</strong><br />
sheath (Fig. 15.2b,c, and d). After<br />
transplanting, infected <strong>seed</strong>lings initially<br />
show yellow-brown<br />
discolorations on their lower leaf<br />
sheaths. Later the discolorations turn<br />
gray-brown to dark brown. Ultimately,<br />
rot results in death <strong>of</strong> the infected<br />
<strong>seed</strong>ling. The infected flag leaf<br />
sheath becomes water-soaked and<br />
necrotic. Other sheaths also exhibit<br />
lesions. Spikelets <strong>of</strong> emerging panicles<br />
are discolored, sterile, or may be<br />
symptomless except for small brown<br />
spots. In acute infections, the sheaths<br />
turn completely grayish brown or<br />
dark brown and panicles shrivel and<br />
dry.<br />
Disease development<br />
Bacterial sheath brown rot is active<br />
in irrigated temperate regions and<br />
rainfed upland <strong>rice</strong> ecosystems.<br />
Cold temperature stress probably<br />
predisposes plants to severe attacks<br />
<strong>of</strong> bacterial sheath brown rot.<br />
In temperate regions,<br />
P. fuscovaginae can survive in <strong>rice</strong><br />
straw only if it is stored indoors. In<br />
the tropics, other host plants and infected<br />
<strong>seed</strong>s harbor the bacterium.<br />
These can be a source <strong>of</strong> primary<br />
inoculum.<br />
The secondary infection or disease<br />
cycle in the field may arise through a<br />
bacterial population already proliferating<br />
on symptomless leaf blades<br />
and sheaths. The secondary occurrence<br />
is most infectious at booting<br />
stage.<br />
Control<br />
Dry heat treatment at 65 °C for 6 d<br />
eliminates P. fuscovaginae from <strong>seed</strong>s<br />
(Zeigler and Alvarez 1987). Streptomycin<br />
alone or combined with<br />
oxytetracyclin (15% + 1.5%) effectively<br />
arrests bacterial sheath brown<br />
rot. Kasugamycin is effective against<br />
P. fuscovaginae but does not eliminate<br />
it completely from the <strong>seed</strong>.<br />
92 <strong>Rice</strong> <strong>seed</strong> <strong>health</strong> <strong>testing</strong> <strong>manual</strong>