RA 00110.pdf - OAR@ICRISAT

Diseased
p l a n t
S p o r a n g i a l r e l e a s e t o a i r
Thermal c o n v e c t i o n
T u r b u l e n t
d i f f u s i o n
Keep t h e
s p o r a n g i a a i r - b o r n e
S e d i m e n t a t i o n due t o g r a v i t y
Figure 3. Air-borne state of Sclerospora graminicola.
at least a few of the sporangia will be viable for
2.5-6.0 h. At a wind speed of 50 m min - 1 , the sporangia
can travel over 3 km in an hour and still be viable.
Further, the environmental conditions best suited
for sporulation are also best suited for efficient dispersal.
Subramanya (1984) demonstrated that during
the hottest nights of the year (April), the sporangia
could remain viable for 3 h 15 min and the
half-life was 1 h. Even at this range, the viable sporangia
can travel at least 1 km. Singh and Williams
(1980) observed the spread of the inoculum up to 340
m in the rainy season, but the disease spread only up
to 80 m from the inoculum source in the postrainy
season. Mayee and Siraskar (1980) observed the
spread of the disease up to 2 km. Obviously, there is
wide variation in the distance the pathogen can
spread by air, perhaps due to the nature of the inoculum
source and the weather. Heavy inoculum sources
and favorable weather place more sporangia in
the air, with more moved from the source.
The fate of sporangia which fall to the ground
after take off from the sporangiophores was not
known until recently. Safeeulla (1976b) speculated
that these sporangia may liberate zoospores in the
wet soil, which in turn may infect healthy plants
through the roots. This is important because zoospores
can very effectively infect through roots.
Ramesh (1981) observed that the sporangia can
germinate in the soil and produce zoospores which
can survive, move against gravity in the soil, and
remain infective up to 5 h. This shows that sporangia/
zoospores deposited on the soil may act as a secondary
source of inoculum, causing infection in the
seedling stage of the plant during the rainy season.
Zoospores exhibited a strong affinity towards
host plant roots, and less affinity towards the nonhost
roots. The complete process of infection starting
from zoospore germination, formation of the
appressorium and infection peg, further development
in the epidermal cells of the host, and subsequent
colonization, was observed in host plant
roots. In nonhost monocotyledenous plants, although
the fungus penetrated the epidermis, it failed to colonize
the root tissue (Subramanya et al. 1983).
Reddy (1973) established the airborne nature of
the sporangia, but details such as horizontal and
vertical diffusion of sporangia in the air have not
been worked out. Shenoi (1976) observed maximum
deposition of conidia of P. sorghi at the tips of the
sorghum leaves. Sporangia may also be deposited at
the leaf tips of pearl millet plants. At Mysore, dew
periods of 2-8 h in a day are frequent (Shenoi and
Ramalingam 1979), and dew deposited on the leaves
can act as a germination medium for sporangia on
the leaves to liberate the zoospores. The zoospores
swim towards the leaf whorl of the plant to cause
infection (Subramanya et al. 1982) (Fig. 4).
As a rule, sporangia germinate by producing
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