RA 00110.pdf - OAR@ICRISAT

however, that wind plays an important role in dislodging
oospores from shredding leaves and disseminating
them to neighboring areas. It is also possible
that whirlwinds, common in the semi-arid tropics
during April and May, carry oospores from field to
field. Dissemination would be effective because the
off-season fields would be free of crops, and oospores
and oospore-laden debris are dry and light.
Frederiksen and Rosenow (1967) and Rajasaab et al.
(1979) reported the airborne nature of P. sorghi
oospores.
Secondary Infection
The progress of the disease in a single season
initiated by primary inoculum sources depends on
recurring infection cycles involving secondary inoculum.
Once the host becomes infected, it produces
several crops of sporangia which cause secondary
infection in the same season.
S. graminicola produces sporangia in large numbers
under humid conditions. Much work has been done
on the influence of environment on sporulation
(Weston 1929, Safeeulla and Thirumalachar 1956,
Suryanarayana 1965).
The author's work on this pathogen has shown
that sporulation isinfluenced by several factors. The
process of sporulation can be classified into two
phases: inductive and formative. During the inductive
phase, the pathogen in the host tissues prepares
itself for asexual reproduction. The formative phase
begins when the sporangiophores start emerging
from the stomata, and continues up to the dissemination
of the sporangia. Environmental factors, particularly
humidity, can influence sporulation during
the formative phases, while temperature can influence
sporulation both during the inductive and the
formative phases. Sporulation can occur between 14
and 30°C (Safeeulla 1976a). No sporulation was
observed below 12°C or above 30°C, and sporulation
was maximum at 23°C and 100% R H . It was
also evident that at low temperatures (< 23°C), the
time required for sporangia production was longer
than at 23°C. The night temperature at Mysore
(South India) during the cropping season ranged
from 20-25°C. At all these temperature profiles,
sporulation can occur within 4-6 h. Maximum sporulation
was observed at 100% R H , but very little at
70% R H . The duration of the inductive period was
almost the same at all RH profiles, but at 100% R H ,
the formative phase was as short as 2 h 25 min. With
RH decreased to 80%, the duration of the formative
phase increased. Probably the desiccation at low RH
and a high rate of metabolic activity are responsible
for the delay in sporulation at low RH and higher
temperature.
Free water on the leaf surface inhibits sporulation
of S. graminicola, but Safeeulla and Thirumalachar
(1956) observed the proliferation of sporangiophores.
Under such conditions, the absence of free aeration
might be responsible for the inhibition of sporulation.
Some of the author's experiments have also demonstrated
that prior exposure of the leaves to light is
a must for sporulation. Infected leaves from plants
stored in darkness for 12 h failed to sporulate. A
minimum 2-hour exposure to light prior to incubation
at 24°C, 100% RH in darkness, was essential for
infected leaves to sporulate. However, with the
increase in exposure duration, the number of sporangia
produced per unit area also increased. This
indicates that the photosynthates produced during
the period of light exposure are utilized for the
actual phase of sporulation, which obviously needs
high nutrition for the process of wall and protoplast
synthesis (Subramanya 1984).
Under favorable conditions Safeeulla (1976b)
recorded the production of 35 000 sporangia cm -2 on
the infected leaf and as many as 11 crops of sporangia
formed on successive nights.
Melhus et al. (1927) found that sporangia are
forcibly discharged up to 2.5 m from the sporangiophores.
Twisting of sporangiophores during the
process of liberation of sporangia into air has not
been observed so far. Sporangial liberation can
occur continuously at 24°C, 100% R H , and darkness.
However, the effect of varying temperatures
and light on spore liberation has not been studied.
After liberation, some sporangia may fall to the
ground and some may be carried by moving air (Fig.
3).
The sporangium remains airborne after take off
from the sporangiophores until it gets deposited on a
substratum. The viability of the sporangium is
determined by temperature, humidity, and wind
speed. Analysis of weather at Mysore indicated that,
during most of the year, nights are best suited for
sporulation. Most days have a high RH (above 95%)
between 00:00 and 06:00, with a temperature of 20-
24°C, and a wind speed of 20-160 m min - 1 . Subramanya
and Safeeulla (1981) showed that at 98% R H ,
22°C, and 50 m min - 1 wind speed, about half of the
sporangial population can remain viable for more
than 1 h and at least some of the sporangia will be
viable for 6 h. At night time extremes for temperature
and RH in Mysore during the cropping season,
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