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

In susceptible varieties, lesions
may also appear over the leaf sheath,
extending to its base. Lesions may
occur on an injured part of a blade.
Sometimes infected blades wilt and
roll up while the other leaves are still
green. Yellow stripes sometimes appear
toward the inner side of the leaf
blade, especially in resistant varieties.
These later become necrotic.
Bacterial ooze—milky or pale amber-colored
droplets—exudes from
the surface of the young lesions in
the early morning (Fig. 15.5d). The
droplets dry up and impart a certain
roughness to the leaf. The dry droplets
fall off easily into the field water,
where they may become a source of
secondary inoculum.
In severe infections, grains are involved.
Glumes show discolored
spots surrounded by water-soaked
margins. Spots can easily be seen
while the grain is young and green;
when the grain is mature, the spots
become gray or yellowish white.
In temperate regions, the disease
appears at the heading stage, rarely
in the seedbed. On seedlings, small
water-soaked spots develop on the
margins of fully developed lower
leaves. Later, the leaves yellow, dry
rapidly, and wither.
Kresek symptoms were described
in Indonesia almost four decades
ago, as a separate bacterial rice disease.
Leaves of the infected plant become
grayish-green, fold up, and roll
along the midrib about 1-2 wk after
transplanting.
Pale yellow leaf symptom is seen
in the youngest leaves of mature
plants. These leaves become uniformly
pale yellow or have yellow or
greenish-yellow broad stripes on
their blades. The mechanism of the
appearance of this symptom is not
well-understood. Bacteria have not
been detected in the infected leaves
but have been found in the stem and
in the internodes of the infected
leaves. Some workers consider this
symptom to be a secondary effect.
Sometimes it is difficult to distinguish
the symptoms of bacterial
blight from symptoms of other dis-
eases. Kresek symptoms may be confused
with stern borer injuries. In
order to confirm bacterial leaf blight
infection, a few tests can be made.
Perform the pathogenicity test for
Xanthomonas (see Chapter 7).
Disease development
Irrigated and rainfed lowland ecosystems
support bacterial leaf blight
development.
Heavy rains with strong winds
facilitate disease development by
causing wounds in plants. Dry
weather helps bacterial exudates fall
into irrigation water and spread the
disease to neighboring fields. Moderately
high temperature (25-30 °C)
increases the disease incidence.
Where and how long Xoo survives
in the environment to cause
disease recurrence is not settled, as
scientists of different rice-growing
areas report varying findings. Other
alternate hosts (weeds), diseased
straw, rice stubble, and irrigation
water (field or canal) may be potent
inoculum carriers for the next sea-
son.
Xoo can enter into the host's body
through numerous points such as
stomata, wounds and other injuries
to leaves, water pores (hydathodes)
on the leaf blades, and cracks at the
base of the leaf shield caused by
emergence of new roots.
Xoo may be detected from the late
seedbed stage to the tillering stage,
or even later.
Excessive use of nitrogen (especially
organic nitrogen) as a late
topdressing, phosphate and potassium
deficiency, and excess silicate
and magnesium are importmt factors
that predispose plants to bacterial
leaf blight infection.
Control
The disease can be avoided effectively
by using resistant varieties.
However, the pathogen has different
races in different areas and it is essential
to know the pathogen’s racial
distribution to plan effective varietal
deployment.
Xanthomonas oryzae pv.
oryzicola
Pathogen: Xanthomonas oryzae pv.
oryzicola (Fang, Ren, Chen, Chu, Faan
& Wu) Dye (Dye 1978)
(Etymology: from Greek xanthus, yellow;
monas, unit; oryza, rice)
Disease: bacterial leaf streak
Detection level: infrequently detected,
high epidemic potential
Where detected: seeds and other plant
parts
How detected: see Chapter 7.
Appearance and characteristics: see
Figure 15.6.
See Chapter 7 and Figure 15.6a for a
comparison between X. oryzae pv.
oryzae and X. oryzicola.
THE DISEASE—BACTERIAL LEAF STREAK
Bacterial leaf streak is widespread in
tropical Asia.
Symptoms
Translucent interveinal streaks of
various length appear on the leaves
(Fig. 15.6b,c). Yellowish droplets of
bacterial ooze may be seen over the
lesions under humid conditions
(Fig. 15.6d). When dry, the ooze
looks like small beads. Windborne
beads then infect healthy leaves. Old
lesions become light brown. A yellow
halo may appear around the lesions
in susceptible varieties.
At advanced stages of the disease,
the leaves turn brown and die.
Necrotic lesions become covered by
saprophytic organisms. At this point,
it is difficult to distinguish the symptoms
of bacterial leaf streak from
those of bacterial leaf blight.
Disease development
Bacterial leaf streak develops in both
lowland and upland ecosystems.
High temperature (more than
30 °C) favors disease development.
High humidity (more than 80%)
favors initiation of infection and dissemination
of the bacteria.
Bacterial pathogens 95