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

Symptoms
Stackburn causes lesions on leaves.
Significant seed infection and discoloration
result in poor germination
and rotting of seeds, roots, and
coleoptile. A. padwickii infects the
endosperm and reduces the rice
quality. Although stackburn is rarely
seen in the Philippines, a high percentage
of seed infection is revealed
by the blotter test.
Where severe infections occur,
symptoms are visible on seedlings,
on leaves of adult plants, and on
grains as discoloration. The fungus
causes typical dark brown spots on
the leaves (Fig. 14.1e). The spots are
oval to circular, with distinct margins
and rings. The spots vary from
1 to 5 mm in diameter. The center of
the spot is pale brown. Later it turns
white and develops minute black
dots, the sclerotia. Similar spots may
appear on seedling roots where they
cause root tissue to rot. In severe infections,
seedlings wilt and finally
die. Infected grains have pale brown
to whitish spots with a dark brown
border and black dots in the center.
Similar symptoms arise from various
other organisms.
The fungus can penetrate deep
into the glumes, causing the kernel
to shrivel and become brittle.
Disease development
Both upland and lowland ecosystems
support stackburn.
The disease cycle has not been determined
yet. A. padwickii is thought
to survive in the soil and on old rice
straw and cause infection in the next
season. Infected seeds may be the
source of primary inoculum. The
stackburn pathogen infects wild
grass in ricefields. The wild grasses
may be a source of inoculum
(Padwick 1950).
Little is known about the influence
of environmental factors on
stackburn. Sreeramulu and Vittal
(1966) found conidia in the air over
ricefields in greater numbers in the
late morning than at other times of
the day. This indicates the role of
temperature in spreading the disease.
Control
Seed treatment with Dithane M-45
(0.3% by seed weight) provides satisfactory
control (Vir et al 1971). Other
fungicides and hot water treatment
at 50-54 °C for 15 min are also suggested.
Burning stubble and rice
straw reduces inoculum potential.
Bipolaris oryzae
Pathogen: Bipolaris oryzae (Breda de
Haan) Shoemaker
Other acceptable names:
Drechslera oryzae and
Helminthosporium oryzae
Teleomorph: Cochliobolus miyabeanus
(Ito and Kuribayashi) Drechsler ex
Dastur
(Etymology: from bipolaris, bipolar,
referring to the bipolar germination of
the spores)
Disease: brown spot
Detection level: frequently detected
(1-65% of seeds tested), with low epidemic
potential
Where detected: infected seeds and
plant parts
How detected: blotter or agar plate
methods, washing test
Appearance: see Figure 14.2.
Infected seeds incubated on blotters
appear dark brown to black, with
profuse growth of mycelia visible to
the unaided eye (Fig. 14.2a). Under a
stereobinocular microscope, at
12-25X magnification, erect dark
conidiophores appear scattered or in
groups over the seeds. Conidia on
the conidiophores are curved both
apically and laterally (Fig. 14.2b).
After 5 d incubation at 30 °C , a
colony on potato dextrose agar
measures 8.1 cm in diameter
(Fig. 14.2c). It is effuse, dark brown
to black, with blackish reverse.
Hyphae are branched, darker, and
measure 8-15 nm in diameter.
Conidiophores occur in small
groups, seldom singly, and are
flexuous, geniculate, light to dark
brown, long, and thick. Conidia are
numerous, curved, naviculate, light
brown, smooth, pseudoseptate with
6-14 septa, 63-153 × 14-22 nm, with
minute hilum (Fig. 14.2d).
14.2a. Habit character of
B. oryzae on severely infected
seeds. b. Habit character of
B. oryzae on lightly infected
seeds. c. B. oryzae colony on
PDA. d. E. oryrae conidia show-
Ing minute hilum at the base of
the conidia (courtesy of
S. Merca). e. Brown spot symp
toms on leaves (courtesy of
S. Merca).
76 Rice seed health testing manual