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

Colonies on potato dextrose agar
at 25 °C measure 4.1 cm in diameter
after 5 d (Fig. 14.9b). Mycelia are
thin, effuse, almost black in the
center with whitish margins; reverse
blackish. Hyphae are hyaline and
branched. Conidiophores are distinct
from vegetative hyphae
(macronematous), single or in
groups, simple, and septate with distinct
geniculations. Conidia are produced
singly in sympodial fashion,
pyriform to obclavate, usually
biseptate, rarely 1-3 septate, almost
hyaline to pale olive-colored, measuring
14-40 × 6-13 nm; with distinct
protruding hilum (Fig. 14.9c).
THE DISEASE—BLAST
Blast is one of the most widely distributed
rice diseases. Losses due to
blast are difficult to estimate because
other factors complicate the disease.
However, losses are always significant.
Symptoms
Pyricularia oryzae produces spots or
lesions on leaves, nodes, panicles,
and grains, but rarely on leaf
sheaths.
Leaf spots are spindle-shaped
with brown or reddish-brown margins,
ashy centers, and pointed ends
(Fig. 14.9d). Fully developed lesions
normally measure 1.0-1.5 cm in
length and 0.3-0.5 cm in breadth.
These characteristics vary with the
spots’ age, susceptibility of the
cultivar, and environmental factors.
When nodes are infected, they become
black and rotten (Fig. 14.9e).
Infection of the panicle base causes
rotten neck or neck rot (Fig. 14.9f) and
causes the panicle to fall off. In severe
infection, secondary rachillae
and grains are also affected.
Disease development
Found in both upland and lowland
environments, blast occurs most often
in upland environments in the
tropics. Water deficiency predisposes
the crop to severe infection in
all environments.
Low night temperatures favor
blast infection and lesion development.
Disease development is very
much affected by the humidity and
soil moisture. The upland ecosystem
with high night humidity, where rice
is grown in dry soil, presents a
favorable environment for development
of blast. Rice grown in moist
and flooded conditions is moderately
to highly resistant to the disease.
Airborne conidia, which may be
present all year in the atmosphere,
are the most potent source of infection.
Conidia may be seedborne, or
they may come from straw, stubble,
or numerous alternate or collateral
hosts (weeds). Environmental factors,
such as temperature and relative
humidity, play significant roles
in conidia production and, thus in
cycling the disease.
High relative humidity (more
than 90%) favors conidial formation.
A typical lesion can produce 2000-
6000 conidia daily for at least 14 d
under laboratory conditions (IRRI,
unpubl. data).
Applying high amounts of nitrogenous
fertilizers induces a heavy incidence
of blast, irrespective of the
supply of phosphorus or potassium.
Control
Blast may be controlled by
planting resistant varieties,
using chemicals, and
using balanced agronomic practices.
Many blast-resistant rice varieties
have been developed, but the pathogen
adapts to them relatively
quickly. Hence, constant efforts are
needed to breed rice varieties with
several resistant genes.
If planting time is suitably adjusted
to avoid low night temperatures
and high humidity, blast infection
can be avoided.
Seed is effectively treated with a
benomyl slurry at 0.3% by seed
weight, or by hot water at 52-57 °C
for 15 min.
Rhizoctonia solani
Pathogen: Anamorph: Rhizoctonia solani
Kuhn
Teleomorph: Thanatephorus
cucumeris (Frank) Donk
(Etymology: from rhiza, root; ketinein,
to kill)
Disease: sheath blight
Detection level: infrequently detected,
with low epidemic potential
Where detected: infected seeds and
other plant parts
How detected: sclerotial bodies seen
during dry seed inspection; blotter test
Appearance: see Figure 14.10.
Under a stereobinocular microscope,
infected seeds on a blotter show dark
brownish spreading mycelia. Under
a compound microscope,
Rhizoctonia’s characteristic branching
pattern identifies the pathogen
(Fig. 14.10a).
Rhizoctonia solani can be grown on
potato dextrose agar or Czapek Dox
agar media at 28-30 °C. Colonies are
initially coloriess, then turn brown
with age (Fig. 14.10b,c). Hyphae are
hyaline when young but turn brown
at maturity, measure 8-12 nm in diameter,
and are septate. Three types
of mycelia may be seen—straight
runner hyphae; short, swollen, and
branched or lobate hyphae which
give rise to penetration pegs; and
mycelia consisting of moniloid cells
which take part in sclerotia formation.
The hyphae can be seen in cultures,
over the edge of the petri
plate, or on the wall of the culture
tube. On culture media, superficial
sclerotia are produced. These are
composed of thick-walled, dark,
compact cells (Fig. 14.10d). Sclerotia
may be irregular to somewhat
spherical, and measure 4-5 mm or
more in diameter. Basidia and
basidiospores are formed under
natural conditions and measure
10-15 × 7-9 nm and 8-11 × 5-6.5 nm,
respectively; however, measurements
between isolates vary.
Sclerotial size and number depend
on concentration and source of nitrogen
in the medium.
84 Rice seed health testing manual