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

Sarocladium oryzae is seedborne.
The fungus enters through stomata
or wounds. Secondary infections
may be windborne, the fungus entering
the host through injured tissue
(Amin et al 1974, Chin 1974).
Plants are most vulnerable at
tillering to panicle initiation stages.
Infection at these stages becomes serious.
Severe infections occur in densely
planted fields, especially where stem
borers have infested the plants, or
the plants are under stress.
Control
No effective control methods are
currently available. However, seed
treatment with fungicides such as
Dithane M-45 and Benlate effectively
eliminates seedborne inocula.
Tilletia barclayana
Pathogen Tilletia barclayana (Bref.) Sacc.
and Syd. (Duran and Fischer 1961)
(Etymology: after Tillet and Barclay,
plant pathologists)
Disease: kernel smut
Detection level: frequently detected
(1-100% of seeds observed), with low
epidemic potential
Where detected: infected seeds
How detected: during dry seed inspection
under a stereobinocular microscope;
washing test; soaking in 0.2%
solution of NaOH
Appearance: see Figure 14.12.
Under the stereobinocular microscope,
infected seeds on a blotter
show blackening of glumes. Dissected
glumes show masses of
spores on and in the kernel. In some
instances, grains burst, revealing the
spore masses. Dark, black, minute
spots (singly or as masses of sporesteliospores)
may be seen on the infested
grain (Fig. 14.12a,b). Under a
compound microscope, white, erect,
primary sporidia may be seen issuing
from spores on incubated seeds
(Fig. 14.12b) and details of
teleospores with primary sporidia
(Fig 14.12c) may be recognized.
Takahashi (1896), who named the
fungus Tilletia horrida, described it as
follows: "spore masses pulverulent,
black produced within the ovaries
and remaining covered by the
glumes. Spores globose, irregularly
rounded, or sometimes broad-elliptical,
the round ones 18.5-23.0 nm in
diameter and the elongated 22.5-26.0
× 18.0-22.0 nm in size. Epispore deep
olive brown, opaque, thickly covered
with conspicuous spines
(Fig. 14.12d). The spines hyaline or
slightly colored, pointed at the apex,
irregularly polygonal at the base,
more or less curved, 2.5-4.0 nm in
height, and 1.5-2.0 nm apart at their
free ends. Sporidia filiform or needle-shaped,
curved in various ways,
10 to 12 in number and 38-53 nm in
length." Subsequently, the fungus
has been studied by various workus.
Padwick and Khan (1944) placed
it with the genus Neovossia as
N. horrida (Tak.) Padwick and Azmat
Khan. Later on, Tullis and Johnson
(1952) renamed it N. barclayana.
Duran and Fischer (1961) returned it
to the genus Tilletia, and renamed it
Tilletia barclayana (Bref.) Sacc. and
Sycl.
THE DISEASE—KERNEL SMUT
Takahashi and Anderson reported
kernel smut in Japan and the USA in
1896 and 1899, respectively. It is now
known in almost all rice-growing
countries.
14.12a. Tilletia
barclayana infected
and -contaminated
rice seeds.
b. T. barclayana
spores germinating.
Note whitish growth
of promycelia and
primary sporidia.
c. Germinating
teliospore giving rise
to promycelin and
primary sporidia
(courtesy of
S. Merca).
d. T. barclayana
spores. e. Smutted
seeds in panicles
caused by
T. barclayana.
Fungal pathogens 87