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

CHAPTER 14
Fungal pathogens
J.K. Misra, S.D. Merca, and T.W. Mew
Fungi are the most numerous of the
seedborne rice pathogens. Their epidemic
potential varies between species,
races, ecosystems, and with
their immediate adaptation to their
environments. Species of concern to
quarantine are Alternaria padwickii,
Bipolaris oryzae, Cercospora janseana,
Curvularia lunata, Ephelis oryzae
Fusarium moniliforme, Microdochium
oryzae, Nakataea sigmoidea, Pyricularia
oryzae, Rhizoctonia solani, Sarocladium
oryzae, Tilletia barclayana, and
Ustilaginoidea virens.
Alternaria padwickii
Pathogen: Alternaria padwickii (Ganguly)
Ellis (Ellis 1971)
Other acceptable names: Trichoconis
padwickii, Trichoconiella padwickii
(Etymology: from Latin alteres, a kind
of dumbbell and Padwick, a scientist)
Disease: stackburn
Detection level: frequently detected
(1-100% of incoming seed lots), with
low epidemic potential
Where detected: infected seeds and
plant parts
How detected: blotter or agar plate
methods
Appearance: see Figure 14.1.
Under a stereobinocular microscope,
restricted to profuse mycelial growth
with conidia can be seen over the
seed on the blotter after 6-8 d incubation
(Fig. 14.1a). The profusely growing
mycelia are grayish brown, a
characteristic of this fungus. Sometimes,
pinkish to brownish areas are
seen over the blotter around the seed
(Fig. 14.1b). Aerial hyphae with
straw-colored to dark brown conidia
with long terminal appendages are
easily discernible at 25X. Figure 14.1c
shows a slide mount of conidia with
long apical appendages.
A colony on potato dextrose agar
is light salmon to dark grayishbrown
and attains 4.1 cm in diameter
after 5 d incubation at 25 °C
(Fig. 14.1d). The reverse is greenish
black with salmon edge. Mycelia are
effuse, thin, well-developed, copiously
branched, hyaline while young
and become salmon to dark brown
at maturity, 3-6 nm thick, and
septate at almost regular intervals of
20-25 nm. Conidiophores are 100-175
× 3-6 nm, swollen apically, and
minutely echinulate at the tip.
Conidia are fusiform, nondeciduous,
measure 103-173 × 9-20 nm (the
broadest cell measuring 9-20 nm),
have three to five (commonly four)
transverse septa, are constricted at
the septa, hyaline, and turn from
straw-colored to grayish-brown at
maturity, with a long terminal appendage.
The appendage is half or
more of the length of the conidium.
One or more septa are seen in the
body of the appendage (Fig. 14.1c).
THE DISEASE—STACKBURN
Stackburn is widely spread. It occurs
in China, several Southeast Asian
countries, Egypt, Nigeria, Madagascar,
Surinam, and the USSR.
14.1a. Alternaria
padwickii mycelial
growth and conidia
on seed. b. Pink to
brownish coloration
rendered by
A. padwickii (courtesy
of S. Merca).
c. Conidia of
A. padwickii.
d. A. padwickii
colony on potato
dextrose agar.
e. Stackburn lesion
on leaves.