Untitled - International Rice Research Institute

. Occurrence/distribution
Rice blast is widely distributed in all rice-growing
countries (Fig. 23). It is most prevalent in temperate
subtropical environments and also in rice
grown in upland conditions.
c. Disease history
Records of this disease can be traced back to as
early as 1637 in China. It was reported in 1704 in
Japan, in 1828 in Italy, and in 1907 in South Carolina,
USA. In India, it was first recorded in 1913.
Its causal fungus, Pyricularia oryzae, was named
in 1891 in Italy. It was recently renamed P. grisea
but P. oryzae has widespread usage.
d. Importance in crop production
Blast is generally considered as the principal disease
of rice because of its wide distribution and
destruction in causing crop failure and epidemics.
The epidemic potential is very high under favorable
conditions where susceptible cultivars are
planted. Blast may cause total crop failure but,
because resistant cultivars are grown widely in
major rice production environments, it accounts
for 1–3% yield loss across all rice production situations
in Asia.
Detection on seed
a. Incubation period on blotter
Using the blotter test, P. oryzae can be observed
on rice seeds 3–4 d after incubation in NUV light
at 21 °C. The detection frequency is about 9.9%
on seeds coming from different regions (Fig.
24a,b).
b. Habit character
Aerial mycelia are rarely present or in most cases
absent. If present, mycelia are branched, hyaline
to olivaceous. If aerial mycelium is absent, conidiophores
arise directly from the seed surface singly
or in small groups or bundles. They are moderately
long, simple, and light brown. Conidia are
hyaline, pale olive or grayish, and borne
sympodially (Fig. 25a-c).
c. Location on seed
P. oryzae is observed mostly on sterile lemmas of
the seed (91%) (Fig. 26).
Microscopic characters
a. Mycelium—septate, branched, and hyaline.
b. Conidiophores—simple to rarely branched, moderately
long, septated, light brown, slightly thickened
at the base with denticles at the apex (Fig.
25d).
c. Conidia (sympodulosphores)—pyriform to
obclavate, hyaline to pale olive; usually 2 septate,
rarely 1 or 3 septate (observed in PJA); apex narrow,
base rounded with a prominent appendage or
hilum (Fig. 25e). Measurements: 15.64–22.54 µ ×
7.82–10.81 µ (PDA); 16.56–31.97 µ × 8.51–12.42
µ (PJA); and 15.18–25.76 µ × 7.13–10.81 µ(VJA).
Fig. 23. Occurrence of blast (Ou 1985, CMI 1981).
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