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

The teleomorph, Sphaerulina
oryzina, was described by Hara
(1918). It is not commonly seen on
seeds during rice seed health testing.
THE DISEASE—NARROW BROWN LEAF
SPOT
Constantinescu (1982) renamed the
fungus Cercospora janseana (Racib.)
O. Const.
Narrow brown leaf spot occurs in
almost all rice-growing countries in
Asia, Latin America, Africa, and in
the USA, Australia, and Papua New
Guinea.
Symptoms
Narrow brown elongated spots or
lesions measuring 2-12 × 1-2 mm appear
on the leaves (Fig. 14.3d), leaf
sheaths, pedicels, and glumes. In resistant
varieties, lesions may be narrower,
shorter, and darker than
those on susceptible varieties (Ou
1985). Spots appear just prior to
flowering stage.
Infection causes severe damage in
susceptible varieties by reducing the
green surface area of the leaves, killing
them and the sheath.
Disease development
Estrada and Ou (1978) reported that
30 d or more are required for symptoms
to develop after artificial inoculation.
This may account for the late
appearance of the disease in the field
although young and old leaves are
equally susceptible.
Both upland and lowland environments
support disease development.
Control
The disease can be controlled by
using resistant varieties and chemicals.
lnformation is not available on
the effectivity of seed treatment to
control the disease.
Curvularia spp.
Pathogen: Curvularia Boedijn (Boedijn
1933)
(Etymology: from curvus, curved, referring
to curved spores)
Disease: black kernel
Detection level: frequently detected
(1-60% of seeds tested), with very low
epidemic potential
Where detected: seeds and other plant
parts; decaying plant parts
How detected: blotter or agar plate
methods; washing test
Appearance: see Figure 14.4.
One of the most commonly encountered
fungal genera during rice seed
health testing, Curvularia spp. may
infect up to 80% of seeds and cause
grain discoloration. In severe infections,
Curvularia may weaken seedlings
and cause leaf spot (Ou 1985).
The most common species infecting
rice is C. lunata; however, C. affinis,
C. geniculata, C. oryzae, and
C. pallescens may also be involved in
black kernel disease.
Identification of the species is not
necessary during rice seed health
testing. However, to identify the species,
view mycelia, spores, and
conidiophores under a compound
microscope. Mount specimen in water
or lactophenol cotton blue. Water
is preferable as it does not interfere
with ascertaining the color.
Under a stereobinocular microscope,
infested seeds on a blotter
show light to dark brown or somewhat
blackish erect conidiophores
(macronematous) scattered or (sometimes)
grouped (Fig. 14.4a), with terminal
or/and laterally borne light to
dark brown conidia (Fig. 14.4b).
Conidia are often curved but may
show other shapes as well.
On infected seeds, dark brown to
almost black, unbranched, and
septate (apically) conidiophores are
seen. They have boat-shaped,
brownish conidia. Conidia are born
terminally, spirally, or in whorls giving
a clustered appearance. Viewed
under a compound microscope in
water or lactophenol cotton blue
mount, the conidia appear triseptate.
The third cell from the base is larger,
shows prominent curvature, and the
basal cell has a scar from its attachment
with the conidiophores.
Conidia measure 19-32 × 8-16 nm
(Fig.14.4c).
Curvularia lunata (Walker) Boedijn
colony on potato dextrose agar at
30 °C attains 7.2 cm diameter
(Fig. 14.4d). The colony is dark
brown to black with hyaline edge.
14.4a. Habit character of
Curvularia showing dark mass of
conidiophore and conidia.
b. Flowerlike conidiophores of
Curvularia spp. c. C. lunata
conidia. Note scar on basal cell.
d. Curvularia spp. colony on PDA.
78 Rice seed health testing manual